A glass plate 0.9cm thick has a reflective index of 1.50. How long does it take for a pulse of light to pass through the plate [c = 3.0 x 10^8 ms‾¹]

A. 4.5 x 10^-11s
B. 4.5 x 10^-10s
C. 4 .5 x 10^-12s
D. 3.0 x 10^-10s

Correct Answer:

Option C = 4 .5 x 10^-12s

Explanation

N = 1.5 = (speed of light in glass)/speed of light in air
(1.5)/1 = (3.0 x 10⁸)/V
V = (3.0 x 10⁸)/1.5
V = 2.0 x 10⁸ ms^-1
But x = Vt
t = (x)/Vt
t = (x)/v = (9 x 10)⁴/2.0x 10⁸ = 4.5 x 10^-2S
∴ Time = 4.5 x 10^-12S