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Fraunhofer diffraction - double slit

For the double slit we simply have light from two adjacent slits meeting at the eyepiece. In this case the formula for a maximum (a place where the light waves 'add up') is:

ml = d sin q

where d is the distance between the centres of the two slits (See Figure 1).

The intensity of the interference pattern produced by two sources is simply varied by the diffraction effects.

We will have cos2 fringes modulated by the diffraction pattern for a single slit. The intensity distribution is shown in Figure 2.

Example problem
A double slit of width 0.5 mm is illuminated with light of wavelength 550 nm. Calculate the diffraction angle for the second maximum.

Using ml = d sin q with m = 2
we have sin q = 2 x 5.5x10-7/0.5x10-3 = 0.0022
Therefore angle of diffraction = 0.13o
© Keith Gibbs 2011