A parallel beam of light of wavelength 500 nm falls on a narrow slit and the resulting diffraction pattern is observed on a screen 1 m away. It is observed that the first minimum is at a distance of 2.5 mm from the centre of the screen. Find the width of the slit.

Wavelength of light beam, λ= 500 nm = 500 x 10⁻⁹ m

Distance of the screen from the slit, D = 1 m

For first minima, n = 1

Distance between the slits = d

Distance of the first minimum from the centre of the screen can be obtained as:

x = 2.5 mm = 2.5 x 10^-3 m

It is related to the order of minima as: d

nλ = x d/D

d= nλ D/x

= 1 x 500 x 10⁻⁹ x 1/(2.5 x 10^-3 )  = 2 x 10^-4 m= 0.2 mm

Therefore, the width of the slits is 0.2 mm