i. What could be the reason behind the disappearance? ii. At what angle of viewing the ring vanishes?

Question

In one of the performances, a magician keeps a gold ring beneath a thick glass slab (µ = \(\frac{3}{2}\)) Then he keeps a flask filled with water (µ = \(\frac{4}{3}\)), over the slab. Now when spectators one by one observe from the open end of the flask, the ring disappears at a certain angle of viewing.

i. What could be the reason behind the disappearance?

ii. At what angle of viewing the ring vanishes?

Answer 1

i. The ring disappears due to total internal reflection of the light at water-air interface.

ii. _aµ_w = \(\frac{4}{3}\)

sin i_c = \(\frac{1}{µ}\)

Hence, for angle of viewing for which angle of incidence of ring from water to air is greater that 48.6°, the ring will vanish.