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Particle Properties of Waves 65

Table 2.1Photoelectric Work Functions

Metal Symbol Work Function, eV

Cesium Cs 1.9

Potassium K 2.2

Sodium Na 2.3

Lithium Li 2.5

Calcium Ca 3.2

Copper Cu 4.7

Silver Ag 4.7

Platinum Pt 6.4

All light-sensitive detectors, including the eye and the one used in this video camera, are based

on the absorption of energy from photons of light by electrons in the atoms the light falls on.

to pull an electron from a free atom of that metal (see Fig. 7.10); for instance, the

ionization energy of cesium is 3.9 eV compared with its work function of 1.9 eV. Since

the visible spectrum extends from about 4.3 to about 7.5  1014 Hz, which corre-

sponds to quantum energies of 1.7 to 3.3 eV, it is clear from Table 2.1 that the pho-

toelectric effect is a phenomenon of the visible and ultraviolet regions.

According to Einstein, the photoelectric effect in a given metal should obey the

equation

Photoelectric effect hKEmax (2.8)

where his the photon energy, KEmaxis the maximum photoelectron energy (which is

proportional to the stopping potential), and is the minimum energy needed for an

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