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Photoelectricity

Science Snack
Photoelectricity
Get electricity from light.
Photoelectricity
Get electricity from light.

Recreate the experiment that proved Einstein’s theory that light waves are made up of quantized energy—what we now call photons.

Tools and Materials
  • Two-liter beaker or large glass jar with a wide mouth
  • Sheet of zinc or galvanized steel slightly larger than the mouth of the beaker
  • Metal paper clip
  • Thin strips of aluminized mylar (sometimes sold as tinsel)
  • Two-foot (50-centimeter) length of PVC tube, 3/4 inch or 1 inch (1.5 or 2.5 cm) in diameter
  • Wool cloth
  • Source of ultraviolet light (the sun or a UV light, also known as a blacklight)
  • Drill (not shown)
  • Tape
  • Optional: black paper
Assembly
  1. Drill a hole in the center of the zinc or steel sheet.
  2. Thread the paper clip through the hole. Bend it so it creates a hook, and tape it in place.
  3. Cut the aluminized mylar into a strip about 4 inches (10 cm) long.
  4. Drape the aluminized mylar over the paper clip so that the two ends hang down.
  5. Place the zinc plate over the mouth of the jar so that the aluminized mylar hangs freely inside the jar. You have just built an electroscope.
To Do and Notice

In a location where there is no ultraviolet light, such as an indoor room with no sunlight, rub the PVC with the wool until it really crackles. This creates a charge on the PVC tube.

Stroke the zinc plate with the charged PVC tube, and notice that the ends of the aluminized mylar spread apart. As the plate discharges over tens of seconds, the aluminized mylar should slowly come back together. Depending on the lighting in your room, covering the back side of the jar with black construction paper (as shown in the photo above) may make things easier to see.

Charge the electroscope again by stroking the plate again with the charged PVC tube. Then turn on an ultraviolet light or allow sunlight to strike the zinc plate. Notice the mylar rapidly comes together as the plate discharges.

What's Going On?

When you rub PVC with wool, it becomes negatively charged. When you touch the charged PVC tube to the zinc plate, some of the negative charge transfers from the PVC to the zinc, making it negatively charged. The negative charge spreads to the two arms of the aluminized mylar. The negative charges repel each other and the mylar pieces spread apart. Over time, the charges leave the mylar, and the pieces eventually come back together.

When ultraviolet light strikes the zinc, the mylar comes together quickly because electrons are knocked out of the zinc when it absorbs ultraviolet photons. Ultraviolet light has higher energy than visible light, whose photons do not have enough energy to knock electrons from zinc. When visible light (such as a room light) strikes the zinc, the mylar stays spread apart. When the plate is exposed to ultraviolet light (either from the sun or a black light), the electrons get knocked out, and the mylar comes back together. The emission of electrons by a piece of metal when light strikes it is called the photoelectric effect.

Going Further

Einstein explained the photoelectric effect by saying electrons in the metal absorb the energy from individual photons. The energy of a photon, E, is proportional to its frequency, f. This relationship is shown in the equation

$$E= hf$$

where h is Planck’s constant.

You can obtain a zinc plate by taking apart a non-alkaline D-cell battery. These are usually labeled “heavy duty.” Inside the outer steel case, there is a pure zinc shell that can be cut with scissors. Caution: Do not take apart an alkaline battery.