Is light a wave or a particle?

Is light a wave or a particle? The short history

Scientists argued about this for centuries. Isaac Newton favoured tiny particles of light. Christiaan Huygens argued for waves. In the early 1800s, Thomas Young's double-slit experiment seemed to settle it in favour of waves. Then, in 1905, Albert Einstein explained the photoelectric effect by treating light as particles. Modern physics accepts both descriptions: which one you see depends on the experiment.

Evidence that light is a wave

The double-slit experiment

Shine light through two narrow slits close together, and the screen beyond shows a pattern of bright and dark stripes, not just two bright lines. Where waves from the two slits arrive in step, crest on crest, they add up to bright light. Where they arrive out of step, crest on trough, they cancel, giving darkness. This interference is something only waves can do. Try it in our Wave Interference Simulator.

• Diffraction: light spreads out after passing through a narrow gap, just as water waves spread through a harbour entrance.
• Polarisation: filters can block light vibrating in one direction, which only works for transverse waves. See our guide to transverse and longitudinal waves.
• Refraction and colour also fit a wave picture, with each colour having its own wavelength.

Evidence that light is a particle

The photoelectric effect

Shine light on a clean metal surface and it can knock electrons out. OpenStax University Physics lists three observations a wave picture can't explain:

• A cut-off frequency: below a certain frequency, no electrons are emitted, however bright the light or however long it shines.
• Energy depends on frequency, not brightness: brighter light releases more electrons, but not faster ones.
• No delay: electrons come out almost instantly, even in dim light.

If light were a smooth wave, dim light should eventually build up enough energy to free an electron. Einstein's 1905 explanation was that light arrives in packets, photons, each with an energy set by its frequency. One photon gives all its energy to one electron. If a single photon hasn't enough energy, nothing happens, no matter how many arrive. His Nobel Prize in Physics for 1921 was awarded especially for his discovery of the law of the photoelectric effect, according to the Nobel Prize site.

Photon energy: E = hf

E is a photon's energy in joules, f its frequency, λ its wavelength, and h is the Planck constant, defined exactly as 6.62607015 × 10⁻³⁴ J s (NIST). Higher frequency, shorter wavelength light carries more energy per photon.

Worked example: a metal needs 3.0 eV to release an electron (its work function). Red and green photons, at 1.77 eV and 2.34 eV, don't have enough energy, so no electrons come out, however bright the light. Each ultraviolet photon has 4.96 eV, so electrons escape with up to 4.96 − 3.0 = 1.96 eV of kinetic energy. That is the photoelectric effect in numbers. Try one: this question asks you to calculate a photon's energy.

So what is light really?

Light is a quantum object: it isn't a classical wave like a ripple on a pond or a classical particle like a tiny ball. The strangest evidence comes from sending light, or even electrons, through a double slit one at a time. Each one lands at a single spot on the screen, like a particle. But as the spots build up, they form the striped interference pattern of a wave. OpenStax describes experiments showing the fringes form gradually even when electrons pass through the slits individually.

A useful way to think of it: light travels like a wave, which decides where it is likely to be found, and it is detected like a particle, which arrives in one place with one packet of energy. Quantum mechanics describes both at once. The Light and Energy course covers waves, photons and the evidence for each.

Common mistakes

• Saying brighter light gives electrons more energy. Brighter means more photons, so more electrons, but not faster ones. Frequency sets the energy.
• Saying light switches between being a wave and a particle. It is one thing that shows both behaviours, depending on how you look.
• Using wavelength in nanometres in E = hc/λ. Convert to metres: 530 nm = 530 × 10⁻⁹ m.
• Thinking only light has duality. Electrons, atoms and even molecules show wave behaviour too.
• Forgetting the photon is all or nothing. One photon gives all its energy to one electron.

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