Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sharp shadow

  1. Apr 27, 2008 #1
    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
     
  2. jcsd
  3. Apr 27, 2008 #2

    pam

    User Avatar

    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.
     
  4. Apr 27, 2008 #3
    Blue light

    Thanks for your response. Will any blue filter do? What about blue LED?
     
  5. Apr 27, 2008 #4

    pam

    User Avatar

    Try it, but it still might be too long a wavelength, in which case you need coarser (probably cheaper) lace.
     
  6. Apr 27, 2008 #5
    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).
     
  7. Apr 28, 2008 #6

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

    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.
     
  8. Apr 28, 2008 #7

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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: Apr 28, 2008
  9. Apr 28, 2008 #8
    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.
     
  10. Apr 28, 2008 #9

    DaveC426913

    User Avatar
    Gold Member

    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.
     
  11. May 1, 2008 #10
    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
     
  12. May 1, 2008 #11

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Awesome! Another victory for physics!!! :smile:

    Glad we could help out.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?