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Help with diffraction flashlight lab activity...

  1. Feb 19, 2017 #1
    I'm updating some poorly written lab activities for an online physics class that I have been given, and I am going through the experiments and doing them myself to better write them. I came across this one, and I have no idea what the results are supposed to be. It comes after the chapter on diffraction, which covers interference patterns, why interference patterns indicate that light is a wave, diffraction, diffraction patterns, and Huygen's principle.
    Here (everything in blue) is the text that the students are given:

    Observing Light Waves
    You’ll answer the following research question:
    • Which light behaviors does the wave model explain?

    1. Using a thin needle, poke a tiny hole in the center of the paper. Stand at about 1 m from your
    screen. Shine the flash light through the whole onto the screen. What do you see? Move the paper
    screen slowly toward the flash light and back toward the screen. Observe what happens. What light
    phenomenon do you observe here? Explain.

    2. Using the same needle, poke two more holes in the paper so that all the three holes make a
    triangle with a side of about 0.3 cm. Shine the flash light through the holes onto the screen. What
    do you see? Move the paper screen slowly toward the flash light and back toward the screen.
    Observe what happens. What light phenomenon do you observe here? Explain.

    3. Organize all of your data in a chart. Record and explain all of your observations in as much detail
    as you can.

    4. Evaluate your data by creating a data analysis focused on light waves. To do this, be sure to
    answer the following questions:
    • Why is the wave model of light most appropriate to explain your observations?
    • What wave behaviors did you observe?

    I understand that lasers make diffraction patterns through a pinhole, but flashlights don't seem to. It doesn't seem to be a pinhole camera since they aren't looking at a discernable image, just the light of a flashlight. And I have no idea what they are doing with the three holes in a triangle shape - I can't find any lab activities online that are similar.

    When I did it myself I found nothing interesting with one pinhole - just a large circle of light - and with three I could see that it was brighter where the circles overlapped but that's it.

    Am I missing something? Should there be more notable results? Or could the point just be that the students should point out that diffraction causes the image to be bigger than the pinhole? Can anyone shed some light on the purpose of the three pinholes arranged in a triangle? Does anyone have any suggestions for updating this activity? (Note that this is for an online class where we don't provide students with materials; we are limited to household materials. I wouldn't even want to require a laser pointer because if they don't have one, we're kind of stuck; though it could be used as an optional addition.)

    Any advice on this is appreciated!

  2. jcsd
  3. Feb 20, 2017 #2


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    2017 Award

    Staff: Mentor

    Lasers are better, but flashlights can work as well. You'll get a multi-colored pattern. A single spot doesn't lead to a very clear pattern, and with white light it will be even harder to see anything.
    It is similar to the double-slit experiment: You can get interference between the different patterns. Even more challenging to see anything with the flashlight.
  4. Mar 3, 2017 #3

    Mark Harder

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    Gold Member

    What is the "screen" mentioned in the book? At first, it sounds like you are looking directly at the pinhole from behind the paper with the pinhole punched in it. Later, it sounds more like you are looking at patterns on a screen on which the diffracted light is projected. It sounds to me like the author of the experiment never performed the experiment himself. Maybe you will have to design the exercise yourself. If you use a colored LED as a light source, the diffraction patterns should be sharper, since LEDs emit over a narrower range of wavelengths than the incandescent bulb in the flashlight. You can find LEDs that are much brighter than the usual flashlight bulb. Do not use a 'white' LED. The white light is actually emitted over a broad spectrum from a phosphor excited by a short-wavelength LED, so it's not that different than an incandescent bulb. LEDs of different colors require different minimum turn-on voltages. Connecting an LED to a voltage source directly will probably burn it out, so wire a resistor in series with the LED in order to limit the current. Obtain the specs for the LED from the merchant or mfr. Choose a resistance that will limit the current to less than the maximum specified, and power with a voltage large enough that the voltage remaining after subtracting the drop across the resistor exceeds the turn-on voltage of the LED. Rather than looking directly into the diffracted light, project it on a screen. A black screen may work better than white. Good luck.
  5. Mar 9, 2017 #4
    They seem to refer to both the black paper with a pinhole as a screen, and the white surface that the light is being projected onto as a screen (as I said, they are poorly written). Thank you for the tips about colored LEDs, wiring the LEDs and using a black screen. I will give that a try!
  6. Mar 10, 2017 #5

    Mark Harder

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    Gold Member

    Good. Let us know how the LED setup works if you use it. I may try it myself.
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