# Explaining light distribution trough a gap using waves and Huygens principle

According Huygens principle when a light wave goes trough a gap that is so narrow that only one wavelet can pass, it would spread out in all directions like in this image:

(image from https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle)

That would mean that the room after the gap is lit equally in all directions. But its never the case in real life. No matter where you stand as an observer the front will be always the brightest and sides darker.

How can that be explained, hanging on to wave principle?

• I think you are seeing an interference pattern. Diffraction happens at the edges of the opening. Each side of the slit acts as a source of expanding, circular waves, and the waves from those two, physically separate sources interfere with each other. Mar 25, 2020 at 10:57

If the opening is greater than one wavelength in size you will have a diffraction pattern, a beam pattern like a sin(x)/x function, and will have a main lobe and side lobes--and so will not have uniform illumination.

To get relatively uniform illumination you would need an opening less than 1/4 wavelength in size.

The opening acts like an aperture or antenna where the main lobe beam width is proportional to the wavelength divided by the aperture size.