Courses for Kids
Free study material
Offline Centres

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

Last updated date: 01st Mar 2024
Total views: 38.1k
Views today: 0.38k
hightlight icon
highlight icon
highlight icon
share icon
copy icon

Physics Experiment - Tracing Path of a Ray of Light Passing Through a Glass Slab

The corpuscular theory explains light refraction. Therefore, investigating light will help comprehend what glass slab does to light. The light changes its direction rather than traveling straight as it moves from one medium to another is termed refraction. This results in the image being either moved or deformed.

Examples of light refraction are a pencil partially submerged in a glass of water and appears deformed, a spoon that seems twisted or broken after being submerged in a glass of water, etc.

Table of Contents 

  • Aim

  • Apparatus Required

  • Theory 

  • Procedure

  • Observation Table

  • Results


To trace the path of a ray of light that is passing through a glass slab. To find the lateral displacement due to refraction in a glass slab. 

Apparatus Required

  1. A drawing - board

  2. 4 - 6 all pins

  3. A plain sheet of paper

  4. A glass - slab

  5. A protractor

  6. A scale

  7. A pencil

  8. Thumb pins


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

  1. 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. 

  2. The ratio of the sin of the angle of incidence (i) to that of sin of angle of refraction (r) is constant or has a specific value. According to Snell's law:

\[\dfrac{{{\rm{Sini}}}}{{{\rm{Sinr}}}}{\rm{ = constant}}\] where the constant value relies on the refractive indices of the two media used, i = angle of incidence, and r = angle of refraction as shown in the Figure below.

Some Important Terms to Understand

  • 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 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.

  • 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.


  1. Use thumb pins to secure a blank sheet of paper to theο soft board.

  2. Put the rectangular glass slab in the centre of the white paper and use a pointed pencil to outline it.

  3. Let PQRS represent the glass slab's rectangle at this point.

  4. Create a perpendicular EN on point E on PQ and mark it as a normal ray.

  5. Using the protractor, draw an angle of 30° along EN.

  6. Take two spots that are 4 to 5 cm apart on the ray produced by the angle, label them A and B, and place pins in each of them.

  7. Place the glass slab over the PQRS figure in the shape of a rectangle.

  8. To fix C and D, look through the glass slab's side RS so that all pins A, B, C, and D are visible as a straight line.

  9. Remove the pins by drawing a small circle with a pencil around A, B, C, and D. Remove the glass slab simultaneously.

  10. To meet RS at point F, points C and D must be connected.

  11. Draw a parallel N1'N2' at point F at RS. Connect points E and F now.

  12. The incidence angle, the refracted angle, and the emerging angle are all angles created at PQ and RS.

  13. Stretching the ray AB in a dotted line parallel to EFD will allow you to determine the lateral displacement. Measure the lateral displacement, d.

  14. The same steps must be followed for angles of 45° degrees and 60°.

Refraction of the light through a glass slab.

Refraction of the light through a glass slab.

Observation Table


Angle Of Incidence

∠i = ∠BEN1

(in ∘)

Angle Of Refraction

∠r = ∠FEN2

(in ∘)

Angle Of Emergence

∠e = ∠DFN2

(in ∘)

∠i ~ ∠e

∠BEN1 ~ ∠DFN2

(in ∘)

lateral displacement 

(d in cm)








  • The angle of incidence is equal to the angle of emergence. 

  • The experiment also demonstrates that the light ray goes away from the normal when it travels from the denser to the rarer medium and towards the normal when it travels in the opposite direction.

  • For various incidence angles, the lateral displacement stays the same, d = …… cm.


  1. The edges of the rectangular glass slab must be completely smooth.

  2. The soft drawing board will allow for easy pin attachment.

  3. It is important to ensure that the incidence angle is between 30o and 60o.

  4. The distance between the pins P and Q or the pins R and S must be kept at a minimum of 5 cm.

  5. It's important to pay attention to the protractor and pencil's quality.

  6. To obtain precise measurements, the protractor must be correctly positioned.

Lab Manual Questions

  1. What will be the angle of incidence if a light ray forms a 30° with the refracting surface?

Ans. The angle of incidence will be 60°.

  1. What happens to the nature of light when a ray of light travels from an optically denser medium to a rarer one? 

Ans. Light deviates from its normal path as it moves from an optically denser to an optically rare medium.

  1. What will be the nature of a light ray if it passes through to oil from water?

Ans. The light will move towards the normal as oil is optically denser than the water. 

  1. What will happen if you replace the glass slab with a water slab?

Ans: Since the water has a lower refractive index than the glass, the bending of the light ray will be smaller than the glass lab, and the lateral displacement will be smaller.

Viva Questions

1. Give a succinct explanation of the connection between the angle of incidence and the angle of emergence.

Ans. The experiment tracked the passage of a light beam and revealed that the angle of incidence and emergence typically remained equal. It has been discovered that, in the event of certain faults brought on by human situations or acts, there may only be a discrepancy of up to one degree.

2. Briefly describe the law of refraction.

Ans. Snell's law is an alternative name for the law of refraction that mentions that:

  • The angle of incidence is equal to the angle of refraction.

  • The incident ray, the refracted ray, and the normal all reside in the same plane. 

3. Explain the relation between the normal and the medium.

Ans. It has been discovered that a light ray bends toward normal when it moves from air to glass or from the rarer medium to the denser ones. A light ray deviates from the normal when it passes from the glass into the air or from the optically denser medium to the rarer one. 

4. Mention the characteristic of light that causes the rainbow to form.

Ans. The rainbow is created because of the reflection, refraction and dispersion of light rays using droplets. The presence of the microscopic water droplets causes the light to refract and create a rainbow from the original white light. 

5. What circumstance stops the light from deviation during refraction?

Ans. When the light ray is perpendicular to the refracting surface, the deviation of light is zero. 

6. What are the inferences drawn from the experiment?

Ans. According to the findings of the aforementioned experiment, the angle of incidence and angle of emergence are equal. And since the angle of refraction is smaller than the angle of incidence when light moves from a rarer to a denser optical medium, it bends in the direction of normal. It was also observed that the lateral displacement remained constant for various incidence angles.

7. What potential causes of mistakes could arise during the aforementioned experiment?

Ans. Most errors come from unintentional human behaviour. Because there are bubbles in the glass slab, one mistake could occur. Therefore, it is important to take precautions to keep the glass slab bubble-free. Second, all necessary measurements must be carried out correctly and precisely to get the right findings.

8. What should the angle of incidence be for the experiment mentioned above?

Ans. For the aforementioned experiment, an angle of incidence between 30° and 60° is needed.

9. Which factor controls the lateral displacement of the emergent rays and Why?

Ans. The lateral displacement of the emergent rays depends upon the refractive index of the optically denser medium (e.g., glass, water, etc.). A higher value of the refractive index will bend the light away from the normal and increase the lateral displacement.  

10. What is the refractive index of the medium?

Ans. The speed of light gives the refractive index of a medium in a vacuum divided by the speed of light in the medium.

Practical-Based Questions

1. Lateral displacement is determined by:

A) Angle of incidence

B) Refraction angle

C) The glass slab's thickness

D) The emerging angle

Ans. C) The glass slab's thickness

2. The incident and emerging rays in the glass slab are:

A) Always parallel

B) Always convergent

C) Occasionally parallel 

D) Occasionally diverging

Ans. A) Always parallel

3. A student does the experiment of following a light beam through a rectangular piece of glass at two distinct angles of incidence i = 30o and i = 45o. The learner is likely to notice the set of values for the angle of refraction and the angle of emergence in the two cases (all values are in degrees):

A) r = 30o, e = 20o

B) r = 45o, e = 28o

C) r = 20o, e = 30o, and r = 28o and e = 45o

D) where r = 20o, e = 20o, and r = 28o, e = 28o

Ans. (C) The values for the angle of refraction and the angle of emergence in the two cases are r = 20o, e = 30o, and r = 28o, and e = 45o.

4. The difference between the incident and the emergent ray path is called as:

A) Horizontal Difference

B) Lateral displacement

C) Vertical Displacement

D) Phase Difference

Ans. (B) The difference between the incident and the emergent ray path is called lateral displacement.

5. When a light beam passes from the air to a glass slab, its wavelength:

A) Increases

B) Remains unchanged 

C) Decrease

D) depending on the thickness of the glass slab

Ans. C) Decreases

6. When light bends as it travels through different media, it is referred to as: 

A) Dispersion 

B) Diffraction

C) Refraction

D) Reflection

Ans. C) Refraction

7. A light ray is incident normally, with the angle of incidence being one of the following:

A) 90° 

B) 60° 

C) 180° 

D) 0°

Ans. D) 0°

8. When a light beam travels from water to air and suffers refraction, the following equations apply: 

A) ∠i = ∠r 

B) ∠i < ∠r 

C) ∠i > ∠r 

D) not defined

Ans. B) ∠i < ∠r 

As the light ray is moving from denser to rarer medium.

9. Four students carried out the following actions in the experiment to follow the course of a light beam through a rectangular glass slab using pins P1, P2, P3, and P4:

A. While installing P3 and P4, as well as P1, P2, and P3 heads, P4 was being placed.

B. While placing P3 and P4, as well as P1, P2, and P3 feet, P4 was being placed.

C. While positioning P3 and P4, she observed the heads of P1 and P2 and the feet of each pin.

D. While putting P3 and P4 together, she observed the feet of P1 and P2 and all the pins heads.

The method used by the student is the proper one:

A) A 

B) B 

C) C 

D) D

Ans. B) B

10. Identify the material having the highest value of the refractive index.

A) Glass

B) Air

C) Water

D) Diamond

Ans. (D) Diamond has the highest value of refractive index among all.


  1. The angle of emergence and angle of incidence are equal.

  2. When light moves from a rarer to a denser optical medium, the angle of refraction is smaller than the angle of incidence.

  3. For various incidence angles, the lateral displacement does not change.

  4. Light bends towards the normal as it moves from an optically rarer to an optically denser medium.

FAQs on Tracing Path of a Ray of Light Passing Through a 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?

A light beam may be refracted when it encounters two transparent materials at their interface. This refraction should cause the ray to emerge bent away from the normal if it is leaving the denser medium.

Refraction is not feasible if this causes the beam to emerge from the object at an angle greater than 90 degrees from the normal. The law of reflection is applied in this case, causing the light to be reflected inside the denser material. This is termed Total internal reflection.

4. Is the path of light rays reversible or irreversible?

The light rays always retrace their traveling path when the direction of travel of the light ray is reversed and this principle is called the principle of reversibility of light. According to the principle of reversibility of light, the light whose direction of travel is reversed suffers a number of reflections and refractions.