Q. The cross-section of a glass prism has the form of an isosceles triangle. One of the refracting faces is silvered. A ray of light falls normally on the other refracting face. After being reflected twice, it emerges through the base of the prism perpendicular it. The angles of the prism are
A) $54{^\circ},54{^\circ},72{^\circ}$
B) $72{^\circ},72{^\circ},36{^\circ}$
C) $45{^\circ},45{^\circ},90{^\circ}$
D) $57{^\circ},57{^\circ},76{^\circ}$
Explanation
(b) [image:9_image73.jpeg]
Form $\mathrm{\Delta}\ abc,A + 90{^\circ} + (90{^\circ} - i) = 180{^\circ}$
Or $\ i = A$
Now, complementary angle at point $d,\theta = 2i$
$\therefore\theta = 2A$
Only option (b) satisfies this
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.