Classical physics established that light exhibits wave behaviour through phenomena such as interference and diffraction. These experiments showed that light travels as a transverse electromagnetic wave with a wavelength and frequency .
However, several experiments demonstrated that light also behaves as a stream of discrete energy packets called photons:
| Phenomenon | Evidence for Particle Nature |
|---|---|
| Photoelectric Effect | Light ejects electrons only above a threshold frequency; intensity affects number of electrons, not their energy. |
| Compton Effect | X-ray photons collide with electrons and transfer momentum, just like billiard balls. |
| Pair Production | A single photon can materialise into an electron–positron pair, behaving as a localised particle. |
This dual behaviour — wave and particle — is called wave-particle duality.
In 1924, Louis de Broglie proposed that matter also has a wave nature. Any particle with momentum has an associated wavelength:
where:
This is called the de Broglie wavelength.
For a cricket ball of mass kg moving at m/s:
This is far smaller than any physical aperture, so diffraction is completely undetectable. Wave behaviour is only observable for subatomic particles such as electrons.
In 1927, Davisson and Germer fired a beam of electrons at a nickel crystal and observed a diffraction pattern — the same pattern produced by X-rays of comparable wavelength. This was the first direct experimental proof that electrons have wave properties.
Key points:
The electron microscope exploits the wave nature of electrons to achieve very high resolution imaging.
Resolution is limited by the wavelength of the probe used — smaller wavelength means less diffraction and finer detail can be resolved.
Increasing the accelerating voltage increases the electron speed , which decreases and increases resolution.