Understanding Dual Nature of Radiation and Matter

The wave nature of light was strongly established by Maxwell’s equations of electromagnetism and Hertz’s experiments on the generation and detection of electromagnetic waves in 1887. The same broad period also saw major discoveries from experiments on electric discharge through low-pressure gases in a discharge tube.

At sufficiently low pressure, about 0.001 mm of mercury column, discharge occurred between the two electrodes when an electric field was applied to the gas in the discharge tube. A fluorescent glow appeared on the glass opposite the cathode, and its colour depended on the type of glass, being yellowish-green for soda glass.

The fluorescence was attributed to radiation originating from the cathode. These rays were called cathode rays.

Cathode rays were discovered in 1870 by William Crookes. Later, in 1879, Crookes suggested that these rays consisted of streams of fast-moving negatively charged particles.

This idea was confirmed by the British physicist J. J. Thomson. By applying mutually perpendicular electric and magnetic fields across the discharge tube, Thomson experimentally determined the speed and the specific charge, that is, the charge-to-mass ratio $$e/m$$, of cathode ray particles.

Key Observations

• Cathode ray particles were found to travel with speeds ranging from about 0.1 to 0.2 times the speed of light.
• The speed of light is given as \[3 \times 10^8 \text{ m/s}\].
• The presently accepted value of \[e/m\] is \[1.76 \times 10^{11} \text{ C/kg}\].
• The value of \[e/m\] was independent of the material used as the cathode.
• The value of \[e/m\] was also independent of the gas introduced in the discharge tube.

Inference

These observations suggested the universality of cathode ray particles.

Around the same time, in 1887, it was found that certain metals emitted low-speed, negatively charged particles when irradiated with ultraviolet light. Certain metals were also found to emit negatively charged particles when heated to a high temperature.

The value of \[e/m\] for these particles was found to be the same as that for cathode ray particles. This established that the particles produced under different conditions were identical in nature.

In 1897, J. J. Thomson named these particles electrons and suggested that they were fundamental and universal constituents of matter.

Important Fact

For his discovery of the electron through theoretical and experimental investigations on the conduction of electricity by gases, J. J. Thomson was awarded the Nobel Prize in Physics in 1906.

In 1913, the American physicist R. A. Millikan performed the oil-drop experiment for the precise measurement of the charge on an electron.

He found that the charge on an oil droplet was always an integral multiple of an elementary charge:

Inference

Millikan’s experiment established that electric charge is quantised.

From the values of charge e and specific charge e/m, the mass m of the electron could be determined.

• Maxwell’s equations and Hertz’s experiments established the wave nature of light.
• Low-pressure discharge tube experiments led to important discoveries in atomic structure.
• Cathode rays were identified as fast-moving negatively charged particles.
• J. J. Thomson measured the specific charge of these particles and named them electrons.
• Millikan measured the elementary charge and established the quantisation of electric charge.