A parallel-sided plate of refractive index 17/16 is illuminated from air at 30°. A ray reflected from the rear surface emerges parallel to the primary reflected ray with separation 4√3 cm. If plate thickness is t metres, find 40t.
Physics · Wave Optics
JEE Mains Advanced

Question

A parallel-sided plate of refractive index 17/16 is illuminated from air at 30°. A ray reflected from the rear surface emerges parallel to the primary reflected ray with separation 4√3 cm. If plate thickness is t metres, find 40t.

Physics · Wave Optics

G1 Physics · Accepted Answer

Use Snell's law to find refraction angle r, then lateral separation between the two parallel reflected rays from the front and rear surfaces. Substituting n=17/16, i=30° and the stated separation gives t=0.075 m, hence 40t=3.