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In some smartphone charger there exists a bulb (LED) most probably which emits light when charger is plugged in. I have noticed that light coming from that bulb causing diffraction pattern on the wall. I think that the setup is similar to single slit diffraction. However to my surprise I am not observing the central maxima. What could go wrong? The images are given here. One is of light source and another is of diffraction pattern observed in the wall opposite to it.

diffraction source, ignore the red light

diffraction pattern

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    $\begingroup$ I believe this is just due to spherical aberration from the front plastic surface of the LED. But I'm not 100% sure. $\endgroup$
    – Michael Seifert
    Commented Oct 1 at 11:34
  • $\begingroup$ its not a diffraction pattern. Its just the "image" of the LED after passing through the clear plastic "lens". Its just the shape of the light. $\endgroup$
    – José Andrade
    Commented Oct 1 at 14:20
  • $\begingroup$ but if i make the light pass through a wider opening like from a tube like structure, it does not show those pattern anymore. $\endgroup$
    – thinking_sapiens
    Commented Oct 1 at 16:06
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    $\begingroup$ I want to add that your assumption about the brightness of the central order of diffraction patterns is not generally true. There are plenty of examples where the central order is dark instead of bright, eg. Newton's rings or Llyod's mirror. This may occur when there is an additional reflection involved in one of the interfering paths which can modify the phase difference condition and thus switch bright and dark interference fringes. $\endgroup$
    – zimmervi
    Commented Oct 1 at 17:04
  • $\begingroup$ I am now not even sure if the phenomena observed above is diffraction or not. I think the phenomena above is reproducible by anyone with mobile charger with on indicator LED $\endgroup$
    – thinking_sapiens
    Commented Oct 4 at 9:11

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On my phone (iPhone Xs Max), the light is bigger than the cameras. It has to produce a lot of light, and this takes a lot of area.

A point source of light would ordinarily produce the same amount of light in all directions. Mine does not. As you rotate the phone, the light is dimmer when seen from the sides.

I can see this better by aiming the light at the wall as you have done. The light is a ring. As I back away, the ring expands. The phone is set up to produce a somewhat hollow cone of light. There is light in the interior and around the cone, but mostly at the cone.

Looking at the light, it appears there is a hexagonal grid of lenses in front of the LED. It seems each lens sends light from one region of the LED in a particular direction. I can confirm this with a pinhole in a piece of paper. As I move the pinhole back and forth over the light, bright spots move across the wall. As I move the pinhole from one lens to another, I get separate fuzzy images.

My wife's phone is a Samsung Note 20. It produces a more filled in expanding cone of light, but there is a dip in the center. It has a Fresnel lens with what appears to be a relatively small LED behind it.

enter image description here

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  • $\begingroup$ The OP is talking about the "on" indicator LED on the phone charger, not a LED built into the phone. $\endgroup$
    – PM 2Ring
    Commented Oct 1 at 15:57
  • $\begingroup$ Yeah, my light was coming out of phone charger, not the phone itself $\endgroup$
    – thinking_sapiens
    Commented Oct 1 at 16:04

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