Dual Nature of Radiation and Matter

Lesson

Part 1 — light that knocks electrons out of metals, and four observations the wave theory of light could not explain.

In this lesson you will

Describe the observations of Hertz, Hallwachs and Lenard on the photoelectric effect
Interpret graphs of photocurrent against intensity, collector potential and frequency
Explain why the wave theory of light cannot account for the observations

Concept

Electron emission and the work function

Free electrons in a metal are held in by the attraction of the positive ions. The least energy needed to remove an electron from the surface is the work function $\varphi_0$ , usually given in electronvolts: $1\ \text{eV} = 1.6 \times 10^{-19}$ J.

Energy can be supplied by heating (thermionic emission), a very strong electric field (field emission) or light (photoelectric emission).
Typical values: caesium about 2.1 eV, potassium about 2.3 eV, zinc about 4.3 eV, platinum about 5.6 eV.

Table of approximate work functions of caesium, potassium, sodium, zinc and platinum in electronvolts
Metal	Work function (approx.)
Caesium	2.1 eV
Potassium	2.3 eV
Sodium	2.8 eV
Zinc	4.3 eV
Platinum	5.6 eV

Concept

Discovery: Hertz, Hallwachs and Lenard

Early observations:

Hertz (1887): sparks jumped more easily across a gap when its electrodes were lit by ultraviolet light.
Hallwachs: a negatively charged zinc plate lost its charge in ultraviolet light; a neutral plate became positive; a positive plate was unaffected.
Lenard: ultraviolet on the emitter of an evacuated tube made a current flow; it stopped at once when the light was cut off. The particles emitted were electrons (photoelectrons).
Zinc and cadmium need ultraviolet; alkali metals respond even to visible light.

Cookies