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:

enter image description here

(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.

enter image description here

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

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    $\begingroup$ 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. $\endgroup$ – Solomon Slow Mar 25 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.

Google "beam pattern" or "radiation pattern".


No matter where you stand as an observer the front will be always the brightest and sides darker.

This is true but the effect can be made so small as to be not noticible by decreasing the size of the opening. For example, decrease the size by a factor of 3 in your simulation. Then you wall get nearly uniform illumination.

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  • $\begingroup$ But that doesn't answer my question :( $\endgroup$ – John T Mar 25 at 17:48
  • $\begingroup$ updated my answer to specifically address your question. $\endgroup$ – user45664 Mar 25 at 18:45
  • $\begingroup$ What do you mean with "uniform illumination"? English is not my native and I could not find decent translation :) $\endgroup$ – John T Mar 25 at 19:18
  • $\begingroup$ uniform illumination means the same brightness everywhere $\endgroup$ – user45664 Mar 25 at 20:57
  • $\begingroup$ same as "lit equally in all directions" $\endgroup$ – user45664 Mar 25 at 20:59

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