JEE Main · 2021 · 18 Mar 2021 · Shift 2 · Geometrical Optics

The three rays strike the face PQ along its normal and enter undeviated, so each meets the hypotenuse PR at $45^{\circ }$, the prism angle at P. A ray escapes through PR only when $45^{\circ }$ lies below the critical angle, that is when $\sin 45^{\circ }<\frac{1}{n}$, or $n<\sqrt{2}=1.414$. Red, with $n=1.27$, meets this condition and refracts out. Green ($n=1.42$) and blue ($n=1.49$) both exceed $\sqrt{2}$ and are totally internally reflected. The light leaving the face PR is red.