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What is light?

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Quick answer

Light is a form of energy that travels as electromagnetic waves and lets us see the world. It moves at about 300,000 kilometres per second — the fastest anything can travel — and behaves as both a wave and a stream of particles called photons.

Key things to understand

  1. Light is electromagnetic radiation; visible light is the small part of the spectrum our eyes can detect.
  2. It travels at roughly 300,000 km/s in a vacuum — the universe's speed limit.
  3. Light acts as both a wave (with wavelength and frequency) and particles called photons — known as wave–particle duality.
  4. Different wavelengths appear as different colours; white light is a mix of all of them, as a prism reveals.
  5. Beyond visible light, the spectrum includes radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.

People also ask

Is light a wave or a particle?
Both. Light shows wave behaviour (like bending and interference) and particle behaviour (photons carrying energy) — a famous idea called wave–particle duality.
How fast does light travel?
About 299,792 kilometres per second in a vacuum — nothing with mass can reach this speed, which is why it's called the cosmic speed limit.
Why does light have colour?
Colour comes from wavelength. Shorter wavelengths look blue or violet, longer ones look red, and a mix of all visible wavelengths looks white.

Part of the guide: Everyday Science: Light, Heat & Matter →