Process of Refraction of Light Through a Glass Prism​

Ray Optics and Optical Instruments 2025 ( Physics Ray Optics and Optical Instruments )

Jaya Sharma
Updated on Sep 6, 2025 22:47 IST

By Jaya Sharma, Assistant Manager - Content

Refraction of light through a prism happens when a light ray enters a triangular-shaped object called a prism. Light ray in a prism bends twice, which results in the light deviating from its original path. As a result, the emergent ray does not remain parallel to the incident ray. Since different wavelengths of light refract at slightly different angles, the white light gets dispersed into constituent colours.

Refraction Through a Prism

In this article, we will explain refraction through a prism in detail along with the derivation of refractive index of prism. Once you have understood the topic, start practising the NCERT excercise on Ray Optics and Optical Instruments

Table of content
  • Process of Refraction of Light Through a Prism
  • Derivation of the Refractive Index of a Prism
  • Reasons For Dispersion of White Light During Refraction of Light Through a Prism
  • Class 12th Physics NCERT Notes
  • Chapterwise NCERT Excercise of Class 12th Physics Chapters
  • About the Content Reviewer
  • FAQs Related to Refraction of Light Through A Prism
View More
Physics Ray Optics and Optical Instruments Logo

Process of Refraction of Light Through a Prism

Let us explain refraction of light through a glass prism in a step-by-step manner:

refraction through a prism
  • The incident light ray will strike the surface AB of the prism at an angle labelled as i, or the angle of incidence.
  • As soon as the incident light ray enters the prism,refraction will take place and light will bend towards the normal since it is coming from a rarer medium (air) to denser medium (glass). 
  • Here, as evident in the image, the angle between incident ray and normal will be known as the angle of incidence. 
  • The angle between refracted ray and normal drawn within the prism, will be known as the angle of refraction (r).
  • Now the light is travelling through the prism, bending at angle r. 
  • This path of refracted light ray within the prism will depend on both the refractive index of glass and angle of incidence.
  • As soon as the refracted ray touches the second surface AC of this prism, it travels back into the air. This ray will be known as an emergent ray. It is basically moving from glass to air.
  • Since the refracted ray is moving from denser to rarer medium, the refracted ray will bend away from the normal. 
  • The angle formed between emergent ray and the normal is known as the angle of emergence (e).
  • If the incident light is white, prism will disperse this light into its constituent colours. Therefore, white light will emerge as a rainbow.

We have also attached a simulation that you can look at to understand how refraction through a prism takes place.

 

Physics Ray Optics and Optical Instruments Logo

Derivation of the Refractive Index of a Prism

Let us understand the step-by-step derivation of the refractive index of a prism:

refractive index of the prism

From the above figure, we can tell that 

𝛿 =  𝛿₁ + 𝛿₂

= ( i₁ - r₁) + ( i₂+ r₂) 

= ( i₁ +i₂ ) - ( r₁+ r₂)              - (1) 

As we know by basic geometry rules, the sum of angles in a triangle is 180° which means that 

∠APR + ∠A + ∠AQR = 180°

(90° - r₁) + ∠A + (90° - r₂) = 180°

( r₁+ r₂)  = ∠A                      - (2)

Let us substitute the values of equation 2 in equation 1:

𝛿 = ( i₁ +i₂ ) - ∠A                  - (3)

For the case of minimum deviation

i₁ =i₂ =i

We will now use this equivalence relation in equation 3. 

𝛿 = ( i +i ) - ∠A 

𝛿 = ( 2i ) - ∠A 

i = (𝛿 + ∠A)/2

Since   r₁= r₂ =r, we will modify equation 2 as mentioned below:

(r₁ +r₂) = ∠A

(r + r) = ∠A

2r = ∠A

r = ∠A/2

Therefore, refractive index for a medium of prism for the minimum deviation angle (𝛿𝑚) will be:

n    =                sin        (                            δ m            +            A                   2               )                  sin        (                 A          2               )
       

Physics Ray Optics and Optical Instruments Logo

Reasons For Dispersion of White Light During Refraction of Light Through a Prism

Knowing this is important for JEE Main exam and NEET exam. As we have seen in the above section, a white light will disperse into a colourful spectrum of light when it passes through a prism. Let us understand why this happens:

  • A white light is composed of different colours. Every colour in this white light has a different wavelength and frequency
  • The light in the prism slows down since glass is denser medium than air. Refractive index of the prism material will be different relative to different wavelights of light. 
  • The shorter wavelights bend more than longer wavelengths because there is more change in speed of shorter wavelengths of light.
  • As per the Snell's law, angle of refraction depends on the refractive index of medium and wavelength of light.
  • Since refractive index differs for every colour, each colour will bend at a different angle while entering and exiting the prism.
  • Due to this reason, the white light disperses and splits into its constituent colours to form a spectrum.
  • One of the main reasons for the dispersion of white light during refraction through a prism is angle of prism as well. 
Physics Ray Optics and Optical Instruments Logo

About the Content Reviewer

This lesson has been reviewed by subject matter expert Raghvendra Kumar, who has years of experience in teaching. He has qualified for the AIEEE and GATE exams. He specialises in preparing students for JEE Main and NEET as well as for Class 11 and 12 subjects. As a reviewer at Shiksha, he ensures that content has clarity, correctness and exam readiness of concepts and explanation sets. To further maintain the quality of the content being shared on our website, we have referred to the study material provided by NIOS, NCERT and standard Physics books.

Physics Ray Optics and Optical Instruments Logo

FAQs Related to Refraction of Light Through A Prism

Let us take a look at the frequently asked questions related to refraction through a prism which may even be important for competitive exams like IIT JAM exam and CUET exam:

Q&A Icon
Commonly asked questions
Q:  

What is total deviation in prims?

A: 

Total deviation in a prism is the total angle by which the light ray gets bent as it passes through the prism. It is an angle between incident ray and emergent ray of the prism. When a light enters the prism, it will bend towards the normal. After that, it will travel through the prism and bend away from the normal as it exits. Total deviation is the sum of these two from which the apex angle is subtracted.

The formula for total deviation for a prism is as follows:

  δ   =   i1   +   e2   -   A

 

  •        i     1    : angle of incidence at first surface
  •        e     2    : angle of emergence at second surface
  • A: apex angle of the prism 
Q:  

How many refractions are in a prism?

A: 

In a prism, the light ray undergoes refraction twice as it passes through a prism. Let us take a look at both types of refraction. 

The first refraction takes place when a light ray enters the prism from air or another medium into glass. It then bends toward the normal if the prism material is denser (n>1)

During the second refraction, the light ray exits the prism again into air, it then bends away from the normal. In case, the angle of incidence is too steep, total internal reflection may occur at the second interface of prism, which prevents the ray from exiting. When this happens, there is only one refraction and one reflection.

Q:  

What is a total refractive prism?

A: 

A total refractive prism is also known as a total internal reflection prism. It is an optical prism that is designed for reflecting 100% of the incident light. This happens since this prism uses the principle of total internal reflection. These prisms are oriented and shaped in a specific way so that the light that enters at a specific angle is completely reflected inside the prism. A right-angle prism, porro prism, dove prism and roof prism are some of the examples of total reflective prism. 

qna

Physics Ray Optics and Optical Instruments Exam

Student Forum

chatAnything you would want to ask experts?
Write here...