Photoelectric emission

Photoelectric emission 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.

Neutral zinc plate joined to golf-leaf electroscope. White light shone at the zinc has no effect. UV light shone at positively-charged zinc has no effect. UV light shone at negative zinc discharges.

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.

Only if the plate is negatively charged and the source has a high enough frequency will it discharge. 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 ionization (the positive one would also discharge).
  • The total amount of energy from the source does not determine whether the plate discharges or not.

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.

The photo electric effect

Question

What evidence is there from the above description that the emitted particles are electrons rather than photons?

Photoemission only occurs from negatively charged surfaces and the charge decreases. This indicating electrons are leaving the surface.