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Pinhole Diffraction results - Young's Experiment

  1. Apr 28, 2009 #1
    Just another twist on the pinhole diffraction lab. This version of the experiment works great for kids to work in groups to make their own target if they are given a cheap red laser pointer and some very inexpensive easy to work with and SAFE materials. The objective is the determine the wavelength of light.

    Young originally did this experiment in the early 1800's with a thin sheet of card paper splitting the light beam. He discovered that light gives off wave interference patterns, when the common thinking of the time was that light rays were made of particles that traveled only in straight lines ala Newton... This is a much easier "pinhole" version of the same experiment.


    -bar of soap or putty or clay
    -paper clip
    -straight pin (of known diameter, mine I used were .60 mm, these are quite standard)
    -3 X 5 inch piece of aluminum foil (size of a notecard)
    -standard laser pointer or laser level, with standard 5mW red laser (known wavelength is 650nm)
    -ruler with cm marks
    -measuring tape - with cm marks, or you can convert from inches (1 inch = 2.54 cm)

    Taking the paperclip, embed it into the soap or clay so that it sticks up vertically. This is the mount you will use to place your little foil sheet.

    Flatten the foil out a little bit by rubbing your fingers on it while it is on a table.

    Poke a hole in the foil sheet with the straight pin, try to keep the foil as straight as possible.

    Slide the foil into the paper clip mount being careful not to bend or crease the foil. Foil target must be perpendicular to the laser ray. Place the target about 1 foot away from the laser pointer -- this particular distance doesn't really matter.

    Turn the laser on and move your foil to center the pinhole on the laser point by manipulating it while it is on the paper clip, slide it up / down or left / right until you hit your target pinhole. Try not to move the laser pointer or the soap - paperclip assembly.. try only to move the foil sheet.

    Foil could be substituted with standard 3 X 5 notecards, but the paper residue from the notecard surrounding the hole that is poked causes bad interference with the actual pattern you have to measure later on. Foil delivers a really clean hole.

    Target the laser through the pinhole at a dark wall I'd say at least 20 or 30 feet away and you'll get the patterns that I got in the pics. The really brilliant pattern I got was an over-exposed pic, so you can see all the multitude of nodes, I guess is what they are called technically. But with just your eyes you can easily see 4 nodes from the center bright spot.

    Using Young's equation:

    wavelength = y * d / m * L

    y = distance from the central bright spot to the 4th bright wave node

    d = pinhole width, which is equal to the known diameter of the pin

    m = the node number, in this case, we're measuring to the 4th node so this is "4"

    L = length from the target pinhole to the wall

    I got pretty close to actual value:

    y = 42 mm
    d = .60 mm
    m = 4
    L = 9194 mm (a little over 30 feet)

    experimental wavelength = 680 nanometers

    actual = 650 nm

    Please see pics - hope this lab helps

    Mr. "O"

    Attached Files:

    Last edited by a moderator: Apr 28, 2009
  2. jcsd
  3. Apr 28, 2009 #2


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    Question: how do you know the actual wavelength is 650 nm? Laser pointer specs can easily be ±5 or ±10 nm.
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