Introduction

11.1 Introduction

The story of how we came to understand that light and matter have a dual nature begins not with a single discovery, but with a crisis. By the late 19th century, Maxwell's equations had unified electricity, magnetism, and light into a single, elegant wave theory. Hertz's experiments in 1887 — generating and detecting electromagnetic waves — seemed to put the final seal on the wave nature of light. Physics appeared nearly complete.

Yet, at the very same moment, experiments on the conduction of electricity through gases at low pressure were quietly opening a door to a new world — one where particles behaved like waves and waves like particles.

The Discharge Tube and the Discovery of Cathode Rays

When an electric field is applied across a gas at sufficiently low pressure — about 0.001 mm of mercury — a discharge takes place between the two electrodes inside a sealed glass tube. A fluorescent glow appears on the glass wall opposite the cathode (the negative electrode). The colour of this glow depends on the type of glass: for soda glass, it is a characteristic yellowish-green.

This fluorescence was attributed to radiation coming from the cathode. These cathode rays were first observed by William Crookes in 1870. By 1879, Crookes proposed that these rays were not radiation at all, but streams of fast-moving, negatively charged particles.

J. J. Thomson's Determination of $e/m$

The British physicist J. J. Thomson (1856–1940) put Crookes' hypothesis to the test. By applying mutually perpendicular electric and magnetic fields across the discharge tube, Thomson became the first to measure both the speed and the specific charge (charge-to-mass ratio, $e/m$) of the cathode ray particles.

This is the presently accepted value. Thomson found that these particles travelled with speeds ranging from about $0.1c$ to $0.2c$, where $c = 3 \times 10^8 \ \text{m/s}$ is the speed of light in vacuum.

The Electron Emerges as a Universal Particle

Around the same time (1887), two other phenomena were observed:

1. Certain metals, when irradiated by ultraviolet light, emitted negatively charged particles with small speeds.
2. Certain metals, when heated to a high temperature, also emitted negatively charged particles.

In both cases, the measured value of $e/m$ for these emitted particles was identical to that of cathode ray particles. This was a profound result: particles produced under completely different conditions — by electric discharge, by light, by heat — were identical in nature.

In 1897, J. J. Thomson named these particles electrons and proposed that they were fundamental, universal constituents of matter. For this epoch-making discovery — both theoretical and experimental — he was awarded the Nobel Prize in Physics in 1906.

Millikan's Oil-Drop Experiment: Quantisation of Charge

While Thomson had measured the ratio $e/m$, the individual values of $e$ and $m$ remained unknown. In 1913, the American physicist R. A. Millikan (1868–1953) performed his famous oil-drop experiment to measure the charge on an electron with precision.

Millikan found that the charge on any oil droplet was always an integral multiple of this elementary charge. This established that electric charge is quantised — it comes in discrete packets, not in a continuous range.