Question

Define interference of light waves.
Draw a diagram of Young's double slit experiment to produce interference fringe pattern. Derive an expression of fringe width for bright fringes.

Answer 1

When two light waves from different coherent sources meet together, then the distribution of energy due to one wave is disturbed by the other. This modification in the distribution of light energy due to super- position of two light waves is called 'Interference of light'.

The path difference of light from the sources $S_1$ and $S_2$ meeting at point P which is at a distance $y$ above the central axis is $\frac{yd}{D}$

Hence for constructive interference, $\frac{yd}{D}=n\lambda$

Therefore bright fringes occur at $y=0,\frac{D\lambda }{d},\frac{2D\lambda }{d},....$

Therefore the fringe width=$\frac{D\lambda }{d}$