In: Physics
in free space (vacuum) light travels at speed c (approximately
3x10^8 m/s)
inside the glass the light travels at a speed v = c/n., n = index
of refraction
L = length of the glass slab
v = d / t (velocity = distance / time)
t = d / v
t = L / (c / n) = n*L/c
c = lambda * f
t = n * L / c = n * L / (lambda * f)
Now, use lambda = L/10, so and get
t = n * L / (lambda * f) = n * L / (L * f / 10) = 10* n / f
so t = 15/f