Physics - Flourescent Tube

Why is there low pressure mercury?

Mercury must not absorb all the electrons, There must be sufficient distance for electrons to reach a high enough enery to cause ionisation and excitation

1

Circuit switched on, electrons at cathode accelerate towards anode.

2

Electrons pass through mercury vapour and excite and ionise the mercury atoms

3

When the mercury atoms de-excite, they release UV photons in multiple directions

4

Atoms in the phosphor coating absorb the UV photons, and are excited. When the electrons fall back to their original energy levels in stages they release visible light photons.

Max KE photoelectrons

Photoelectrons with max KE will be the ones at the surface of the metal, as they do not require as much energy to leave the metal.

Low KE photoelectrons

Photoelectrons with lower KE are those deeper within the metal, as some of the energy absorbed from the photon is used to approach the metal surface and overcome the work function, meaning less KE is available once they have left the metal.