Practical Activity for 14-16
A little something to enliven teaching the photoelectric effect.
Apparatus and Materials
- Butthead game
- Disc shooter or disc rifle
- Foam discs of different colours (red, yellow, green, blue, violet)
- Safety glasses
- Buttons, 2
- Fishing line
- Airzooka (optional)
Health & Safety and Technical Notes
Wearing safety glasses will stop discs hitting people in the eye – uncomfortable rather than dangerous – but it also makes them look even dafter. Be careful of firing the Airzooka at or near ears. Be careful that the fishing line doesn't cut through fingers. Don't leave the cells in the gun (or rifle) for a year in between uses!
Do not be tempted to use a CO2 fire extinguisher which should NEVER be aimed at anyone's head.
The Butthead game consists of a hat with Velcro pieces on it and three fluffy balls. The balls stick to the Velcro. To make one of the balls definitely come off the helmet at the appropriate time you can thread fishing line through the ball in a loop. Put a button on the fluffy ball and have the fishing line go through it to stop the line cutting through the ball. Put a button on the other end of the line to stop it cutting through fingers.
Coloured discs can be hard to get hold of and they may not be standard sizes. You may have to cut your own from other coloured foam or colour lightly coloured ones different colours with marker pens.
You can take this further with a smoke machine and an Airzooka (see below).
- Select a
volunteer. Get them to put the Butthead hat on.
- Stick the fluffy balls to the hat explaining that these are the electrons bound in the atom.
- Produce a camera and take pictures of the victim for future use. Posting these pictures on the intranet can hasten production of coursework.
- Fire red photon (discs from the gun) at the electrons. They won
t dislodge them as the dont have a high enough frequency - E=hf.
- Try yellow and then green photons - still no good - you can't remove an electron.
- Finally try blue and violet (if you have it). These still don't work so we need UV photons.
- At this stage you can pretend that you can magically make your gun fire UV photons and get the victim to pull on the fishing line at the right moment and hence remove one of the electrons or you can use an Airzooka.
- Careful placement of the fluffy ball will enable the Airzooka to knock it off. You can then use smoke in a can to demonstrate that the Airzooka is firing UV photons because when you fill it with smoke and fire it you get a smoke ring which is the same shape as the photons/discs from the gun/rifle.
- This explains the photoelectric effect from a particle perspective. It doesn't explain this as evidence for light being particles rather than waves.
- Emphasize that it doesn't matter how many red discs you fire they just don't have a high enough frequency to dislodge the electrons. Only when you're past the threshold frequency will the photons dislodge the electrons.
- When the balls fall off they are moving and you can bring in the idea of kinetic energy and use E=hf to account for what is going on.
This experiment was submitted by Gary Williams, editor of Physics Education.