I was up late working with my red LED headlamp on and when I was looking at the black part of my LCD (Apple Retina Macbook Pro) screen I noticed this interesting diffraction pattern. I'm confused because all I can think of that would cause this are Fourier and Fraunhofer patterns which I thought required more advanced optics the light to be collimated. One friend suggested it might be the polarizer. Any thoughts?
Unfortunately, the picture doesn't show this, but along the x and y axes there were discrete points not a continuous streak. Sorry for the horrible photo quality.
Answer
The diffraction seems to form from the pixels (basically a diffraction grating). The pixels have a translational symmetry in x and y directions, so the pattern also exhibits this symmetry. On a 15-inch macbook retina display, the pixels are separated by
d=15.6 inch√28802+18002=0.396 m√28802+18002=1.17⋅10−4 m
From elementary geometry and optical path lengths (and small angle approximation), you can deduce that constructive interference occurs when there is an angle change from usual reflection of
Δα=λ/d
If you hold the your laser at for example b from the screen and your eyes are also at b from the screen, the dots should appear to have a width
a=bΔα/2=bλ/2d
where the the factor of Δα/2 came from the fact that the beam then has to reflect back at angle −Δα/2 relative to normal to reach your eyes, so the difference is Δα.
For b=0.5 m and λ=700 nm, this gives a=1.5 mm.
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