Spark image

The phasor treatment of diffraction

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 3(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 3(b)).

A further movement across the pattern will give the first minimum (Figure 3(c)) where the intensity is zero and the second maximum (Figure 3(d)).

© Keith Gibbs 2011