LED Photocell

An LED connected to a voltmeter generates a measurable voltage when illuminated by certain colors of light. White and green light produce a reading, but red light does not, showing that photon energy depends on frequency rather than brightness - evidence of light behaving as particles.

1. Insert a clear green LED into a piece of card and mount it in a clamp stand.
2. Connect the LED to a high-resistance voltmeter using crocodile clips.
3. Shield the LED from ambient light using a black tube or darkened room until the voltmeter reads zero.
4. Shine a white torch directly at the LED and observe the small voltage produced.
5. Repeat with a red laser pointer and note no measurable voltage.
6. Repeat with a green laser pointer and observe a non-zero voltage again.

📄 LED photocell - Institute of Physics: https://spark.iop.org/led-photocell

• Try LEDs of different colors to see how each responds to different photon energies.
• Use multiple light sources of varying frequency (blue, yellow) for comparison.
• Collect quantitative data of voltage produced vs. wavelength to plot a threshold graph.

• Use only class 2 lasers from reputable suppliers.
• Clamp lasers securely and direct beams away from eyes.
• Conduct the activity in a controlled, darkened environment to reduce stray light.
• Avoid touching LED leads directly while in circuit to prevent inaccurate readings.

• Why does the red laser not produce a reading, even though it is brighter than the torch? (Because red photons have lower frequency and therefore less energy.)
• What determines whether photons can release electrons in the LED? (Their frequency/energy, not their intensity.)
• How does this experiment demonstrate light behaving as a particle? (Each photon delivers a packet of energy, and only sufficiently energetic photons release electrons.)
• How is this related to the photoelectric effect studied by Einstein? (Both show a threshold frequency below which no electrons are emitted, regardless of light intensity.)