Photoelectric Effect

Physics:	Photoelectric Effect
Grade Range:	Elementary School, Middle School, High School
Format:	Stage

This demonstration is a part of the Quantum Mechanics Show This demonstration is not typically performed by itself, since it does require background knowledge in the topic to be understood. In the Quantum Mechanics Show, this demonstration is scripted, so although this write-up does not provide a script, one should be aware of the differences that happen between scripted and non-scripted demonstrations.

Materials

• Variable Transformer
• Photoelectric Effect Circuit
• Small Flashlight
• Red and Blue Color Filters
• Small Balls (Red, Green, Blue)
• Power Strip

Non-Scripted Show Optional

• Einstein Wig
• Labcoat

Safety Precautions

Please read the General Safety Precautions section of the Demonstration Safety page before performing this demonstration. This demonstration uses a variable transformer, and is capable of delivering a dangerous shock. Do not let the alligator clips touch while it is on! If they touch, they can cause permanent damage to the transformer or to the outlet they are plugged into.

This demonstration requires: Safety Glasses

Demonstration

Preparation

• Set up the circuit: prop the circuit board against either a box or the transformer, with the wire ends facing up. Connect the alligator clips to the wires, being sure that the metal parts are not touching. Once connected, make sure the knob for the transformer is turned all the way down before plugging it into the power strip. Turn on the power strip, then turn on the transformer using the switch on the back.
• Calibrate the transformer: The variable transformer usually needs to "warm up" before the show, so first slowly turn the knob until you reach a voltage that triggers the circuit. Once that happens, turn the knob backwards about 10%, and leave the transformer on. wait 5 minutes to see if it starts triggering again, and if not then re-find the point where it triggers. Once you do, turn it back about 2-3% and leave it on. It should stay at that setting for the performance.

Presentation

1. Introduce the demonstration by talking briefly about how light can act like a particle as well as a wave. If dressed like Einstein, talk about how you discovered this property by using an experiment similar to what is now set up.
2. Use the balls to illustrate what happens with the photoelectric effect: The red and blue balls are different wavelengths of light, and the green ball is an electron. Hold the red and blue balls in one hand, and the green in the other. Hit the electron with the red and explain that it doesn't move, because the red has low energy. Hit the electron with the blue ball and throw it, explaining that the blue has high energy.
3. Show the circuit, and explain the parts of the circuit to the audience. There is the switch, a small red bulb with two metal plates in it. There is a resistor, to get the right voltage, and there is the small speaker, which will indicate if the circuit is triggered. Point out that the circuit is getting some power, but it isn't enough to trigger it.
4. Hold the flashlight and the red filter, and shine the red light on the switch. Nothing happens!
5. Hold the flashlight and the blue filter, and shine the blue light on the switch. It triggers! Can anyone identify the sound? (Homer Simpson saying "Mmm, Burger")
6. Ask the audience to answer the following: why did the blue light trigger the switch, but not the red? Remind them that the intensity of the light was the same, since you used the same light source. The only thing you changed was the color of the light!

Additional Information

• This demonstration is a part of the Quantum Mechanics Show