Dual Nature of Radiation and Matter

11.1 INTRODUCTION

The late 19th century witnessed significant progress in understanding light and atomic structure. Maxwell's equations of electromagnetism predicted electromagnetic waves, and Hertz experimentally generated and detected these waves in 1887, firmly establishing the wave nature of light. Concurrently, studies on electric discharge through gases at low pressure in discharge tubes led to landmark discoveries such as X-rays by Roentgen in 1895 and the electron by J. J. Thomson in 1897. When a discharge occurs at very low pressure (~0.001 mm of mercury), a fluorescent glow appears on the glass opposite the cathode, with color depending on the glass type. This glow was attributed to radiation from the cathode, known as cathode rays, discovered by William Crookes in 1870 and later suggested to be streams of fast-moving negatively charged particles. J. J. Thomson experimentally measured the speed and charge-to-mass ratio (e/m) of these particles using perpendicular electric and magnetic fields. The speed was about 0.1 to 0.2 times the speed of light (3 × 10^8 m/s), and the e/m value was 1.76 × 10^11 C/kg, independent of cathode material or gas type, indicating universality. Around the same time, metals irradiated by ultraviolet light or heated emitted negatively charged particles with the same e/m ratio, confirming these particles as electrons, fundamental constituents of matter. Millikan's oil-drop experiment in 1913 precisely measured the electron charge as 1.602 × 10^-19 C, establishing quantization of electric charge. Knowing charge and e/m allowed determination of electron mass. These discoveries laid the foundation for modern atomic physics.

• Maxwell's equations and Hertz's experiments established light's wave nature.
• Electric discharge in low-pressure gases led to cathode ray discovery.
• Cathode rays are streams of negatively charged particles (electrons).
• Charge-to-mass ratio (e/m) of electrons is universal and independent of cathode material.
• Millikan's oil-drop experiment measured the elementary charge, confirming charge quantization.
• Electron mass calculated from charge and e/m ratio.
• 📌 Cathode rays: Streams of fast-moving negatively charged particles emitted from cathode.
• 📌 Electron: Fundamental negatively charged particle discovered by J. J. Thomson.
• 📌 Charge-to-mass ratio (e/m): Ratio of charge to mass of a particle.

11.2 ELECTRON EMISSION

Metals contain free electrons responsible for electrical conductivity. However, these electrons cannot normally escape the metal surface because when an electron attempts to leave, the metal surface becomes positively charged and pulls the electron back due to electrostatic attraction. Thus, electrons are held inside the metal by attractive forces from the positive ions. To escape, an electron must gain sufficient energy to overcome this attractive pull. The minimum energy required for an electron to escape the metal surface is called the work function (φ₀), measured in electron volts (eV). One electron volt is the energy gained by an electron accelerated through a potential difference of 1 volt (1 eV = 1.602 × 10^-19 J). The work function depends on the metal's properties and surface nature. Electron emission can be induced by: (i) Thermionic emission – heating the metal to provide thermal energy to electrons; (ii) Field emission – applying a very strong electric field (~10^8 V/m) to pull electrons out; (iii) Photoelectric emission – illuminating the metal with light of suitable frequency to eject electrons (photoelectrons).

• Free electrons in metals are bound by attractive forces to the metal surface.
• Work function (φ₀) is the minimum energy needed for electron emission.
• Work function is measured in electron volts (eV).
• Electron emission can occur via thermionic, field, or photoelectric emission.
• Photoelectric emission involves light-induced electron ejection.
• 📌 Work function (φ₀): Minimum energy required for an electron to escape a metal surface.
• 📌 Thermionic emission: Electron emission due to heating.
• 📌 Field emission: Electron emission due to strong electric fields.