The phasor treatment of diffraction is a useful visual way of explaining the
diffraction pattern at a point. Imagine the intensity of the light coming from each part of the slit to
be represented as a vector. The intensity of the light in the diffracted pattern at any point will
then be the sum of these vectors at that point.
If we consider the single slit to be broken
up into a large number of very small parts then all points on a given strip have the same path
length to a point P on the screen where the diffraction pattern is formed. At the centre of the
pattern light from each point is in phase and so the phase shift between adjacent strips is zero -
the resulting intensity is then given simply by the algebraic sum of the intensities due to each
strip (Figure 1(a)). As we move away from the centre of the pattern there is a definite phase
difference between adjacent strips and the final intensity (E) is shown by (Figure 1(b)).
