8

This evening, looking at the television in my apartment, I noticed that the light reflected on it manifested some kind of regular pattern. When the source of light is sufficiently far, this consists of six straight rays that appear to be symmetric under reflections about the vertical axis, as can be seen in the following picture:

Far source pattern

My first thought on this was that light propagates within the screen along certain preferred directions. However, when the reflection angle between the source and the observer is such that the source is not directly reflected, the straight lines are still visible, and converge to a point outside of the surface of the television:

Picture with a focal point outside the television

Finally and oddly enough, when the source gets very close to the reflection surface, some strong bending of the rays becomes visible, and the rays become shorter. This would suggest that the origin of this phenomenon may simply be due to some reflection, rather than diffraction taking place within the screen of the television.

Closest picture, where some curvature is visible

I was wondering what the origin of this peculiar phenomenon might be. A possible interpretation I thought of is that some kind of crystal structure may reflect light in certain preferred directions, but this does not seem to explain the bending of the rays. Any ideas?

Qmechanic
  • 201,751
CoughCourse
  • 81

2 Answers2

6

This happens because the screen acts like a diffraction grating, due to the periodic arrangement of pixels. You will likely see the same phenomenon if you look at the reflection of a distant point-like source of light on your smartphone. Note also the lines you see are "colored", because the diffraction maxima occur at different angles for different wavelengths.

The cause of the "bending" you see is more subtle. When the light source is distant, the angle of incidence (or more precisely, the direction of the incident ray of light) is approximately the same on each point of the screen. Likewise, when the observer (i.e. you) is also distant, each diffraction maximum you see has the same angle of reflection. This means that, in a sense, you are directly observing the directions of the diffraction maxima for a fixed angle of incidence, hence the simpler, more regular pattern you see. If either you or the source is close to the screen, the angle of reflection or incidence changes throughout the screen, manifesting as an apparent bending of the lines you see.

Puk
  • 13,526
  • 1
  • 22
  • 42
3

This appears to be due to diffraction and in the middle of the screen the source is brightest there due to the first (zero order) maximum. Note that light of different colors further from the center image exist next to each other in bands (corresponding to the second, third, etc., maxima).

Such bands are characteristic of diffraction. Maybe there are small scale structures or periodicity in the pixels in the screen that cause the light from your source to diffract. There also appears to be spectral dispersion as well since light of specific colors are deflected at different angles.

Without knowing more details about the screen's design and composition, it is hard to tell what's causing all this exactly, but from these images, it looks like both disperse reflection and diffraction are occurring simultaneously.

joseph h
  • 29,356
  • The "direct reflection" you refer to is consistent with diffraction, it's just the zeroth order of the diffraction grating. – Puk Apr 08 '22 at 06:31
  • Yep. Thanks for that. But due to it's intensity I'm think a lot is reflection. – joseph h Apr 08 '22 at 06:33
  • It is normal for different diffraction orders to have different intensities. The direction of the zeroth order is also the same for all wavelengths, which also partly explains why it appears much brighter. – Puk Apr 08 '22 at 06:35
  • @Puk Yeah. Sounds right. I've made an edit. Thanks. – joseph h Apr 08 '22 at 06:41
  • @Puk isn't the zeroth order of diffraction grating the same as ordinary reflection? – John Dvorak Apr 08 '22 at 13:57
  • @JohnDvorak I wasn't arguing otherwise. Yes, the zeroth order is consistent with ordinary specular reflection from a flat surface. Describing the phenomenon as a mix of reflection and diffraction just seems a bit awkward to me (although not necessarily wrong) since this "reflection" is fully included in diffraction grating theory. Technically you could argue that all orders of the grating are due to reflections from the grating surface. – Puk Apr 08 '22 at 14:38