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!

Why This Works

The Photoelectric Effect is the phenomenon when a photon with sufficiently high energy can strike an electron and free it, resulting in an electric charge. When Einstein introduced this idea, he was the first to suggest the idea of a Photon, which is a light wave behaving like a particle. Einstein suggested that a light wave of sufficiently high energy could strike an electron on a metal surface and free it, and that for this to happen the light wave would have to act similarly to a particle. He built this idea off of the work of Max Planck, who developed this formula for the energy of an emitted electron:

Kmax = ħ(f - f0)

Kmax: Max Kinetic Energy for emitted electron

f: Frequency of incoming light

f0: Minimum frequency of light needed

ħ: Planck Constant = h/2(pi)

For this equation, the incoming light needs to be at a higher frequency than what is needed to free the electron. If it isn't, then the equation would give a negative value, which means that nothing would happen. From this to happen, Einstein proposed that the light waves would have to act like particles. He called these light particles Photons, and proposed that only the higher energy photons would be able to strike an electron and free it, while low energy photons would be unable to do so. What made this idea revolutionary is that it is unaffected by the Intensity of the light, or how bright the light is, and only affected by the Wavelength, or color of the light.

Our circuit setup has electrons ready to jump across the gap in the bulb, but they don't have enough energy to do so. By using the flashlight, one can show how the color of the light affects it, while the brightness of the light, which stays the same for both colors, does not. The red photons have a longer wavelength, and therefore a lower frequency, which is why they cannot trigger the circuit. The blue photons have a shorter wavelength, and therefore a higher frequency, which is why they can trigger the circuit!

Additional Information

• This demonstration is a part of the Quantum Mechanics Show