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Young's Double Slit Experiment

This is a classic example of interference effects in light waves. Two light rays pass through two slits, separated by a distance d and strike a screen a distance, L , from the slits,
Double slit diffraction

If d < < L then the difference in path length r1 - r2 travelled by the two rays is approximately:

r1 - r2 $\displaystyle\approx$ dsin $\displaystyle\theta$

where $\theta$ is approximately equal to the angle that the rays make relative to a perpendicular line joining the slits to the screen.

If the rays were in phase when they passed through the slits, then the condition for constructive interference at the screen is:

dsin $\displaystyle\theta$ = m$\displaystyle\lambda$ ,m = $\displaystyle\pm$ 1, $\displaystyle\pm$ 2,...

whereas the condition for destructive interference at the screen is:

dsin $\displaystyle\theta$ = (m + $\displaystyle{1\over 2}$)$\displaystyle\lambda$ ,m = $\displaystyle\pm$ 1, $\displaystyle\pm$ 2,...

The points of constructive interference will appear as bright bands on the screen and the points of destructive interference will appear as dark bands. These dark and bright spots are called interference fringes. Note: