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Photoemission is a process during which electrons are ejected from a metal surface when exposed to electromagnetic radiation, (eg ultraviolet).

If a clean, positively charged zinc plate is exposed to intense white light it will not discharge.

If a clean, negatively charged zinc plate is exposed to intense white light it will not discharge.

Neither the intensity of light nor the charge of the plate matter - the plate will not discharge.

If a clean, positively charged zinc plate is exposed to an ultraviolet source it will not discharge.

However, if a clean, negatively charged zinc plate is exposed to an ultraviolet source it will discharge.

This shows that the plate will only discharge if:

• the plate is negatively charged
• the source is of a high enough frequency

The intensity of the source only affects how long it takes the plate to discharge.

The above points lead to the following conclusions:

• the plate is not being discharged by ionisation (the positive one would also discharge)
• the total amount of energy from the source does not determine whether the plate discharges or not

What does this tell us?

This leads to the theory that the beams of light and ultraviolet are made up of individual units called photons.

Before an electron can be removed from a negatively charged plate each photon must have enough energy on its own to remove it.