Dual Nature of Radiation and Matter

Lesson

Part 1 — light that knocks electrons out of metals, and the idea of the photon.

In this lesson you will

Describe Hertz’s and Lenard’s observations and how photocurrent depends on intensity, frequency and collector potential
Explain why the wave theory of light fails to explain the photoelectric effect
Use Einstein’s equation K_max = hf − W₀ to find maximum kinetic energy, stopping potential, threshold frequency and threshold wavelength

Diagram

Hertz’s observation (1887)

While producing radio waves with a spark gap, Hertz noticed that sparks jumped across the small receiver gap more readily when ultraviolet light from the transmitter spark fell on it.

A glass sheet in between (glass absorbs ultraviolet) made the receiver sparks shorter; a quartz sheet (transparent to ultraviolet) did not.
Hallwachs then found that a negatively charged zinc plate in ultraviolet light loses its charge: light frees negative charges from metals.
These charges were later shown to be electrons: photoelectrons.

Diagram

Lenard’s apparatus

An evacuated bulb holds an emitter C and a collector A. Monochromatic light enters through a quartz window (it passes ultraviolet) and falls on C. A battery across a rheostat with a reversing key sets the potential V of A relative to C; a microammeter reads the photocurrent.

The vacuum lets electrons reach A without colliding with gas molecules.
A positive: A attracts the electrons. A negative: A repels them (a retarding potential).

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