CBSE 2026 · Region 5 · Set 3 · Q18 · 2 marks
A ray of light MN is incident normally on the face corresponding with side AB of a prism with an isosceles right-angled triangular base ABC. Trace the path of the ray as it passes through the prism when the refractive index of the prism material is (i) $\displaystyle \sqrt{2}$, and (ii) $\displaystyle \sqrt{3}$.
Marking-scheme solution
(i)
$\displaystyle \sin i_{c}=\dfrac{1}{\mu}=\dfrac{1}{\sqrt{2}}$, so $\displaystyle i_{c}=45^{\circ}$
Since $\displaystyle i=i_{c}$, the emergent ray grazes alongside AC.
(ii)
$\displaystyle \sin i_{c}=\dfrac{1}{\mu}=\dfrac{1}{\sqrt{3}}$, so $\displaystyle i_{c}<45^{\circ}$
Since $\displaystyle i>i_{c}$, the ray suffers Total Internal Reflection (TIR).
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CBSE Class 12 Physics past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.