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AimThe objective is to study the trajectory of a light ray while it passes through a rectangular glass slab at varying angles of incidence. The measurements to be taken are the angle of incidence, angle of refraction, and angle of emergence. Finally, the results will be interpreted to understand the behavior of light in this scenario.
Apparatus RequiredA drawing board
4-6 all pins
White sheet of paper
Rectangular glass slab
A protractor
A scale
A pencil
Thumb pins
Theory When a ray of light passes through a rectangular glass slab, it undergoes refraction or bending of the light. This bending depends on the angle of incidence, the angle at which the ray strikes the surface of the glass, and the refractive index of the glass. The refractive index is a measure of how much the speed of light changes when it passes through a medium.
Let’s consider a rectangular glass slab with parallel sides, as shown in the diagram below. The incident ray of light is represented by the red line, and the refracted ray is represented by the blue line. The angles of incidence, refraction, and emergence are labeled as θ1, θ2, and θ3, respectively.

Now, let’s consider different angles of incidence and see how the path of the ray changes.
When the incident ray is perpendicular to the surface of the glass (θ1 = 0°), it passes straight through the glass without bending.
When the incident ray is at a small angle to the surface (θ1 < critical angle), it undergoes refraction and bends towards the normal (the line perpendicular to the surface of the glass) as it enters the glass. As the ray leaves the glass, it bends away from the normal. The angle of incidence and angle of refraction are related by Snell’s law:

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sin θ1 / sin θ2 = n
where n is the refractive index of the glass. The angle of emergence can be calculated using the same formula.
When the incident ray is at a large angle to the surface (θ1 > critical angle), total internal reflection occurs. This means that the ray is reflected back into the glass instead of passing through it. The critical angle is given by:

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sin θc = 1/n
If the angle of incidence is greater than the critical angle, then the ray is reflected back into the glass at an angle equal to the angle of incidence. In this case, the angle of refraction is undefined.
When the incident ray is at an angle of 45° to the surface of the glass, it undergoes equal angles of refraction and emergence, and the refracted ray is parallel to the surface of the glass.
In summary, the path of a ray of light passing through a rectangular glass slab depends on the angle of incidence and the refractive index of the glass. At small angles of incidence, the ray bends towards the normal as it enters the glass and away from the normal as it leaves. At large angles of incidence, total internal reflection occurs. At an angle of 45°, the refracted ray is parallel to the surface of the glass.
ProcedureSet up a rectangular glass slab on a flat surface.
Fix a ray box in such a way that it emits a thin, straight ray of light.
Place the rectangular glass slab in the path of the ray of light emitted from the ray box.
Adjust the angle of incidence by changing the position of the ray box and measure the angle of incidence using a protractor.
Observe the path of the ray of light as it passes through the glass slab.
Measure the angle of refraction and angle of emergence using a protractor.
Repeat steps 4 to 6 for different angles of incidence.
Record the values of angle of incidence, angle of refraction, and angle of emergence in a table.
Analyze the results to understand the behavior of light in this scenario.
Draw a graph of angle of incidence vs. angle of refraction to visualize the relationship between these two variables.
Draw a conclusion based on the experimental results and their analysis.


observation and resultThe observations and results of the experiment of tracing the path of a ray of light passing through a rectangular glass slab for different angles of incidence are as follows:
Observations:
As the angle of incidence increases, the ray of light bends more while passing through the glass slab.
At small angles of incidence, the ray of light bends towards the normal while entering the glass and away from the normal while leaving it.
At large angles of incidence, the ray of light undergoes total internal reflection and is reflected back into the glass slab.
At an angle of 45°, the refracted ray of light is parallel to the surface of the glass.
Results:
The angle of incidence and angle of refraction are related by Snell’s law: sin θ1 / sin θ2 = n, where n is the refractive index of the glass.
The angle of emergence can be calculated using the same formula.
At the critical angle, sin θc = 1/n, the angle of incidence for total internal reflection is achieved.
The graph of angle of incidence vs. angle of refraction is a straight line, which proves that the two variables are directly proportional to each other.
Interpretation: The experiment demonstrates that the trajectory of light passing through a rectangular glass slab is dependent on the angle of incidence and the refractive index of the glass. The experiment also confirms Snell’s law and the critical angle for total internal reflection. The results obtained from the experiment can be useful in understanding and predicting the behavior of light in different mediums.