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

The LED is mounted on a ring stand and placed on the overhead projector so that the LED is illuminated by the projector. The LED is connected to the current to voltage converter (10 microamps to 1 volt) which is connected to the overhead projection meter. The position of the LED should be adjusted to give a full scale deflection of the meter. Holding a red filter over the LED reduces the meter reading to zero. Holding a single green filter over the LED reduces the meter reading to approximately 36%. Two green filters reduce the meter reading to approximately 12% (= 0.36 squared).

The LED may be connected to a DC power supply and the turn on voltage measured.

Cautions:

Concepts demonstrated:

Setup Time: Short

Difficulty/Commitment: Straightforward

Visibility: on overhead projector

Related demonstrations:

References: PIRA 7A10.??