The Photoelectric Effect
The experiment that revealed light's particle-like behavior
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Experiment in the late 1800's baffles scientists.
Light is not acting like a wave!
Experiment scientists
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The light’s energy causes
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Effect, Leaf Blower, Volleyball
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Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. What is the photoelectric effect?
2. Electrons emitted from a metal surface due to light are called…
3. The photoelectric effect is evidence that light can behave like…
4. What causes the electrons to leave the metal in the photoelectric effect?
5. What is the fastest speed in the universe?
Automatic Doors and Light Sensors
Some light sensors use the photoelectric effect: when light strikes a metal surface, it can knock electrons out of the metal. These electrons form a current that can trigger a circuit. A sensor with a sodium surface works with violet light but not with red light, no matter how bright the red light is.
Which statement best explains why red light does not trigger the sensor?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. The brightness of the violet light is doubled. What happens?
C2. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response.
C3. Red light has a frequency of 4.3 × 1014 Hz. Substitute numerical values into a mathematical representation that can be used to determine the energy of one red photon. Then calculate the energy. (h = 6.63 × 10−34 J·s)
C4. Place a check mark in two boxes to indicate changes that would allow the sensor to be triggered when the original red light fails.
C5. Which device uses a version of the photoelectric effect to produce electrical energy?
C6. An engineer considers replacing the sodium surface with cesium, which has a smaller work function, so the sensor responds to more colors of light. Identify one benefit and one drawback of this design change.