Question

A light wave of wavelength  500 nm falls upon three slits a distance 0.5 mm each from one another. A screen is placed at a distance 2 m from slits. Find

(a) the distances from P where intensity reduces to zero

(b) the distance from P where next bright fringe are observed 

(c) the ratio intensities of bright fringes observed on the screen.

Answer 1

(a) In case of three slits, intensity becomes zero, when phase difference between any two waves is,

The corresponding path difference,

Substituting the values, we have

(b) Bright fringes are obtained on the screen where

These are called primary maximas

Proceeding in the similar manner, we have

These are called secondary maximas.

Note that y = 0 is also a secondary maxima because at P, ϕ  = pi

(c) At principal; maximas (ϕ = 2pi, 4pi,.....etc)