State one feature by which the phenomenon of interference can be distinguished from that of diffraction.
A parallel beam of light of wavelength 600 nm is incident normally on a slit of width ‘ a ’. If the distance between the slits and the screen is 0.8 m and the distance of 2 nd order maximum from the centre of the screen is 15 mm, calculate the width of the slit.
Difference between interference and diffraction: Interference is due to superposition of two distinct waves coming from two coherent sources . Diffraction is produced as a result of superposition of the secondary wavelets coming from different parts of the same wavefront.
Numerical: Here, λ = 600 nm = 600 × 10 −19 = 6 × 10 −7 m
D = 0.8 m, x = 15 mm = 1.5 × 10 −3 m,
n = 2, a = ?
The formula is absolutely correct but there are a couple of mistakes...
for constructive interference
a(x/D) = nλ
or
the slit width would be
a = (nλD) / x
here
n = 2
λ = 600 nm = 600 X 10-9 m = 6 X 10-7 m
D = 0.8 m
and x = 15 mm = 15 X10-3 m (not 1.5 X10-3 m)
so,
a = (2 X 6 X 10-7 X 0.8) / 15 X10-3
or
the slit width is
a = 0.64 X 10-4 m = 6.4 X10-5 m