Question

Draw a ray diagram to produce interference fringe pattern in Young’s double slit experiment. Derive an expression for fring width of bright fringes. 

Answer 1

Suppose S₁ and S₂ are two fine slits, as small distance d apart. They are illuminated by a strong source S of monochromatic light of wavelength. MN is a screen at a distance D from the slits.

Young’s double slit arrangement to produce interference pattern

Consider a point P at a distance y from I, the centre of screen.

The path difference between two waves arriving at point P is equal to S₂P – S₁P. 

For constructive interference (Bright fringes)