# What is causing the separation of light in this image?

Reflection of keychain LED light on Macbook Pro (2014) screen

I have also observed this phenomenon on my mobile phone and on a tablet. I assume this means it is somehow related to backlit screens.

The further the light source from the screen, the more separation that is observed.

Each distinct reflection appears to be the result of a prism with imperceptible depth.

This is not repeatable on other common reflective surfaces, so I assume it is a property of some material or materials in this particular class of technology.

Clearly the image does not fully represent the observed phenomenon, but it was the best I could capture with the equipment I have on hand.

The photograph was taken with a mobile phone camera.

• You need to say more about what the light source is, what's in front of the source, and what you're photographing with. It looks a diffraction pattern but I'm sure there are many other explanations. – Brandon Enright Nov 19 '14 at 23:27

This looks like an interference pattern like the type created by a diffraction grating.

Note that to the left and right there are little rainbows with the red outermost and then green and blue innermost.

Also the further away from the centre the wider the rainbows get.

These features are characteristic of diffraction grating type phenomena.

Now here the picture has a complex two dimensional array of dots that must be caused by a two dimensional structure. The one thing to say for certain is that the dimensions of the structure causing the pattern are close to the wavelength of visible light (roughly $750$ to $350 nm$)

The structure which is acting like a diffraction grating is, of course, in the screen you are shining the LED on.

I suggest you try pointing the LED light at the shiny surface of a CD or DVD. You can get beautiful rainbows... as you said it looks a bit like as if it were from a prism.

Final point. Because you get continuous rainbows suggest that the LED light source is giving broad bands of light over many wavelengths. If you can do the same experiment with a fluorescent light source you may find some individual bright sharp lines due to emission of light at individual wavelengths by isolated atoms in the gas inside the fluorescent lamp.

Looking at past posts I have not seen this question asked before, but did find this past question about whether or not light should be coherent for this effect to be observed

• I'm not sure what you mean by "smeared out rainbows". – Shmiddty Nov 20 '14 at 1:03
• @Shmiddty sorry good point.. will try to make it clearer – tom Nov 20 '14 at 1:49