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

PhysicsPhysics Experiments


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
  • Scale
  • Thumb pins
  • White paper
  • A rectangular glass slab
  • Pencil
  • Protractor

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 300 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 300 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

Angle of incidence Angle of emergence Angle of refraction
600 59.80 560 0.20
450 44.80 430 0.20
300 300 280 00

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.

Updated on 13-Oct-2022 11:19:47