Refraction Through A Prism : Introduction & Explanation

Key Concepts

• Refraction through a triangular prism

Introduction: 

A ray of light bends towards the normal when it gets into a denser medium and bends away from the normal on entering a rarer medium. This is true whenever light gets refracted at any interface separating any two mediums and it does not depend upon the shape of the surfaces or their inclination. This is true even when it gets refracted through a prism. 

Explanation: 

Refraction through a triangular prism: 

A triangular glass prism has two triangular bases and three rectangular lateral surfaces. 

The angle between its two lateral surfaces is called the angle of the prism (A). 

The refraction of light through a prism occurs in the way shown in the figure below. 

The light ray seems to bend towards the normal on entering the prism at the interface AB. The angle of incidence here is greater than the angle of refraction. 

The light ray seems to bend away from the normal on emerging from the prism at the interface AC. The angle of incidence here is smaller than the angle of refraction (emergence, e). 

The angle between the incident ray produced and the emergent ray produced is called the angle of deviation (D). It is a measure of deviation of the incident ray from its actual path on emergence. 

Thus, it can be concluded that, refraction takes place at both the interfaces of a triangular prism, when light passes through it. However, the refraction of light through a triangular prism is somewhat different from that of the refraction through a glass slab. 

The differences between the refraction of light through a prism and a glass slab are as follows: 

• As the index of refraction of the material of the prism is decreased by keeping it constant for the surrounding, it is observed that the emergent ray shows lesser and lesser deviation. Thus, the angle of deviation decreases on increasing the refractive index of the prism. 
• As the index of refraction of the surrounding is decreased by keeping it constant for the prism, it is observed that the emergent ray shows lesser and lesser deviation. Thus, the angle of deviation decreases on increasing the refractive index of the surrounding. 
• As the color of the incident light is changed from red to green and green to violet, it is observed that the emergent ray shows more and more deviation. Thus, the angle of deviation increases on decreasing the wavelength of the incident light. 

Questions and answers: 

Which of the following would happen if the refractive index of the surrounding medium is greater than that of the material of the prism? 

1. The incident ray would be parallel to the emergent ray. 

2. The light ray would bend towards the normal at the first interface. 

3. The light ray would bend away from the normal at the first interface. 

4. The light ray would bend away from the normal at the second interface.  

Answer: 

3. The light ray would bend away from the normal at the first interface. 

Which of the following would happen if the refractive index of the surrounding medium is equal to that of the material of the prism? 

1. The incident ray would coincide with the emergent ray. 

2. There will be no deflection of the light ray from its usual path. 

3. The angle of deviation will be zero. 

4. All of the above. 

Answer: 

4. All of the above. 

Summary

1. A triangular glass prism has two triangular bases and three rectangular lateral surfaces. The angle between its two lateral surfaces is called the angle of the prism (A).

2. The angle between the incident ray produced and the emergent ray produced is called the angle of deviation (D). It is a measure of deviation of the incident ray from its actual path on emergence.

3. The refraction takes place at both the interfaces of a triangular prism, when light passes through it.

4. The incident ray is not parallel to the emergent ray when light gets refracted from a triangular prism unlike refraction through a glass slab.

5. As the index of refraction of the material of the prism is decreased by keeping it constant for the surrounding, it is observed that the angle of deviation decreases.

6. As the index of refraction of the surrounding is decreased by keeping it constant for the prism, it is observed that the angle of deviation decreases.

7. As the color of the incident light is changed from red to green and green to violet, it is observed that the angle of deviation increases on decreasing the wavelength of the incident light