refraction through a glass slab experiment

• Physics Article
• Tracing Path Of A Ray Of Light Passing Through A Glass Slab

Tracing Path of a Ray of Light Passing Through a Glass Slab

Refraction is the property of light due to which it bends its path while travelling from one medium to another. The law of refraction is also known as Snell’s law which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to constant. Below is an experiment to trace the path of a light ray passing through the rectangular glass slab.

To trace the path of a ray of light passing through a rectangular glass slab for different angles of incidence. Measure the angle of incidence, angle of refraction, and angle of emergence, and interpret the result.

What are the laws of refraction?

Following are the laws of refraction:

• The incident ray, the normal at the point of incidence, and the refracted ray lie in the same plane.
• Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. \(\begin{array}{l}\frac{sin\;i}{sin\;r}=constant\end{array} \)

What is refraction of light?

The refraction of light is a property of light due to which it changes its path when it passes from one medium to the other.

What is lateral displacement?

Lateral displacement is defined as the perpendicular shift in the path of light when it emerges out from the refracting medium.

Watch the video and learn more about Snell’s Law

Materials Required

• A drawing board
• 4-6 all pins
• White sheet of paper
• Rectangular glass slab
• A protractor
• Fix a white sheet on the soft drawing board using thumb pins.
• Place the glass slab at the centre of the white paper and draw its outline boundary using a sharp pencil.
• Let ABCD be the rectangular figure obtained by drawing.
• Mark a point E on AB and draw a perpendicular EN and label it as a normal ray.
• Draw one angle of 30° with the help of protractor with EN. Fix pins at P and Q at 4-5 cm on the ray that is obtained by the angle.
• Place the glass slab on the rectangular figure ABCD.
• To fix R and S, see through the glass slab from side CD, such that when seen through the glass slab, all the pins P, Q, R, and S should lie in a straight line.
• Draw small circles around the pins P, Q, R and S and remove the pins.
• Remove the glass slab.
• Join points R and S such that it meets CD at point F. Draw a perpendicular N’M’ to CD at point F.
• Using a pencil, join the points E and F.
• Measure the angles formed at AB and CD, i.e, the incident angle, refracted angle, and emergent angle.
• The lateral displacement is obtained by extending the ray PQ in a dotted line which is parallel to ray FRS.
• Measure the lateral displacement.
• Repeat the same procedure for angles 45° and 60°.

Ray Diagram

Observation Table

Due to human error the value of ∠ i – ∠ e may not be equal to zero.

• The angle of incidence and the angle of emergence are almost equal.
• As the light is traveling from rarer to denser optical medium, the angle of refraction will be lesser than the angle of incidence.
• For different angles of incidence, the lateral displacement will remain the same.
• The light will bend towards the normal when it travels from an optically rarer medium to an optically denser medium.

Recommended Videos

Previous year questions in the chapter light reflection and refraction.

Numerical questions in the chapter Light: Reflection and Refraction

Precautions

• The rectangular glass slab used should have perfectly smooth faces.
• The drawing board should be soft so that pins can be easily fixed on it.
• The angle of incidence should lie between 30° and 60°.
• All pins base should be in a straight line.
• The distance between the pins P and Q or the pins R and S, about 5 cm gap should be maintained.
• Using a sharp pencil, draw thin lines.
• The quality of the protractor should be good.
• The placement of the protractor should be correct to get correct measurements.
• The perpendiculars should be drawn with care.

Sources of Error

• There shouldn’t be any air bubbles in the glass slab.
• The measurements done by the protractor should be accurate.

Watch the video and revise all the important concepts in the chapter Light Reflection and Refraction Class 10

Viva Questions

Q1. How is the angle of incidence related to the angle of emergence? Ans: The angle of incidence is equal to the angle of emergence or may differ by very small amount due to human error.

Q2. What should be the angle of incidence for the above experiment? Ans: The angle of incidence required for the above experiment is between 30° to 60°.

Q3. On which condition does the light not deviate during the refraction of light? Ans: When the light ray is perpendicular to the refracting surface, there is no deviation in the light ray.

Q4. Which property of light causes the rainbow formation? Ans: The refraction of light causes the rainbow formation due to tiny water droplets in the atmosphere.

Q5. On which condition will the emergent and incident rays be parallel? Ans: When the angle of incidence is equal to the angle of the emergent ray, the emergence and incident ray will be parallel.

The below video provides the theory and important questions of the chapter Light: Reflection and Refraction Class 10

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CBSE Class 10 Science Lab Manual - Refraction Through Glass Slab

December 02, 2022 by Veerendra

CBSE Class 10 Science Lab Manual – Refraction Through Glass Slab

Aim To trace the path of a ray of light passing through a rectangular glass slab for different angles of incidence. Measure the angle of incidence, angle of refraction, angle of emergence and interpret the result.

Materials Required Some drawing pins, a white sheet of paper, a rectangular glass slab, a protractor, a measuring scale, a drawing board, a sharp pencil and adhesive tape.

Cause of Refraction Speed of light is different in different media. It is lesser in denser medium and higher in rarer medium. So when light enters a denser medium, its speed reduces and it bends towards the normal and when it enters rarer medium, its speed increases and it bends away from the normal.

Lateral Displacement The perpendicular distance between the emergent ray and incident ray when the light passes out of a glass slab is called lateral displacement. Note: Angle of incidence = angle of emergence, i.e. ∠i = ∠e if the incident ray falls normally to the surface of glass slab, then there is no bending of the ray of light, it goes straight without any deviation.

• Fix a white sheet of paper on a drawing board.

• Remove the rectangular glass slab. Draw a thin line XO directing towards O and inclined to the face AD of the glass slab at any angle preferably between 30° and 60°. It is advisable to take point 0 in the middle of the face AD. Replace the glass slab exactly over the boundary marked on the paper.
• Fix two pins P 1 and P 2 vertically about 5 cm apart by gently pressing their heads with thumb on the line XO.
• Observe the images of pins P1 and P2 through the face BC of the rectangular glass slab. While observing the images of the pins P 1 and P 2 through the face BC of the glass slab, fix two more pins at points P 3 and P 4 such that feet of all the pins appear to be in a straight line. In other words, the pins P 3 and P 4 are collinear with the images of pins P 1 and P 2 .
• Remove the pins and the glass slab and mark the positions of the feet of all the four pins. Join points that mark the positions of the pins P 3 and P 4 and extend the line upto point O’ where it meets the face BC. Also, join the points O and O’ as given in the Fig. 3, where XOO’Y show the path of a ray of light passing through the glass slab. The line XP 1 P 2 O represents the incident ray. Line OO’ shows the path of refracted ray in glass slab while line O’P 3 P 4 Y shows the emergent ray.
• Draw the normal NOM to the face AD at the point of incidence O and similarly, the normal M’O’N’, to the face BC at point O’. Measure the angle of incidence XON (∠i), angle of refraction MOO’ (∠r) and angle of emergence M’O’Y (∠e).
• Record the values of ∠i, ∠r and ∠e in the observation table.
• Repeat the experiment for two more angles of incidence in the range 30° to 60° and record the values of angles r and e in each case.

Observations and Calculations

And we know that, Angle of deviation (∠δ) = Angle of incidence – Angle of emergence For observation 1, ∠δ 1 = ∠i 1 – ∠e 1 = …………. For observation 2, ∠δ 2 = ∠i 2 – ∠e 2 = ……….. For observation 3, ∠δ 3 = ∠i 3 – ∠e 3 = ……………..

• The path of a ray of light passing through a rectangular glass slab is traced on the plane sheet and is shown by the ray XOO’Y in Fig. 3.
• The relations between the angle of incidence, angle of refraction and angle of emergence are obtained through different observations.
• From observations, ∠r < ∠i in each case, thus the ray entering from air to glass bends towards the normal.
• From observation, ∠i = ∠e, the emergent ray emerging out of the rectangular glass slab, is parallel to, but laterally displaced with respect to the incident ray.
• Angle of refraction (∠r) increases with increase in angle of incidence (∠i).

Precautions

• The glass slab should be perfectly rectangular with all its faces smooth.
• The tips of pins P 1 , P 2 , P 3 and P 4 should be sharp. These pins fixed on the sheet of paper may not be exactly perpendicular to the plane of paper. Thus, if their heads appear to be collinear, their feet may not be so, Therefore, it must be important to look at the feet of pins and their images while ascertaining collinearity between them. The mark of the pointed end or the foot of a pin on the paper must be considered while marking its position.
• The distance of 5cm between the pins P 1 and P 2 or P 3 and P 4 must be carefully maintained to obtain an accurate direction of incident ray and that of emergent ray.
• Take the angle of incidence preferably between 30° and 60°.
• Thin lines should be drawn using a sharp pencil to obtain accuracy.
• The angles should be measured accurately using a good quality protractor having clear markings by keeping the eye above the marking.

Sources of Error

• Drawing board may be hard.
• Drawing pins may not be fixed perpendicularly.
• Eyes may not be kept along the plane of paper in line, while observing the images.

Question 1. Why are the incident and emergent rays parallel to each other, in case of refraction though a rectangular glass slab? [NCERT] Answer: As the angle of incidence (∠i) is equal to the angle of emergence (∠e) in case of glass slab, so incident ray is parallel to emergent ray.

Question 2. Why does a ray of light bend towards the normal, when it enters from air into glass slab and bends away from the normal, when it emerges out into air? [NCERT] Answer: It bends towards the normal, when it travels from rarer to denser medium or speed of light decreases in denser medium and vice-versa.

Question 4. While tracing the path of ray of light through a glass slab, the angle of incidence is generally taken between 30° and 60°. Explain the reason on the basis of your performing this experiment for different angles of incidence. [NCERT] Answer: These are following reasons:

• If the angle of incidence is less than 30°, bending of light at glass-air interface will be very less.
• If the angle of incidence is greater than 60°, then emergent ray may emerge from the side surface as angle of emergence also increases with the increase in angle of incidence or vice-versa.

Question 5. How does the lateral displacement of the emergent ray depend on the width of the glass slab and angle of incidence? [NCERT] Answer: Lateral displacement is directly proportional to the width or thickness of the glass slab. It is also directly proportional to the angle of incidence.

Question 6. Give the relation between angle of incidence (∠i), angle of refraction (∠r), angle of emergence (∠e), in this experiment. Answer: In this experiment, the relations between ∠i, ∠r and ∠e are ∠e = ∠i and ∠r < ∠i

Question 7. What happens to the refraction angle, when angle of incidence decreases? Answer: On decreasing angle of incidence, angle of refraction also decreases.

Question 8. What distance should be maintained between the pins P 1 and P 2 or P 3 and P 4 to obtain the accurate result? Answer: The distance between the pins should be maintained at 5cm, to obtain the accurate result.

Question 9. Why is it necessary to observe the feet of the pins for collinearity of the points in this experiment? Answer: The pins fixed on the sheet of paper may not be exactly perpendicular to the plane of paper. Thus, if their heads appear to be collinear, their feet may not be. So, it is important to observe the collinearity by looking at the feet.

Question 10. Define an angle of emergence. Answer: The angle made by the emergent ray with the normal to the glass-air interface is called an angle of emergence. It is denoted by ∠e.

Question 11. State some precautions that one should take while performing the experiment on tracing the path of a ray of light passing through a rectangular glass slab for different values of angle of incidence. Answer:

• Pins should be fixed exactly vertically.
• Angles should be fixed between 30° and 60°.
• Glass slab should be placed well within the outline marked.

Question 12. When a ray of light refracts through glass slab with parallel surfaces, what is the difference between angle of incidence and angle of emergence? Answer: When a ray of light refracts through glass slab with parallel surfaces, then the difference between angle of incidence and angle of emergence is zero.

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Tracing path of a ray of light passing through a glass slab

Introduction.

The nature of a light’s path-changing process is evaluated based on the experiment in which the path of light is traced when it crosses a glass slab. According to Snell’s law of refraction, the nature of light varies from its traveling medium. Interpreting the result helps an individual to understand the reason behind changes in the value of refraction.

Figure 1: Light passing through a glass slab

The aim of this experiment is to trace the exact path in which light passes if it has to pass through a rectangular glass slab. Another aim of this experiment is to measure the exact angle of incidence, emergence, and refraction, and to finally interpret the result.

The theory is based on Snell’s law associated with refraction. The two most significant facts of this law help in depicting the basic concept of refraction. The first law of refraction states that the normal, incident, and refracted rays lie in the same line (Rehm, 2020). According to Snell, the ratio of the angle of incidence’s sine is constant to the sine of refraction angle. This fact can also be presented as sin i /sin r = constant.

Lateral displacement

The perpendicular shift within the path of light at the time when its emergence is found from a refracting medium is associated with lateral displacement.

Figure 2: Incident ray passing through a glass slab

Refraction of light

Light often changes its path as it travels from one medium to another medium. The density of the medium plays an important role in creating the refraction of light (Ahmadi et al. 2020). Refraction is found to accuse mainly when the light moves from a visually dense medium to a not-so-dense medium.

Required materials

• 5-6 all-purpose pins
• A drawing board
• White paper
• A rectangular glass slab

Process of the experiment

Figure 3: Refraction of the path of light

In this experiment, the refraction takes place in both the air-glass and glass-air interface. As presented in this figure, the ray incident on DC in an oblique manner and bends towards its normal path of the ray (Zhdanov et al. 2018). Inside the glass slab, the incident ray refracted parallel. After the refraction takes place, ray moves from its normal way. Both of these refractions take place by obeying the law of refraction.

Figure 4: Tracing the path of light

At first, a white sheet is to be taken and set on the drawing board with the help of thumb pins. The outline of the glass slab is to be drawn with the help of a pencil. In this experiment, four outline points of the rectangular glass slab are ABCD. On the side of AB, E is to be pointed and the perpendicular EN is to be drawn in the label to the normal ray. An angle of 30 0 is drawn with a protractor. At the point P and Q, pins are fixed that is around 4-5 cm from the normal ray. These two points are situated at the ray which is obtained by creating those 30 0 angles.

After that, the glass slab is to be placed at ABCD and looked through its CD side to fix R and S. these pins are to be set in such a manner so that all pins should lie in a straight line. A small circle is also to be drawn around PQR and S then all the pins is to be removed. The glass slab is to be removed at that point.

The points R and S are to be joined properly so that these lines meet at F point, situated at CD. A perpendicular is to be drawn on NM at point F. All the points, including E and F, are joined at this point with the pencil. There are two different angles, formed around AB and CD. Refracted, emergent, and incident angles are to be measured. In this process, a lateral displacement is also found to have occurred that can be getting by extending the PQ ray in parallel to FRS. This same process is repeated for 45 and 60 degrees as well.

Observation table

Table 1: Observation table of the experiment

Some precautions are to be followed for doing this experiment properly and ensuring the smoothness of the rectangular glass slab is an important one. The softness of the glass slab is to be maintained as well so that pins can be placed easily. The angle of incidence is to be placed within 30 and 60 degrees. The base of all the pins is to be in a straight line. The lateral displacement values are to be fixed also.

Q1. What is the relation between angle of emergence and angle of incidence?

The value of these two angles is constant. The value of one of these angles is equal to the other one. A small error can cause a difference in the value of these angles.

Q2. What is the reason behind the creation of the rainbow?

In the formation of the rainbow, refraction plays an important role. The tiny water droplets cause this refraction of light that finally results in creating a rainbow.

Q3. What is the condition of no deviation in the refraction of light?

At a certain time, to the refracting surface, the light ray is perpendicular. At this time, the light ray faces no derivation.

Q4. Which common errors need to be avoided in the experiment of tracing a light’s path?

No air bubbles are to be ensured in the glass slab. The accuracy in measurement by protractor is to be ensured as well.

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Refraction through a Rectangular Glass Slab

Last updated at April 16, 2024 by Teachoo

Before looking at Refraction through a Glass slab

We look at what happens when

• Light travels from rarer to denser medium
• Light travels from denser to rarer medium

Rarer and Denser medium we will study in Refractive Index 

Some definitions

• Incident Ray Light Ray which travels into a Medium is called Incident Ray
• Refracted Ray Light Ray which bends after refracting is called Refracted Ray
• Normal Ray Ray which is perpendicular to Surface at Point of Intersection is called Normal Ray

Refraction when Light Travels from Rarer Medium towards Denser Medium

When Light travels from Air to Glass (Rarer Medium to Denser Medium)

It does not move in straight Direction

It bends towards the Normal

Refraction when Light Travels from Denser Medium towards Rarer Medium

When Light Travels from Glass to Air (Denser Medium to Rarer Medium)

it does not move in straight Direction

It bends away from Normal

Now, lets look at

Refraction through a Glass Slab

In this case, Light First Travels from Air to Glass and then Back from Glass to Air

Hence there are 2 refractions

First, the ray of light travels from air to glass.

Hence , it travels from a rarer medium to a denser medium.

So, the First Refracted Ray bends towards Normal

After travelling in the glass slab

The ray of light travels from glass to air

Hence, it travels from a denser medium to a rarer medium.

Therefore, the Second Refracted Ray (called Emergent Ray) bends away from Normal

Combining the two refractions in the above case, the refraction of light through a glass slab can be represented as

Now, we extend the original incidence ray and bring it towards emergent ray

We observe that, the original incident ray and the emergent ray are parallel to each other.

The angle made by the emergent ray with the normal is called the angle of emergence.

And the Perpendicular distance between the 2 rays is called Lateral Displacement

Why does a light ray incident on a rectangular glass slab emerges parallel to itself?

The ray of light emerges parallel to itself because

the bending of the ray of light on top face AB (air-glass interface) 

is equal and opposite to 

the bending of the ray of light on bottom face CD (glass-air interface)

So, the ray of light emerges parallel to itself

Thus, Angle of incidence = Angle of Emergence

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CBSE Library

CBSE Class 10 Science Lab Manual – Refraction Through Glass Slab

Aim To trace the path of a ray of light passing through a rectangular glass slab for different angles of incidence. Measure the angle of incidence, angle of refraction, angle of emergence and interpret the result.

Materials Required Some drawing pins, a white sheet of paper, a rectangular glass slab, a protractor, a measuring scale, a drawing board, a sharp pencil and adhesive tape.

Cause of Refraction Speed of light is different in different media. It is lesser in denser medium and higher in rarer medium. So when light enters a denser medium, its speed reduces and it bends towards the normal and when it enters rarer medium, its speed increases and it bends away from the normal.

Lateral Displacement The perpendicular distance between the emergent ray and incident ray when the light passes out of a glass slab is called lateral displacement. Note: Angle of incidence = angle of emergence, i.e. ∠i = ∠e if the incident ray falls normally to the surface of glass slab, then there is no bending of the ray of light, it goes straight without any deviation.

• Fix a white sheet of paper on a drawing board.

• Remove the rectangular glass slab. Draw a thin line XO directing towards O and inclined to the face AD of the glass slab at any angle preferably between 30° and 60°. It is advisable to take point 0 in the middle of the face AD. Replace the glass slab exactly over the boundary marked on the paper.
• Fix two pins P 1 and P 2 vertically about 5 cm apart by gently pressing their heads with thumb on the line XO.
• Observe the images of pins P1 and P2 through the face BC of the rectangular glass slab. While observing the images of the pins P 1 and P 2 through the face BC of the glass slab, fix two more pins at points P 3 and P 4 such that feet of all the pins appear to be in a straight line. In other words, the pins P 3 and P 4 are collinear with the images of pins P 1 and P 2 .
• Remove the pins and the glass slab and mark the positions of the feet of all the four pins. Join points that mark the positions of the pins P 3 and P 4 and extend the line upto point O’ where it meets the face BC. Also, join the points O and O’ as given in the Fig. 3, where XOO’Y show the path of a ray of light passing through the glass slab. The line XP 1 P 2 O represents the incident ray. Line OO’ shows the path of refracted ray in glass slab while line O’P 3 P 4 Y shows the emergent ray.
• Draw the normal NOM to the face AD at the point of incidence O and similarly, the normal M’O’N’, to the face BC at point O’. Measure the angle of incidence XON (∠i), angle of refraction MOO’ (∠r) and angle of emergence M’O’Y (∠e).
• Record the values of ∠i, ∠r and ∠e in the observation table.
• Repeat the experiment for two more angles of incidence in the range 30° to 60° and record the values of angles r and e in each case.

Observations and Calculations

And we know that, Angle of deviation (∠δ) = Angle of incidence – Angle of emergence For observation 1, ∠δ 1 = ∠i 1 – ∠e 1 = …………. For observation 2, ∠δ 2 = ∠i 2 – ∠e 2 = ……….. For observation 3, ∠δ 3 = ∠i 3 – ∠e 3 = ……………..

• The path of a ray of light passing through a rectangular glass slab is traced on the plane sheet and is shown by the ray XOO’Y in Fig. 3.
• The relations between the angle of incidence, angle of refraction and angle of emergence are obtained through different observations.
• From observations, ∠r < ∠i in each case, thus the ray entering from air to glass bends towards the normal.
• From observation, ∠i = ∠e, the emergent ray emerging out of the rectangular glass slab, is parallel to, but laterally displaced with respect to the incident ray.
• Angle of refraction (∠r) increases with increase in angle of incidence (∠i).

Precautions

• The glass slab should be perfectly rectangular with all its faces smooth.
• The tips of pins P 1 , P 2 , P 3 and P 4 should be sharp. These pins fixed on the sheet of paper may not be exactly perpendicular to the plane of paper. Thus, if their heads appear to be collinear, their feet may not be so, Therefore, it must be important to look at the feet of pins and their images while ascertaining collinearity between them. The mark of the pointed end or the foot of a pin on the paper must be considered while marking its position.
• The distance of 5cm between the pins P 1 and P 2 or P 3 and P 4 must be carefully maintained to obtain an accurate direction of incident ray and that of emergent ray.
• Take the angle of incidence preferably between 30° and 60°.
• Thin lines should be drawn using a sharp pencil to obtain accuracy.
• The angles should be measured accurately using a good quality protractor having clear markings by keeping the eye above the marking.

Sources of Error

• Drawing board may be hard.
• Drawing pins may not be fixed perpendicularly.
• Eyes may not be kept along the plane of paper in line, while observing the images.

Question 1. Why are the incident and emergent rays parallel to each other, in case of refraction though a rectangular glass slab? [NCERT] Answer: As the angle of incidence (∠i) is equal to the angle of emergence (∠e) in case of glass slab, so incident ray is parallel to emergent ray.

Question 2. Why does a ray of light bend towards the normal, when it enters from air into glass slab and bends away from the normal, when it emerges out into air? [NCERT] Answer: It bends towards the normal, when it travels from rarer to denser medium or speed of light decreases in denser medium and vice-versa.

Question 4. While tracing the path of ray of light through a glass slab, the angle of incidence is generally taken between 30° and 60°. Explain the reason on the basis of your performing this experiment for different angles of incidence. [NCERT] Answer: These are following reasons:

• If the angle of incidence is less than 30°, bending of light at glass-air interface will be very less.
• If the angle of incidence is greater than 60°, then emergent ray may emerge from the side surface as angle of emergence also increases with the increase in angle of incidence or vice-versa.

Question 5. How does the lateral displacement of the emergent ray depend on the width of the glass slab and angle of incidence? [NCERT] Answer: Lateral displacement is directly proportional to the width or thickness of the glass slab. It is also directly proportional to the angle of incidence.

Question 6. Give the relation between angle of incidence (∠i), angle of refraction (∠r), angle of emergence (∠e), in this experiment. Answer: In this experiment, the relations between ∠i, ∠r and ∠e are ∠e = ∠i and ∠r < ∠i

Question 7. What happens to the refraction angle, when angle of incidence decreases? Answer: On decreasing angle of incidence, angle of refraction also decreases.

Question 8. What distance should be maintained between the pins P 1 and P 2 or P 3 and P 4 to obtain the accurate result? Answer: The distance between the pins should be maintained at 5cm, to obtain the accurate result.

Question 9. Why is it necessary to observe the feet of the pins for collinearity of the points in this experiment? Answer: The pins fixed on the sheet of paper may not be exactly perpendicular to the plane of paper. Thus, if their heads appear to be collinear, their feet may not be. So, it is important to observe the collinearity by looking at the feet.

Question 10. Define an angle of emergence. Answer: The angle made by the emergent ray with the normal to the glass-air interface is called an angle of emergence. It is denoted by ∠e.

Question 11. State some precautions that one should take while performing the experiment on tracing the path of a ray of light passing through a rectangular glass slab for different values of angle of incidence. Answer:

• Pins should be fixed exactly vertically.
• Angles should be fixed between 30° and 60°.
• Glass slab should be placed well within the outline marked.

Question 12. When a ray of light refracts through glass slab with parallel surfaces, what is the difference between angle of incidence and angle of emergence? Answer: When a ray of light refracts through glass slab with parallel surfaces, then the difference between angle of incidence and angle of emergence is zero.

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• Refraction of Light Through Glass Slab

Refraction Through Glass Slab - Introduction

Sir Isaac Newton agreed with Pierre Gassendi that light is made up of particles. Both perceived light as corpuscles that emanated in all directions and travelled in straight lines. The corpuscular theory explains light refraction . Therefore, investigating light will help comprehend what glass slab does to light. When it passes through the glass slab, it changes its original path and deviates.

What is Refraction of Light and Examples of Refraction of Light

The light changes its direction rather than travelling straight as it moves from one medium to another is termed as Refraction. This results in the image being either moved or deformed. The refraction of light examples are listed below :

You might see that a pencil that is partially submerged in a glass of water appears deformed.

How a spoon seems twisted or broken after being submerged in a glass of water.

How the covered letters appear shifted when a portion of the word PHYSICS is covered with a glass slab.

What is Refractive Index?

A number that indicates the speed of light in a certain medium is called its refractive index . Also, the ratio of the speed of light in a vacuum to that in a medium is represented by this dimensionless number. Assume that a given medium has a refractive index of n, speed of light in the medium v, and the speed of light in air is c . So, its refractive index is as follows by the definition:

\[{\rm{n = c/v}}\]

Laws of Refraction

When a ray enters into air from a glass slab, follow the laws of refraction listed below.

At the point of incidence, the normal ray, the incident ray, and the refracted ray to the interface of two media located on the same plane.

The ratio of the sin of the angle of incidence to that of the sin of refraction is constant or has a specific value. It is termed as Snell's law:

${{\dfrac{sin i}{sin r}} = constant = RI }$

where the constant value relies on the refractive indices of the two media used, i = angle of incidence, and r = angle of refraction.

Some Related Terms to Understand

Transparent Surface: The term "transparent surface" refers to a planar surface that refracts light. The segment of a plane with a transparent surface labelled PQ in the diagram.

Point of Incidence: The point of incidence is where a light ray intersects a transparent surface. E is the point of incidence in the diagram.

Incident Rays: The incident ray is the light beam that impinges obliquely on one of the glass slab's surfaces. For observing the refraction occurring through a glass slab, we can produce incident rays from a variety of light sources . AB is the incident ray in the diagram.

Refracted Rays: A light beam is refracted by one of the surfaces that the incident light first hit. If light enters from a rarer medium into one that is denser, the refracted ray moves closer to the normal ray and if it exits from a denser medium into a rarer one, it moves farther from the normal ray. EF is the refracted ray in the diagram.

Normal Rays: We can determine the angles of incidence, refraction, and emergence by drawing two normal rays at the slab's two opposite parallel surfaces.

Emergent Rays: The emergent ray is the light ray that appears from the glass slab's other opposite face. It has been noticed or observed that the path from which the incident ray could have emerged if it had not undergone any alteration is roughly parallel to that of the emerging ray. CD is the emergent ray in the diagram.

Rectangular Glass Slab

Now let's look at the situation of what will happen when a ray of light enters from medium air to a glass slab .

Refraction Through Different Mediums

As seen in the figure above, a beam AE strikes surface PQ at an angle i. It refracts with an angle r when it enters the glass slab, bending toward normal and moving along EF. An angle of incidence of r' separates the refracted ray EF from the surface SR. When a ray enters into air from a glass slab at a refraction angle of e, the emerging ray FD deviates from the normal. As a result, although the emergent ray FD is laterally displaced with regard to the incident ray AE, it is parallel to the latter. The direction of the light changes as it leaves a parallel-faced refracting material.

Let n1 represent the refractive index of the air and n2 represent that of the glass.

According to Snell's rule,

\[{\mathop{\rm Sin}\nolimits} i/{\mathop{\rm Sin}\nolimits} r = n2/n1\]………………………………………(1) at surface P

\[{\mathop{\rm Sin}\nolimits} r/{\mathop{\rm Sin}\nolimits} e = n1/n2\]……………………………………..(2) at surface S

Since Equations (1) and (2) are reciprocals of one another, multiplying Equations (1) and (2) results into,

\[({\mathop{\rm Sin}\nolimits} i/{\mathop{\rm Sin}\nolimits} r)*({\mathop{\rm Sin}\nolimits} r/{\mathop{\rm Sin}\nolimits} e) = 1\],

Calculating further,

\[{\mathop{\rm Sin}\nolimits} i/{\mathop{\rm Sin}\nolimits} e = 1\], if i = e .

This results in the emergent ray which must be always parallel to the incident ray. Despite being parallel, the incident and emerging rays do not share the same line. This indicates that the incident ray and emerging ray have different lateral positions.

Interesting Facts

In optics and technology, refraction is used extensively. The following is a list of some of the popular applications:

Refraction allows a lens to create a picture of an object for a variety of functions, including magnification.

The concept of refraction is used in spectacles worn by those with vision problems.

Refraction is a technique used in telescopes, cameras, movie projectors, and peepholes on home doors.

Solved Problems

If the angle of incidence is 35° and the angle of refraction is 15°, what is the constant value?

Ans: \[{\mathop{\rm Sin}\nolimits} i = 35^\circ \] , and \[{\mathop{\rm Sin}\nolimits} r = 15^\circ \] . We obtain the corresponding values of the provided angles from the log table.

\[{\mathop{\rm Sin}\nolimits} 35^\circ /{\mathop{\rm Sin}\nolimits} 15^\circ = 2.19\]

The 2.19 is the value of the refractive index of rays passing from one media to another.

A light beam passes through two glass slabs of the same thickness. In the first slab n1 and the second slab n2, waves form separately. Hence, what will be the refractive index of the second medium with respect to the first.

Ans: Let’s assume t, \[\lambda \] , and n are the thickness of the glass slab, wavelength, and the number of waves, respectively.

\[\begin{array}{l}n*\lambda = t\\\lambda = t/n\end{array}\]

\[\lambda \alpha 1/n\] (as t is the same)

\[\lambda 1/\lambda 2 = n2*n1\]​

Thus, the refractive index of the second medium w.r.t. first medium is\[{\rm{1}}\mu 2 = \lambda 1/\lambda 2 = n2/n1\]

Key Features

Due to the fact that light is moving from a rarer to a denser optical medium, the angle of refraction is smaller than the angle of incidence.

The light bends in the direction of normal when it moves from an optically rarer medium (air) to an optically denser medium (glass).

The angle of emergence and incidence are almost equal.

We noticed that when a ray strikes one of the parallel slab surfaces, it first experiences refraction at the surface, where it then bends toward the normal before striking the other surface that is identical to the first one. Before returning to the air through a glass slab, light experiences two refractions. The refracted light deviates from the norm the second time. The light will travel through the glass slab without deviating if it is incident at a straight angle. As we can see, the emergent ray of light emerged parallel to the incident ray preceding it. Due to its two opposed surfaces, a glass slab possesses this unique quality.

FAQs on Refraction of Light Through Glass Slab

1. Why do incident and emergent rays in the instance of a rectangular glass slab go parallel to one another?

In the case of a rectangular glass slab, the angle of incidence and angle of emergence is the same at the opposing parallel slab surfaces, so the incident and emergent rays are parallel to one another.

2. Which light from VIBGYOR bends at the least angle and at the greatest angle under refraction?

In response, violet light bends the most and red light the least.

3. What is total internal reflection?

Total internal reflection is an optical phenomenon where waves arriving at the interface between two media entirely reflect back into the first medium instead of being refracted into the second.

Lab Manual: Refraction Through Glass Slab | Lab Manuals for Class 10 PDF Download

Materials Required Some drawing pins, a white sheet of paper, a rectangular glass slab, a protractor, a measuring scale, a drawing board, a sharp pencil and adhesive tape.

Theory/Principle

Refraction through a Rectangular Glass Slab When a light ray enters in a glass slab, then the emergent ray is parallel to the incident ray but it is shifted sideward slightly. In this case, refraction takes place twice; first when ray enters glass slab from air and second when exits from glass slab to air. Both refractions have been shown in Fig. 2 (here glass slab is denser medium and air is rarer medium). The extent of bending of the ray of light at opposite parallel faces AB and CD of rectangular glass slab is equal and opposite. So, the ray emerging from face CD is parallel to incident ray but shifted sideward slightly.

Lateral Displacement The perpendicular distance between the emergent ray and incident ray when the light passes out of a glass slab is called lateral displacement.

Note:  Angle of incidence = angle of emergence, i.e. ∠i = ∠e

if the incident ray falls normally to the surface of glass slab, then there is no bending of the ray of light, it goes straight without any deviation.

• Fix a white sheet of paper on a drawing board.

Fig. 3 The images of pins P 1 and /P 2 appear to be at I 1 and l 2 when viewed through the face BC while I 3 and I 4 show the position of the images of pins P 3 and P 4 when viewed through the face AD 

• Remove the rectangular glass slab. Draw a thin line XO directing towards O and inclined to the face AD of the glass slab at any angle preferably between 30° and 60°. It is advisable to take point 0 in the middle of the face AD. Replace the glass slab exactly over the boundary marked on the paper.
• Fix two pins P 1 and P 2 vertically about 5 cm apart by gently pressing their heads with thumb on the line XO.
• Observe the images of pins P 1 and P 2 through the face BC of the rectangular glass slab. While observing the images of the pins P 1 and P 2 through the face BC of the glass slab, fix two more pins at points P 3 and P 4 such that feet of all the pins appear to be in a straight line. In other words, the pins P 3 and P 4 are collinear with the images of pins P 1 and P 2 .
• Remove the pins and the glass slab and mark the positions of the feet of all the four pins. Join points that mark the positions of the pins P 3 and P 4 and extend the line upto point O’ where it meets the face BC. Also, join the points O and O’ as given in the Fig. 3, where XOO’Y show the path of a ray of light passing through the glass slab. The line XP 1 P 2 O represents the incident ray. Line OO’ shows the path of refracted ray in glass slab while line O’P 3 P 4 Y shows the emergent ray.
• Draw the normal NOM to the face AD at the point of incidence O and similarly, the normal M’O’N’, to the face BC at point O’. Measure the angle of incidence XON (∠i), angle of refraction MOO’ (∠r) and angle of emergence M’O’Y (∠e).
• Record the values of ∠i, ∠r and ∠e in the observation table.
• Repeat the experiment for two more angles of incidence in the range 30° to 60° and record the values of angles r and e in each case.

Observations and Calculations

And we know that, Angle of deviation (∠δ) = Angle of incidence – Angle of emergence For observation 1, ∠δ 1 = ∠i 1 – ∠e 1 = …………. For observation 2, ∠δ 2 = ∠i 2 – ∠e 2 = ……….. For observation 3, ∠δ 3 = ∠i 3 – ∠e 3 = ……………..

• The path of a ray of light passing through a rectangular glass slab is traced on the plane sheet and is shown by the ray XOO’Y in Fig. 3.
• The relations between the angle of incidence, angle of refraction and angle of emergence are obtained through different observations.
• From observations, ∠r < ∠i in each case, thus the ray entering from air to glass bends towards the normal.
• From observation, ∠i = ∠e, the emergent ray emerging out of the rectangular glass slab, is parallel to, but laterally displaced with respect to the incident ray.
• Angle of refraction (∠r) increases with increase in angle of incidence (∠i).

Precautions

• The glass slab should be perfectly rectangular with all its faces smooth.
• The tips of pins P 1 , P 2 , P 3 and P 4 should be sharp. These pins fixed on the sheet of paper may not be exactly perpendicular to the plane of paper. Thus, if their heads appear to be collinear, their feet may not be so, Therefore, it must be important to look at the feet of pins and their images while ascertaining collinearity between them. The mark of the pointed end or the foot of a pin on the paper must be considered while marking its position.
• The distance of 5cm between the pins P 1 and P 2 or P 3 and P 4 must be carefully maintained to obtain an accurate direction of incident ray and that of emergent ray.
• Take the angle of incidence preferably between 30° and 60°.
• Thin lines should be drawn using a sharp pencil to obtain accuracy.
• The angles should be measured accurately using a good quality protractor having clear markings by keeping the eye above the marking.

Sources of Error

• Drawing board may be hard.
• Drawing pins may not be fixed perpendicularly.
• Eyes may not be kept along the plane of paper in line, while observing the images.

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Refraction Through a Rectangular Glass Slab

Ray Optics is a branch of optics that describes light propagation in the form of rays. Ray Optics is also called geometrical optics which deals with the geometry of falling lights. this article is about refraction, its laws, refraction through glass slab and its various causes, and practice tracing the path of light rays passing through glass slab. Refraction is the property of the change in direction of the wave while entering from one medium to another medium. This change in direction is due to a change in the speed of the wave. 

What is Refraction? 

Refraction is the phenomenon of change in direction of a wave passing from one medium to another medium because of the change in its speed. Refraction refers to the bending of a wave when it is passed from one medium to another medium.

The wave bends because of the change in its speed from one medium to another medium. Here are two laws of Refraction that can be stated as:

• The incident ray refracted ray, and normal at a point of incidence lie on the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. This is known as Snell’s Law of refraction.

sin i / sin r = constant      

What is Glass Slab?

A glass slab is a substance with a cuboidal shape that is made of glass. A glass slab is made of glass with three dimensions length, breadth, and height.

Some important properties of a glass slab can be stated as:

• The glass slab does not deviate from the light rays passing through it.
• The glass slab does not disperse the light rays passing through it.
• Since it does not deviate and disperse the light rays passing through it that’s why the incident ray and the emergent ray emerging from the glass slab are parallel. The glass slab only produces a lateral displacement to the direction of light.

Refraction through the Glass Slab

• When the incident ray is incident on the surface of the glass slab from air to the glass making an angle of incidence i with the normal the refracted ray bends towards the normal as the ray enters from rarer to denser medium. 

• Now, after traveling the glass slab the refracted ray makes an angle of incidence r on the other surface of the glass slab resulting in the emergent ray which bends away from the normal as it travels from glass (denser medium) to air (rarer medium) forming an angle of emergence e between the emergent ray and normal. 
• The emergent ray is parallel to the incident ray and the perpendicular distance between them is called lateral displacement. 
• Since the angle of incidence is equal to the angle of emergence, therefore the emergent ray is parallel to the incident ray. 
• In the glass slab, the light ray gets refracted two times firstly, from rarer to denser medium and secondly from denser to rarer medium. The displacement created in the emergent ray is due to refraction.

Therefore, the formula for the lateral displacement d is given by,

d = [t sin(i – r)]/ cos r = t sin(i – r) sec r 

where t is the thickness of the glass slab, i is the angle of incidence, r is the angle of refraction.

Experiment to Trace the path of a ray of light passing through a glass slab

Aim: Tracing the path of a ray of light passing through the glass slab.

Materials Required:- Drawing board, all pins, white paper, rectangular glass slab, protractor, scale, pencil, thumb pins.

Tracing the path of a ray of light  passing through a glass slab 

Procedure: 

• Fix the white paper on the drawing board with the help of thumb pins.
• Place the glass slab on the paper at the center and draw an outline of the glass slab and label it PQRS.
• Draw a point E on PQ and draw perpendicular MN and label it as a normal ray.
• Draw one angle of 30° with a protractor with MN. Fix pins A and B, 4-5 cm apart from each other on the ray that is obtained by formed angle.
• Place the glass slab on the outline PQRS and see through the glass slab from side RS and fix C and D pins such that A, B, C, and D should lie on the same line.
• Draw small circles on A, B, C, and D and remove the pins.
• Join C, D such that it meets at point F on RS and draw perpendicular M’N’ at point F.
• Now, join the points E and F.
• Measure the angles formed at PQ and RS which are the angle of incidence, angle of refraction, and angle of emergence.
• The lateral displacement d is obtained by extending the ray AB in the dotted line parallel to FCD and then, measuring it.

Conclusion:

• The angle of incidence is almost equal to the angle of emergence.
• As the light travels from rarer to a denser medium, the angle of refraction will be lesser than the angle of refraction.
• For different angles of incidence, the lateral displacement will be the same.
• The light bends towards the normal when it travels from rarer to denser medium.

 Different Cases of Refraction through Glass Slab

Case 1: Refraction when the object is in a denser medium and the observer is in a rarer medium

Consider a glass slab with refractive index and thickness μ and t respectively. The observer (eye) is in the air and the object (O) is in the glass slab.

Refraction when the object is in a denser medium and the observer is in a rarer medium

Hence for this case,

Virtual Depth = t / μ

Virtual displacement (OI) = OA – AI = t[1-(1/ μ)]  

Case 2: Refraction through the successive slabs with different thicknesses and refractive index

Consider three successive slabs s 1 , s 2 , and s 3 with thickness t 1 , t 2 , t 3, and refractive indices μ 1 , μ 2 , and μ 3 respectively. These slabs are arranged one after the other with s 1 at the top, s 2 at the middle, and s 3 at the bottom. The object(O) is in s 3 and the observer (eye) is outside in the air (rarer medium).

Refraction through the successive slabs with different thicknesses and refractive index

Virtual depth (AI) = (t 1 / μ 1 ) + (t 2 / μ 2 ) + (t 3 / μ 3 ) + ……. 

Virtual displacement (OI) = t 1 [1 – (1/ μ 1 )] + t 2 [1 – (1/ μ 2 )] + t 3 [1 – (1/ μ 3 )] + ………

Case 3: Refraction when object and observer both are in a rarer medium

Consider a glass slab with thickness t and refractive index μ. In this case, the object (O) and observer(eye) both are in a rarer medium (air) separated by the glass slab.

Refraction when object and observer both are in rarer medium

Hence for this case, 

Virtual displacement (OI) = [t – (1/ μ)]

Case 4: Refraction when the object is in a rarer medium and the observer is in a denser medium

Consider the observer(eye) is in water(denser medium) and the object (O) is in the air (rarer medium).

 Refraction when object is in a rarer medium and observer is in a denser medium 

Real height / Virtual height = 1 / μ

Virtual displacement (OI) = AI – AO = (μ – 1)AO 

Sample Questions

Question 1: Determine the position of the object kept at bottom of the glass slab with the thickness of 12 cm and the refractive index of glass is 1.5.

Answer:  

The given case is refraction when the object is in a denser medium and the observer is in the rarer medium.  We have to find virtual depth i.e. position of the object.  Here, t = 12cm and μ = 1.5 Position of the object  = t / μ = 12/1.5  = 120/15 Position of the object  = 8 cm

Question 2: Two immiscible liquids of refractive indices 3/2 and 4/3 filled in the vessel up to 9 cm and 12 cm then, What will be the virtual depth of the bottom of the vessel.

Answer: 

The given case is like refraction through successive slab with different thickness and refractive index.  Here, t 1 = 9cm, μ 1 = 3/2, t 2 = 12cm, and μ 2 = 4/3 Virtual depth = (t 1 /μ 1 ) + (t 2 /μ 2 ) = (9/(3/2)) + (12/(4/3)) = (18/3) + (36/4) =  6 + 9 Virtual depth = 15 cm

Question 3: An object is kept at the bottom of the empty vessel and a microscope is focused to observe it. Now, water is filled in it up to 16cm then, at what amount microscope should move so that it can again focus on the object. The Refractive index is 4/3.

The given case is refraction when the object is in a denser medium and the observer is in the rarer medium.  Here, t = 16 cm , μ = 4/3 The amount microscope should be moved so that it can again focus the object = Virtual displacement = t[1- (1/μ)]  = 16[1 – {1/(4/3)}] = 16 [1 – 3/4] = 16 × (1/4) The amount of microscope should be moved so that it can again focus the object = 4 cm.

Question 4: An air bubble in a glass slab (μ = 1.5) is 5 cm deep when viewed from one face and 2cm deep when viewed from the opposite face then find the thickness of the slab.

Given:    μ = 1.5 Virtual depth v 1 when observed from one face = 5 cm Virtual depth v 2 when observed from opposite face = 2 cm   We have to find the real depth. This case is when the object is in a denser medium and the observer is in the rarer medium. When observed from one face:   v 1 = r 1 /μ  r 1 = v 1 × μ = 5 × 1.5 = 7.5 cm When observed from opposite face:   v 2 = r 2 /μ r 2 = v 2 × μ = 2 × 1.5 = 3.0 cm Thickness of the glass slab = r 1 + r 2 = 7.5 + 3.0 = 10.5 cm

Question 5: What is lateral displacement. State the factors on which it depends.

The perpendicular distance between the incident ray and the resulting emergent ray is called the lateral displacement.

Factors on which lateral displacement depends:

• The thickness of the glass slab
• Angle of incidence
• Refractive index of glass 
• The wavelength of light used

Question 6: Why emergent ray is parallel to the incident ray in the refraction through the glass slab.

The emergent ray is parallel to the incident ray in the refraction through the glass slab because the angle of incidence and the angle of refraction are equal. 

Question 7: Find the lateral displacement when refraction through a glass slab takes place where the angle of incidence is 45° and angle of refraction is 30° and the thickness of the glass slab is 10 cm.

Here, t = 10cm, i = 45°, r = 30° Lateral displacement (d) = [t sin(i – r)]/ cos r = [10 sin( 45° – 30°)]/ cos 30° = [10 sin 15°]/ cos 30° = 10 × (sin 15° / cos 30°) = 10 × (0.25/0.86) = 10 × 0.29 Lateral displacement (d)  = 2.9 cm 

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