The gold leaf electroscope is charged negatively and the leaf raises due to electrostatic repulsion between the leaf and the fixed bar.

The metal target is also charged negatively as the charge spreads out across the conducting surface.

When light of a high enough frequency is shone onto the metal the electroscope discharges as electrons are released from the metal surface. The leaf then falls.

For most target metals this only happens if the light used is ultra violet and nothing happens with visible light.

Light was thought to behave like a wave.

The evidence for this can be thought to be interference and diffraction which can not be explained by particle behaviour.

Waves can meet and if the are coherent and out phase they will cancel creating a minima. This is not possible with particles.

Acoording to wave theory the intensity of the light means energy will be continupously incident on a surface if light is incident on it. The amount of energy dependent on the amplitude and intensity of the waves.

This can not explain the photoelectric effect which is highly dependent on the frequency of the light.

Max Planck arrived at an explanation for black body radiation which necessitated accepting that light energy came in packets of size \( E = hf \).

This idea of the discrete packets of energy or quanta allowed Einstein to solve the photoelectric effect.

In 1905 Einstein published a paper that accepted light was quantised and that light was a stream of particles called photons as Planck described.

According to this idea the photoelectric effect is explained as an electron in the metal absorbing a single photon which if the energy is large enough will allow the electron to escape the metal surface.

One photon is completely absorbed by one electron in the metal.

An electron can not absorb many small energy photons and build up enough energy to escape.

If a low energy photon is incident on the electron it is just scattered immediately.

If a single high energy photon is incident on the metal an electron will be emitted immediately with no delay.