Class 12 CBSE Notes
Worried about the pending board syllabus?
Revise 12th Class Notes.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.
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.
Let us explain refraction of light through a glass prism in a step-by-step manner:
We have also attached a simulation that you can look at to understand how refraction through a prism takes place.
Class 12 CBSE Notes
Worried about the pending board syllabus?
Revise 12th Class Notes.11th CBSE Notes
Class 11th topics left before exams?
Revise 11th CBSE notes.Let us understand the step-by-step derivation of the refractive index of a 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:
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:
Those students who are going to take class 12th CBSE board exams must remember that they need to have the knowledge of each topic of the subject. This is why we have created a note bank for students to revise all the chapters quickly:
Name of Chapter |
Name of Chapter |
---|---|
Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits |
Here, we have shared the NCERT Exercise of Class 12th Physics chapters for students who are planning to take their board exams next year.
Name of Chapter |
Name of Chapter |
---|---|
Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits |
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.
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:
What is total deviation in prims?
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:
How many refractions are in a prism?
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.
What is a total refractive prism?
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.
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