Dual nature of radiation; electron emission and work function
Light behaves as a wave in interference and diffraction, and as a stream of particles (photons) in the photoelectric effect. This is its dual nature.
Wave or particle?
By the end of the nineteenth century, interference and diffraction had shown that light is a wave. Maxwell's theory showed that it is an electromagnetic wave. Then came experiments in which light gives energy to electrons in separate packets. The photoelectric effect can be explained only if light gives its energy in packets, as if made of particles called photons. So light has a dual nature. It behaves as a wave when it travels and spreads (interference, diffraction). It behaves as a particle when it exchanges energy with matter (photoelectric effect). Neither picture alone explains everything.
Electron emission and work function
A metal has free electrons that move about inside it. They do not normally leave the surface, because the positive ions pull them back. An electron can escape only if it gets at least a certain minimum energy. This minimum energy is the work function, φ₀. It depends on the metal and on the nature of its surface. Work function is usually given in electron volts (eV). One electron volt is the energy an electron gains when accelerated through 1 volt: 1 eV = 1.6 × 10⁻¹⁹ J.
Three ways to free electrons from a metal
| Method | How the energy is given | Example |
|---|---|---|
| Thermionic emission | Heating the metal gives the electrons enough thermal energy | The hot filament of an X-ray tube |
| Field emission | A very strong electric field (about 10⁸ V/m) pulls electrons out | A spark plug |
| Photoelectric emission | Light of high enough frequency falls on the metal | A photocell in a burglar alarm |
Worked example: electron volts and joules
The work function of a metal is 2.5 eV. Express it in joules. A photon has energy 4.8 × 10⁻¹⁹ J. What is its energy in eV, and can it free an electron from this metal?
- Use 1 eV = 1.6 × 10⁻¹⁹ J.
- φ₀ = 2.5 × 1.6 × 10⁻¹⁹ J = 4.0 × 10⁻¹⁹ J.
- Photon energy = (4.8 × 10⁻¹⁹ J) ÷ (1.6 × 10⁻¹⁹ J/eV) = 3.0 eV.
- 3.0 eV is more than 2.5 eV, so this photon can free an electron.
φ₀ = 4.0 × 10⁻¹⁹ J; photon energy = 3.0 eV, enough to free an electron
Wave nature vs particle nature of light
Which picture suits best depends on the experiment. In seeing, both matter: the eye lens focuses light as a wave, and the retina absorbs it as photons.
Think of the solar panels on Indian rooftops and the light sensor that dims your phone screen. Both work because light hands its energy to electrons in packets: the particle side of light.
1 marks: “Define the work function of a metal.” The minimum energy an electron needs to escape from the surface of the metal.
2 marks: “Name three ways in which electrons can be emitted from a metal surface, giving the source of energy in each.” Thermionic emission: heat. Field emission: a very strong electric field. Photoelectric emission: light of suitable frequency.
2 marks: “What is meant by the dual nature of radiation? Name one phenomenon that shows each nature.” Light behaves both as a wave and as particles. Wave: interference or diffraction. Particle: photoelectric effect.
Travels as a wave, pays as a particle. Light spreads like a wave but hands over its energy in hν packets.