Diffraction Patterns: Sunlight vs. Laser Beam

[bhav007]

Hi,

I was wondering what the diffraction pattern would be like if the suns light is diffracted through a pin hole or double slit diffraction grating compared to that of a laser beam?

thanks.

 

[Nugatory]

I was wondering what the diffraction pattern would be like if the suns light is diffracted through a pin hole or double slit diffraction grating compared to that of a laser beam?

Next sunny day... try it. All you need is two sheets of heavy black paper and a pin to poke holes in one them, use the other one as a screen. Let the sunlight shine through the pinhole in one sheet onto the second, look at the pattern on the second (DO NOT, of course, ever look directly at the sun!).

 

[mfb]

The coherence length is short. With most diffraction gratings and direct sunlight, I would not expect a visible diffraction pattern. If you collimate the sunlight and filter the frequency range first, it might get better.

 

[Nugatory]

The coherence length is short. With most diffraction gratings and direct sunlight, I would not expect a visible diffraction pattern. If you collimate the sunlight and filter the frequency range first, it might get better.

Yep... I considered mentioning that OP would probably be disappointed he followed my advice to "try it", then decided that the "why the disappointing result?" question might actually be interesting in its own right.

 

[technician]

It is fairly straight forward to produce a diffraction pattern! Let the light pass through a pinhole onto a screen of grease proof paper. You can safely look at the pattern on the other side of the grase proof paper.
place the head of a pin in the light beam and you will see the diffraction pattern of an obstacle...you might even see a bright (relatively speaking) spot at the centre of the geometrical 'shadow'.

extra: With a fine beam of sunlight you will get a diffraction pattern with any diffraction grating. It will produce a spectrum ! you will get a diffraction pattern with a torch beam ! It is what diffraction gratings do.
It is so easy to do...just try it. I don't know what coherence length has to do with this simple practical.

 

[mfb]

If you just use a pinhole and the full sunlight, you get a white spot on the wall. This is not diffraction.
If you just use two narrow slits (with usual sizes) and the full sunlight, you get a white spot on the wall - might be more like a bar, depending on the geometry, but not a diffraction pattern.
If your slits have a size of a few micrometers and the spacing between is of the order of few micrometers as well, you might see a diffraction pattern. Most double slits are larger, however.

 

[bhav007]

Thanks a lot! Would that then mean the diffraction pattern of any chromatic light be the same as the suns effect as chromatic light is not deemed to be coherant? Obviously Lasers show a nice diffeaction pattern as they are coherant.

 

[Andy Resnick]

I was wondering what the diffraction pattern would be like if the suns light is diffracted through a pin hole or double slit diffraction grating compared to that of a laser beam?

This is an interesting question with a surprisingly complex answer. Consider these images- first, sunlight through leaves during a solar eclipse:

https://www.google.com/search?q="so...KW0QGN2YGIDQ&ved=0CAcQ_AUoAQ&biw=1372&bih=791

And these images made with a 'camera obscura':

http://danhume.wordpress.com/2010/10/12/camera-obscura/

Aside from chromatic effects, laser light (or any source with a high spatial coherence) would behave very differently- illuminating a camera obscura with a plane wave would result in a Airy disc at the 'image plane' instead of an image of the source. Similarly, illuminating a tree with laser light would not produce multiple images of the beam.

The reason has to do with the spatial extent of the source. The angular diameter of the sun is about 0.5 degrees, while a source producing a pure plane wave subtends 0 degrees (think of distant stars). Typically, far-field diffraction patterns are calculated using a plane wave incident on the aperture, and this is not the case with sunlight.

The quantitative measure of illumination relevant here is the 'coherence area', which is infinite for a plane wave and in general A = λ^2/Ω, where Ω is the angular size of the source. For the sun, A = 0.003 mm^2, while for a distant star, A = 6 m^2. The coherence area is a measure of how far apart two slits can be and still produce an interference pattern.

For the camera obscura and illumination through leaves, the 'pinhole size' is larger than the coherence area, and so there are no interference fringes- geometrical optics holds. If you make the pinhole size smaller than the coherence area, you will obtain the usual interference patterns, and you can observe speckle with sunlight as well- the 'speckles' are about 0.06 mm in diameter:

http://www.itp.uni-hannover.de/~zawischa/ITP/diffraction.html

The relevant theory (Coherence theory, http://en.wikipedia.org/wiki/Coherence_theory) is fairly broad, but a good introduction is here:

http://maxwell.uncc.edu/gjgbur/papers and CV/030gbur.pdf
https://www.amazon.com/dp/0521822114/?tag=pfamazon01-20

And the standard reference text is here:

https://www.amazon.com/dp/0521417112/?tag=pfamazon01-20

 

[bhav007]

Thank You very much Andy that was very helpful

 

[technician]

If you just use a pinhole and the full sunlight, you get a white spot on the wall. This is not diffraction.
If you just use two narrow slits (with usual sizes) and the full sunlight, you get a white spot on the wall - might be more like a bar, depending on the geometry, but not a diffraction pattern.
If your slits have a size of a few micrometers and the spacing between is of the order of few micrometers as well, you might see a diffraction pattern. Most double slits are larger, however.

It is quite easy to get diffraction and interference with 2 slits. It is a standard A level experiment and most textbooks give instructions for making suitable slits.
They need to be narrow and close together (less than 1mm).
A blackened microscope slide with fine straight scratches works well. The screen needs to be about 1m away from the slits so that the diffraction patterns overlap and produce interference.
If only 1 slit is used then a diffraction pattern is seen.
These experiments work well and it is possible to get a good estimate of the wavelength of light from measurement of the fringe spacing.

 

[mfb]

If you use a laser, I fully agree with your post.

 

[technician]

If you use a laser, I fully agree with your post.

The arrangement described certainly works with a laser as well as with a car headlamp light source and the sun.
Have you not come across this classic physics demonstration?
It is well documented in physics textbooks.
I did it when I was at school...just before lasers were invented!

 

[bhav007]

I am familiar with this experiment, I am basically trying to create this experiment using a DSLR camera and a diffraction grating placed inside a housing which is then attached to the DSLR. The device is working with a laser as I am getting a diffraction pattern on the camera but it is not working with chromatic light.

 

[technician]

I am familiar with this experiment, I am basically trying to create this experiment using a DSLR camera and a diffraction grating placed inside a housing which is then attached to the DSLR. The device is working with a laser as I am getting a diffraction pattern on the camera but it is not working with chromatic light.

Have you tried projecting the pattern onto a screen...something like grease proof paper?
If you are using a white light source you may need to use a converging lens to produce an image of the source on the screen.
The image is a diffracted image of the light source.

 

[bhav007]

Thanks I will try that, I think I need to take into account the focal length of the cameras lens in order for me to get better results

 

[technician]

Thanks I will try that, I think I need to take into account the focal length of the cameras lens in order for me to get better results

I will photograph the practical arrangement I use and post it tomorrow.
I have not tried to photograph the diffraction pattern directly, I produce the pattern on a screen for demonstration. It seems sensible that a photograph of the pattern on the screen would be straight forward.

 

[bhav007]

I will photograph the practical arrangement I use and post it tomorrow.
I have not tried to photograph the diffraction pattern directly, I produce the pattern on a screen for demonstration. It seems sensible that a photograph of the pattern on the screen would be straight forward.

Thank you very much for your help

 

[Johnahh]

If you use a white light bulb you should get a diffraction pattern where the central minima is white as the path difference is the same for each wave. The other maxima will be of different colours due to different path lengths and white light being made up of different wavelengths of light. I would imagine this would be the same for the sun?

 

[sophiecentaur]

The arrangement described certainly works with a laser as well as with a car headlamp light source and the sun.
Have you not come across this classic physics demonstration?
It is well documented in physics textbooks.
I did it when I was at school...just before lasers were invented!

Yes. There was optical life before lasers were available. You just needed to be a better experimenter!

 

[technician]

If you use a white light bulb you should get a diffraction pattern where the central minima is white as the path difference is the same for each wave. The other maxima will be of different colours due to different path lengths and white light being made up of different wavelengths of light. I would imagine this would be the same for the sun?

Absolutely correct, the 'other' maxima are essentially white but the inner edge shows blue and the outer edge shows red.
Cheers

 

[technician]

Yes. There was optical life before lasers were available. You just needed to be a better experimenter!

bring those days back ! no calculators, only slide rules and log tables