Photoelectric Effect and Photons · PhotonLab

The photoelectric effect

Light shining on a metal surface can make electrons escape from it. This is the photoelectric effect, and the electrons that escape are called photoelectrons. Hertz first noticed it in 1887 while experimenting with electromagnetic waves.

Connect a zinc plate to an electroscope, charge it negatively and shine an ultraviolet lamp on it: the leaves slowly close. Then try a positively charged plate, put a sheet of glass between lamp and plate, or switch to a very bright red light.

To see how electrons move in an electric field, visit FieldLab.

The phototube and its I–U curve

Light knocks photoelectrons out of the phototube's cathode K, and the anode A collects them as a photocurrent. With forward voltage the current rises to a saturation current and stops growing; with reverse voltage it falls, reaching zero at a voltage called the stopping voltage Uc.

The findings: every metal has a cut-off frequency, and below it no photoelectrons come out however intense the light; the maximum kinetic energy of photoelectrons rises with frequency and does not depend on intensity; the more intense the light, the larger the saturation current; and photoelectrons are emitted almost instantly. Einstein explained this with photons: Ek = hν − W₀.

Here intensity means the power of the light. At equal power, the higher the frequency, the more energy each photon carries and the fewer photons there are.

Measuring Planck's constant

From eUc = hν − W₀ we get Uc = (h/e)ν − W₀/e: the stopping voltage is linear in frequency, the slope is h/e, and the line crosses the horizontal axis at the cut-off frequency. In 1916 Millikan set out to disprove Einstein's equation and ended up confirming it precisely.

Choose a mercury line and slowly increase the reverse voltage until the galvanometer needle just returns to zero; record that voltage. Record 3 or more lines, then fit a straight line. Change the cathode metal and compare the two lines.

Wave–particle duality of light

The photoelectric effect shows that light behaves as particles; interference and diffraction show that it behaves as waves. Make the light so faint that only one photon at a time passes the double slit: each photon makes a single dot on the screen and where it lands cannot be predicted, yet the dots of many photons build up interference fringes. The probability of a photon landing anywhere is set by the intensity of the light wave.

de Broglie proposed that particles of matter such as electrons are also waves, with wavelength λ = h/p. An electron beam passing through a thin graphite film produces diffraction rings on a fluorescent screen, confirming matter waves.

For the wave optics of the double slit, see OpticsLab.

In-class testing

Look back over the four topics and answer the multiple-choice questions below.