Three rays of light, namely red $(R)$, green $(G)$ and blue $(B)$ are incident on the face $PQ$ of a right angled prism $PQR$ as shown in figure. ![JEE Main 2021 Physics, Ray Optics — question figure](https://prepforbharat.s3.ap-south-1.amazonaws.com/exam/615f0e999476412f48314daf/Physics/images/Ray_Optics/648b5a69417cc3fb48d67033/question_1__q_648b5a69417cc3fb48d67033__cdn-question-pool.getmarks.app__ced3db70-d3eb-4121-a996-ca3d025ed244-image__338f9cf140_final_ppt_sync.png) The refractive indices of the material of the prism for red, green and blue wavelength are $1.27,1.42$ and $1.49$ respectively. The colour of the ray(s) emerging out of the face $PR$ is :

JEE Main 2021 — Physics Optics

2021mcqeasy

Three rays of light, namely red $(R)$ , green $(G)$ and blue $(B)$ are incident on the face $PQ$ of a right angled prism $PQR$ as shown in figure.

JEE Main 2021 Physics, Ray Optics — question figure

The refractive indices of the material of the prism for red, green and blue wavelength are $1.27,1.42$ and $1.49$ respectively. The colour of the ray(s) emerging out of the face $PR$ is :

Official previous-year question

Held on 18 Mar 2021 · Verified 6 Jul 2026.

Options

A
green
B
red
C
blue and green
D
blue

Did you get this right?

Solution

JEE Main 2021 Physics, Ray Optics — solution figure

Assuming that the right angled prism is an isoceles prism, so the other angles will be $45^{\circ}$ each.

$\Rightarrow$ Each incident ray will make an angle of $45^{\circ}$ with the normal at face $PR$ .

$\Rightarrow$ The wavelength corresponding to which the incidence angle is less than the critical angle, will pass through $PR$ .

$\Rightarrow {\theta }_{C}=$ critical angle

$\Rightarrow {\theta }_{C}={\mathrm{sin}}^{-1}(\frac{1}{\mu })$

$\Rightarrow$ If ${\theta }_{C}\geq 45^{\circ}$

the light ray will pass

$\Rightarrow {({\theta }_{C})}_{\mathrm{Red}}={\mathrm{sin}}^{-1}(\frac{1}{1.27})=51.94^{\circ}$

Red will pass.

$\Rightarrow {({\theta }_{C})}_{\mathrm{Green}}={\mathrm{sin}}^{-1}(\frac{1}{1.42})=44.76^{\circ}$

Green will not pass

$\Rightarrow {({\theta }_{C})}_{\mathrm{Blue}}={\mathrm{sin}}^{-1}(\frac{1}{1.49})=42.15^{\circ}$

Blue will not pass

$\Rightarrow$ So only red will pass through $PR$ .

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Solution

JEE Main 2021 Physics, Ray Optics — solution figure

Assuming that the right angled prism is an isoceles prism, so the other angles will be $45^{\circ}$ each.

$\Rightarrow$ Each incident ray will make an angle of $45^{\circ}$ with the normal at face $PR$ .

$\Rightarrow$ The wavelength corresponding to which the incidence angle is less than the critical angle, will pass through $PR$ .

$\Rightarrow {\theta }_{C}=$ critical angle

$\Rightarrow {\theta }_{C}={\mathrm{sin}}^{-1}(\frac{1}{\mu })$

$\Rightarrow$ If ${\theta }_{C}\geq 45^{\circ}$

the light ray will pass

$\Rightarrow {({\theta }_{C})}_{\mathrm{Red}}={\mathrm{sin}}^{-1}(\frac{1}{1.27})=51.94^{\circ}$

Red will pass.

$\Rightarrow {({\theta }_{C})}_{\mathrm{Green}}={\mathrm{sin}}^{-1}(\frac{1}{1.42})=44.76^{\circ}$

Green will not pass

$\Rightarrow {({\theta }_{C})}_{\mathrm{Blue}}={\mathrm{sin}}^{-1}(\frac{1}{1.49})=42.15^{\circ}$

Blue will not pass

$\Rightarrow$ So only red will pass through $PR$ .

JEE Main 2021 — Physics Optics

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JEE Main 2021 — Physics Optics

Options

Did you get this right?

Solution

Your note

JEE Main Optics in other years

More JEE Main Optics questions

Other Physics topics for JEE Main

Mechanics

Electromagnetism

Modern Physics

Thermodynamics

Waves & Oscillations

Keep practicing Optics