How to photograph a diffraction pattern?

  • #26
sophiecentaur
Science Advisor
Gold Member
25,185
4,814
It shouldn't be necessary to use a camera to find out the result. All you should need to do would be to use your eye directly. But the pupil of the eye is only a couple of mm wide so you would not see anything but a small portion of any pattern. It is easy enough to see some diffraction patterns directly, though. LEDs on cars can be seen through net curtains and other regular woven fabrics and they appear as fringes. However, the diffracting structure is very wide so the eye must be seeing different bits of the pattern, caused by different bits of the fabric weave. You are not looking at just a single hole / pair of slits etc. in that case.
 
  • #27
sophiecentaur
Science Advisor
Gold Member
25,185
4,814
I would think that defocusing would project the pattern onto the sensor. The diffraction pattern is, after all, a specific case of unfocused light falling on a surface.
I just read that again and it made me think. The diffraction pattern that falls on a screen is due to the phase relationships between the various contributions of waves from different parts of the originating 'structure'. The screen reveals the phase relationships by scattering the light and taking away any coherence that existed in that plane. Those phase relationships are entirely different for the (still coherent) light travelling further and arriving at the lens of a camera that would have been focussed on the screen when the screen has been removed - so you will get a totally different diffraction pattern. Adding a lens will again alter the phase relationships but in a coherent way, so you would expect some sort of diffraction pattern - but you won't be actually 'focussing' any image. However, as I have mentioned before, it is only in the unobstructed, direct path through the lens that you will get some portion of the screen pattern. The lens will vignette the pattern and reduce how much of it gets to the sensor, compared with the hole left when you take the lens away.
I think I conclude that the camera should be put quite close to the diffracting structure for a useful result. This will result in a small 'throw' and a consequentially small pattern. But a high res sensor will allow you to magnify that image and get a reasonable result.
 
  • #28
Andy Resnick
Science Advisor
Education Advisor
Insights Author
7,509
2,078
I would think that defocusing would project the pattern onto the sensor. The diffraction pattern is, after all, a specific case of unfocused light falling on a surface.
That works *if* the entrance pupil of the camera lens is located at (or really close to) the diffracting aperture. Then, by focusing the lens to infinity, the far-field diffraction pattern of the aperture will be projected onto the sensor (because the lens is 1 focal length away from the sensor).
 
  • Like
Likes sophiecentaur
  • #29
sophiecentaur
Science Advisor
Gold Member
25,185
4,814
That works *if* the entrance pupil of the camera lens is located at (or really close to) the diffracting aperture. Then, by focusing the lens to infinity, the far-field diffraction pattern of the aperture will be projected onto the sensor (because the lens is 1 focal length away from the sensor).
That makes sense. Thanks.
 

Related Threads on How to photograph a diffraction pattern?

  • Last Post
Replies
15
Views
4K
  • Last Post
Replies
2
Views
685
Replies
9
Views
2K
  • Last Post
Replies
7
Views
2K
  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
6
Views
7K
Replies
3
Views
4K
Replies
2
Views
638
Replies
2
Views
2K
Replies
16
Views
4K
Top