Sharp shadow

  • Thread starter alejandra
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  • #1
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I am an artist. I am trying to cast a large sharp (in focus) shadow through a piece of lace onto a wall. It either turns out large and blurry if it is far from the wall, or sharp and small if it is closer to the wall. I have tried the following experiments:

1. Increased the focus and direction of the lamp. This improved the contrast of the shadow, but not the sharpness.

2. Reflected the light with a mirror. This allowed the piece of lace to be further from the wall, but in order to be sharp it still needed to be small, a 1 to1 size ratio.

3. Tried to focus the shadow with an eyeglass lens. This did nothing. I dont have a magnifying glass, but this is my next experiment. Perhaps also a convex lens?

Does anyone have any advice about how to cast a large, in focus shadow with a spotlight? What physics is going on here?

Thanks,
Alejandra
 

Answers and Replies

  • #2
pam
455
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You're getting diffraction, which bends the light at different angles.
You might try blue or violet light, which has a shorter wavelength.
If that doesn't help, you would need thicker thread, maybe with a larger space between the threads.
 
  • #3
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Blue light

Thanks for your response. Will any blue filter do? What about blue LED?
 
  • #4
pam
455
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Try it, but it still might be too long a wavelength, in which case you need coarser (probably cheaper) lace.
 
  • #5
2,006
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You're getting diffraction
Doubtful with no report of coloured fringes; look up "penumbra" then put a card with a very small aperture in front of the lamp beam and try not to put the subject (lace) too close to this aperture (move everything away from the wall instead).
 
  • #6
Andy Resnick
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If you place a lens in between the lace and wall, such that the lens images the lace onto the wall, the outline of the 'shadow' should be sharp.
 
  • #7
Redbelly98
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If alejandra could get access to an overhead projector, I think that would do exactly what is wanted (and exactly what Andy is suggesting). Just place the lace on the projector where you would normally place a transparency.

Otherwise, a simple solution might be a hole in a piece of cardboard held in front of the lamp. Reduce the effective size of the source to get a sharper shadow. Experiment with hole size to find an optimimum; too small a hole will not provide much illumination.
 
Last edited:
  • #8
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new tries

Thanks everyone for your replies.

No colored fringes, just blurry, same shadow color. I tried a blue LED light. It looks better, but still a little blurry. I tried a hole in a card, but the light isn't strong enough to make much of a shadow through the hole, will get a stronger light and try again.

I thought of an overhead projector, or a video feed connected to a video projector, but the extra gear takes the fun out of a simple, effective set up.
 
  • #9
DaveC426913
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What if you used a laser pointer as the light source? Even if you broadened it with a lens so it covered a larger portion of the lace, you'd still have a very bright but very small light source, making for crisp shadows.
 
  • #10
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Thanks!

I tried directing a strong light through a small hole. It works, the shadow is sharp and big. I didn´t realize that even a spotlight sends light out at many angles, enough to blur a shadow.

Thanks again everyone for your responses.

Alejandra
 
  • #11
Redbelly98
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... It works ...
Awesome! Another victory for physics!!! :smile:

Glad we could help out.
 

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