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 cos
2
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
