A thin prism of refractive index 1.5 when placed in air deviates a light ray by a minimum angle of \( 5^\circ \). The angle of minimum deviation when it is immersed in oil of refractive index 1.25 is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( 2^\circ \)
For a thin prism the deviation is \( \delta = (\mu - 1)A \), where \( \mu \) is the refractive index of the prism relative to the surrounding medium.
In air (\( \mu = 1.5 \)):
\( \displaystyle 5^\circ = (1.5 - 1)A \;\Rightarrow\; A = 10^\circ \)
In oil, the relative refractive index is
\( \displaystyle \mu_{\mathrm{rel}} = \frac{1.5}{1.25} = 1.2 \)
\( \displaystyle \delta' = (1.2 - 1) \times 10^\circ = 2^\circ \)