Diffraction of monochromatic light I am learning the diffraction these days, and I am quite curious about the energy distribution of the light. We know that for single slit diffraction the monochromatic light will form some bright and dark areas. Where does the energy of the dark place go? Does it just disappear?
 A: The light doesn't reach the centre of the dark spots where intensity is 0 because of Huygen's principle.
Each wave front is made up of an infinite number of wavelets, and these wavelets interfere to produce the single slit diffraction pattern. 
The maxima are the points where the wavelets interfere constructively, and the minima are the points where the wavelets interfere destructively, meaning no light reaches the minima. 
If this answer doesn't make sense to you then research single slit diffraction and Huygen's principle and it'll make more sense. 
A: You can see diffraction as the interference between all the small spherical waves emitted from the different points of the slit (the Huygens-Fresnel principle). 
On the resulting diffraction pattern you show, in the dark regions, there is no power due to destructive interference while in the bright regions, you have constructive interference. So the power which passes through the slit is distributed on the bright fringes and never gets to the dark regions.
A: The energy does not disappear. The energy that would have gone to the dark spots--if not for diffraction--is diverted to the bright spots. If you were to remove the slits and use a lens to make a band of light as wide as the diffraction pattern, you would find that the light at the center is dimmer than the central peak of the diffraction pattern.
The light bands are concentrated light that was diverted from the dark bands. The total amount of light is the same.
There's a great video from MIT to show that interference just moves light around. It does not create or destroy light.
Where Does the Light Go?
