7A10.10-Photoelectric Effect

Video: Watch this demo

Description: This shows the photoelectric effect. .
If you shine the UV light on a charged plate, the charge will leave the plate (indicated by the needle moving).
This is due to the photoelectric effect, whereby certain frequencies of light excite the electrons in certain metals.

Equipment:

• Phtotoelectric apparatus (electroscope and metal plate
• UV light source
• Flashlight
• Rod and fur
• Sand paper

Setup Procedure:

1. All equipment should be in an open box.
2. Sand one side of the metal plate.
3. Place the metal plate into the top of the electroscope.
4. Take the filter off the UV lamp.
5. Test to make sure it works.
6. When the professor is done with it, place everything back inside the same box.

Demonstration Procedure:

1. Put charge on the electroscope plate using the rod and fur.
2. Wait for the needle to calm down.
3. Shine the flashlight on the plate (shiny side). Note that nothing happens.
4. This is because the wavelength of the visible light (and some IR) is not enough to kick electrons from the metal plate.
5. Turn on the UV light and shine it on the metal plate (the shiny side). Note that the needle moves down.
6. This is because charge (excess electrons) are getting excited enough to leave the metal due to the UV lite.
7. Turn off the UV light and put charge on the electroscope.
8. Now, place the piece of glass in between the plate and the UV light source. Note that nothing happens.
9. This is because the glass plate blocks UV lite.
10. When finished, turn off the UV light.

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