The Photoelectric Effect

Key Points

What is it ?

• Electrons are emitted from zinc when ultraviolet radiation shines on it.

• Other metals emit electrons with other kinds of radiation.

Why was it a problem ?

• If the frequency of the radiation was reduced below the Threshold Frequency (fo), no electrons were emitted.

• No-one could explain this using wave theory.

What did Einstein say ?

• We must think of light as a particle and not as a wave.

• Each particle carries energy given by E=hf where h is Planck’s Constant.

• There is a certain minimum energy needed to release an electron – called the Work Function given by W=hfo.

Equations

• To release an electron, hf > hfo.

• If hf < hfo, no electrons will be released.

• The kinetic energy of the released electrons is given by Ek = hf – hfo.