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
Aim
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
A drawing - board
4 - 6 all pins
A plain sheet of paper
A glass - slab
A protractor
A scale
A pencil
Thumb pins
Theory
When a ray enters into the air from a glass slab, follow one of 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 (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.
Procedure
Use thumb pins to secure a blank sheet of paper to theο soft board.
Put the rectangular glass slab in the centre of the white paper and use a pointed pencil to outline it.
Let PQRS represent the glass slab's rectangle at this point.
Create a perpendicular EN on point E on PQ and mark it as a normal ray.
Using the protractor, draw an angle of 30° along EN.
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.
Place the glass slab over the PQRS figure in the shape of a rectangle.
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.
Remove the pins by drawing a small circle with a pencil around A, B, C, and D. Remove the glass slab simultaneously.
To meet RS at point F, points C and D must be connected.
Draw a parallel N1'N2' at point F at RS. Connect points E and F now.
The incidence angle, the refracted angle, and the emerging angle are all angles created at PQ and RS.
Stretching the ray AB in a dotted line parallel to EFD will allow you to determine the lateral displacement. Measure the lateral displacement, d.
The same steps must be followed for angles of 45° degrees and 60°.
Refraction of the light through a glass slab.
Observation Table
Results
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.
Precautions
The edges of the rectangular glass slab must be completely smooth.
The soft drawing board will allow for easy pin attachment.
It is important to ensure that the incidence angle is between 30o and 60o.
The distance between the pins P and Q or the pins R and S must be kept at a minimum of 5 cm.
It's important to pay attention to the protractor and pencil's quality.
To obtain precise measurements, the protractor must be correctly positioned.
Lab Manual Questions
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°.
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.
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.
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.
Conclusion
The angle of emergence and angle of incidence are equal.
When light moves from a rarer to a denser optical medium, the angle of refraction is smaller than the angle of incidence.
For various incidence angles, the lateral displacement does not change.
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. What are the important steps to trace the path of a light ray through a rectangular glass slab for the Class 10 board practicals?
For the CBSE Class 10 practical exam for the 2025-26 session, follow these crucial steps:
- Fix a white sheet of paper on a drawing board and place the glass slab in the centre.
- Draw the outline of the slab, mark a point O on one side, and draw a normal (NN') at O.
- Draw an incident ray AO at a specific angle of incidence (∠i) with the normal.
- Fix two pins, P1 and P2, vertically on this incident ray.
- Observe the images of P1 and P2 through the opposite face of the slab.
- Fix two more pins, P3 and P4, such that they appear to be in a straight line with the images of P1 and P2.
- Remove the slab and pins. Join P3 and P4 to draw the emergent ray, which meets the slab's face at O'. Join OO' to get the refracted ray.
- Finally, measure the angle of emergence (∠e) and the lateral displacement to verify the laws of refraction.
2. What are the three main observations a student must record from the glass slab experiment to score full marks?
The key observations from this experiment, crucial for board exams, are:
- The angle of incidence (∠i) is approximately equal to the angle of emergence (∠e).
- The emergent ray is parallel to the direction of the incident ray, but it is shifted sideways.
- The light ray undergoes lateral displacement, and the extent of this shift depends on the slab's thickness, its refractive index, and the angle of incidence.
3. Define lateral displacement and list the factors on which it depends. This is a frequently asked 2-mark question.
Lateral displacement is the perpendicular distance between the original path of the incident ray and the emergent ray coming out of the glass slab.
The factors affecting lateral displacement are:
- Thickness of the glass slab: A thicker slab causes greater displacement.
- Angle of incidence: As the angle of incidence increases, so does the lateral displacement.
- Refractive index of the glass: A medium with a higher refractive index causes more bending and thus greater displacement.
4. Why does the emergent ray emerge parallel to the incident ray in a glass slab but not in a prism?
In a rectangular glass slab, the two refracting surfaces where light enters and exits are parallel to each other. Due to this geometry, the bending of light at the first surface is exactly reversed by an equal and opposite bending at the second surface. This ensures the angle of incidence (∠i) equals the angle of emergence (∠e), making the rays parallel. In a prism, the two refracting surfaces are inclined at an angle, which causes the emergent ray to be deviated and not parallel to the incident ray.
5. What happens if a ray of light is incident normally on the surface of a glass slab? Explain why, based on Snell's law.
If a ray of light is incident normally (perpendicularly) on a glass slab, it passes through undeviated and emerges without any lateral displacement. This is because the angle of incidence (∠i) is 0°. According to Snell's law (n₁sin(i) = n₂sin(r)), if i = 0°, then sin(i) = 0. This forces the term n₂sin(r) to also be 0, which means the angle of refraction (∠r) must also be 0°. Therefore, the ray travels straight through both surfaces without changing its path.
6. What is the principle of reversibility of light, and how is it an important concept for the glass slab experiment?
The principle of reversibility of light states that the path of a light ray is reversible. In the context of the glass slab experiment, it means if we were to reverse the emergent ray and send it back into the slab, it would retrace its exact path through the refracted ray and emerge along the path of the original incident ray. This principle fundamentally confirms the symmetric nature of refraction through parallel-sided media.
7. A student performs the glass slab experiment first with a slab of thickness 't' and then with another of the same material but with thickness '2t'. For the same angle of incidence, how will the lateral displacement change and why?
The lateral displacement will be greater for the slab with thickness '2t'. This is because lateral displacement is directly proportional to the thickness of the slab. A thicker slab means the refracted ray travels a longer distance inside the glass before it emerges. This increased path length within the denser medium results in a greater perpendicular shift between the incident ray's original path and the final emergent ray.
