The photoelectric work function

The work function W is a measure of the minimum amount of work required to draw an electron out of the metal. It varies from metal to metal, and determines the minimum photon energy required for the photoelectric effect to take place at that metallic surface.

The most energetic photons of visible light are at the violet end of the spectrum. It can be seen from the table that such photons have enough energy to liberate electrons from caesium, potassium and sodium, but not from aluminium, copper, mercury or lead.

Table 11.1 Photoelectric work function of some elements.

Element Al Cs

Cu Hg

K

Na

Pb

W (eV) 4.28 2.14

4.65 4.49

2.30

2.75

4.25

1 eV = 1.60 x 10-19 coulombs x 1 volt = 1.60 x 10-19 J

Wavelength (nm)

400

550

700

Photon energy, hf (eV)

3.11

2.26

1.61

An example with numbers

When light of wavelength 480 nm strikes a photosensitive surface, the emitted electrons have a stopping potential of 0.45 V.

What is the photoelectric work function of the metal? (1 eV = 1.60 x 10-19 J)

l = 480 nm fi f = - = 3 X 10" Q = 6.25 x 1014 s-1 l 480 x 10-9

Looks like caesium!

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