Spark image

Young's double slit experiment

Light from a monochromatic line source passes through a lens and is focused on to a single slit S. It then falls on a double slit (S1 and S2) and this produces two wave trains that interfere with each other. The interference pattern at any distance from the double slit may be observed with a micrometer eyepiece or by using a screen.

For a bright fringe:     ml = xmd/D

For a dark fringe:     (2m + 1)l/2 = xmd/D

Where m = 0,1,2,3 etc. and so the mth bright fringe for m = 3 is 3lD/d from the centre of the pattern. The distance between adjacent bright fringes is called the fringe width (x).

Wavelength (l) = xd/D

Fringe width (x) = lD/d

© Keith Gibbs 2011