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.
- Phtotoelectric apparatus (electroscope and metal plate)
- UV light source
- Rod and fur
- Sand paper
- Optional: Flood light
- Sand one side of the metal plate.
- Place the metal plate into the top of the electroscope.
- Take the filter off the UV lamp.
- Charge the rod by rubbing it with the fur.
- Slide the rod against the plate; this should charge the plate and cause the needle to move
- Wait for the needle to calm down.
- Shine the flashlight on the plate (shiny side); the needle should not move.
- Optional: Shine the flood light onto the plate; the needle should still not move.
- Turn on the UV light and shine it on the metal plate (the shiny side); the needle should move.
- Turn off the UV light and put charge on the electroscope.
- Now, place the piece of glass in between the plate and the UV light source; nothing should happen.
- When finished, turn off the UV light.
- There are two different electroscopes that you can use for this demonstration. One is quite large, but requires more charge to deflect the needle.
- The electroscope should hold its charge during your lecture. However, when using the larger one, it is recommended to have another person charge the electroscope right before you demonstrate it.