Question
State Huygens’s principle. Show, with the help of a suitable diagram, how this principle is used to obtain the diffraction pattern by a single slit.
Draw a plot of intensity distribution and explain clearly why the secondary maxima becomes weaker with increasing order (n) of the secondary maxima.
Solution
Huygen’s principle states that

Each point on a wave front behaves as a source of secondary wavelets

The secondary wavelets travel with the speed of light in that medium

The position of new wave front at a later time can be found out by drawing a common tangent to all these secondary wavelets
Intensity distribution of single slit distribution
For the first secondary maxima (n = 1), only onethird of the slit contributes to its intensity.
Secondly, for n = 2, only onefifth of the slit contributes to the intensity.
Therefore, with increasing n, the intensity decreases.