AL physics:Diffraction

Why the central maximum is of highest intensity but the other maxima are of much lower intensities,when plane light waves are incident normally on the slit?

In the analysis of diffraction under the Fraunhofer conditions, plane waves arrive at the single slit are divided into segments, each of which can be regarded as a point source, the amplitudes of the segments will have a constant phase displacement from each other. Such constant phase displacement, when added together, will form the intensity variation pattern as observed.

Detailed analysis needs the application of vectors, you may refer to the following web-page for information,


http://hyperphysics.phy-astr.gsu.edu/Hbase/phyopt/sinint.html#c2

In Young's double slit experiment,why diffraction effect cause variation in intensity?

The interference pattern of bright and dark fringes in a double slit is a result of interference effect, not by diffraction. Diffraction at the two slits brings the two light beams to interfere.