Question

Thickness of the glass of a spectacle is 2 mm and refractive index of its glass is 1.5.

Calculate time taken by light to cross this thickness. Express your answer with most convenient prefix attached to the unit ‘second’.

Answer 1

Speed of light in vacuum, c = 3 × 10⁸ m/s

Given that:

Refractive index, (n_glass) = 1.5

Thickness of the glass = 2 mm

= 2 × 10 sup > -3 m

∵ Re fractive index (n_glass) =

As v = \(\frac{s}{t}\)

time taken (t) to cross the thickness (s),

Most convenient unit to express this small time is nano second. (1 ns = 10^-9 s)

∴ t = 0.01 × 10^-9 s = 0.01 ns