Concepts

Photoelectric effect

Overview

A green LED with a clear casing is used as a photodetector. Red and green filters are used to demonstrate the wavelength dependence of the photodetection.

Details

Equipment

• [1] Green LED with a clear casing (used as a photodetector)
• [1] Lab stand
• [1] Current-to-voltage converter (10 microamps to 1 volt)
• [1] Overhead projection meter
• [1] Red filter
• [2] Green filters
• (Optional) [1] DC power supply

Classroom Assembly

1. Mount the green LED on the lab stand and place it on the overhead projector so that the LED is illuminated by the projector.
2. Connect the LED to the current-to-voltage converter and the current-to-voltage converter to the overhead projection meter.

Script

1. Position the LED to give a full scale deflection of the meter.
2. Hold a red filter over the LED. The meter will read zero.
3. Hold a green filter over the LED. The meter will be reduced to approximately 36% of its maximum reading.
4. Hold a second green filter over the LED. The meter will be reduced to approximately 12% (= 0.36²).

Optional:

1. Connect the LED to a DC power supply.
2. Increase the voltage slowly until the LED turns on to measure the turn-on voltage of the LED.

Additional Resources