What Light Is
Light is electromagnetic radiation visible to the human eye — a narrow slice of a vast spectrum that extends from radio waves to gamma rays. In a vacuum, all electromagnetic radiation travels at the same speed: approximately 299,792 kilometres per second, commonly written as c. This is the fastest speed in the universe. Nothing with mass can reach it. Information cannot exceed it. It is, in a deep sense, the universe's speed limit.
Light is generated whenever charged particles accelerate or whenever electrons in atoms transition between energy levels, releasing energy as photons. A photon is the fundamental unit of light — a quantum of electromagnetic energy with no mass and no electric charge, travelling always at c.
Wave and Particle
Looking at distant objects means looking back in time. The light from the Andromeda Galaxy left it 2.5 million years ago.
For most of the nineteenth century, physicists believed light was a wave — a position supported by its behaviour in experiments involving diffraction and interference. In 1905, Einstein explained the photoelectric effect by proposing that light comes in discrete packets of energy: photons. Light, it turned out, behaves as both a wave and a particle, depending on how you observe it.
This wave-particle duality is one of the foundational facts of quantum mechanics. It is deeply counterintuitive, and no classical analogy adequately captures it. A photon is not a wave that sometimes acts like a particle, nor a particle that sometimes acts like a wave. It is something for which classical language has no good word. When physicists say it is both, they mean that neither description alone is complete.
The double-slit experiment demonstrates this clearly. When light passes through two closely spaced slits, it produces an interference pattern on a screen behind them — evidence of wave behaviour. When physicists detect which slit individual photons pass through, the interference pattern disappears, and the photons behave like particles. The act of measurement changes the result. This is not a limitation of the equipment; it is a feature of reality.
The Spectrum
Visible light — the light we see — occupies only a tiny portion of the electromagnetic spectrum. Below visible light, in order of decreasing frequency, are infrared radiation, microwaves, and radio waves. Above it are ultraviolet radiation, X-rays, and gamma rays. All of these are light, in the sense that they are electromagnetic radiation; only their frequencies differ.
The visible spectrum runs from red, at the low-frequency end, through orange, yellow, green, blue, and violet, to ultraviolet at the high end. White light is the combination of all visible wavelengths. A prism separates them — each wavelength is bent by a slightly different angle — revealing the constituent colours. This is also what happens in a rainbow, where water droplets serve as the prism.
Light and Gravity
According to general relativity, light is affected by gravity. Massive objects bend the fabric of spacetime, and light follows the curvature of spacetime. When light from a distant galaxy passes close to a massive object on its way to Earth, it is deflected. This gravitational lensing was first confirmed during the solar eclipse of 1919, when observations showed that starlight was bent by the Sun's gravity by exactly the amount Einstein had predicted.
Near a black hole, the gravitational field is so strong that even light cannot escape. The boundary beyond which nothing — not even light — can return is called the event horizon. The radius of the event horizon, for a non-rotating black hole, depends only on the mass of the black hole. It is called the Schwarzschild radius, and for a black hole as massive as the Sun, it would be approximately three kilometres.
Light and Time
Because light travels at a finite speed, looking at distant objects means looking back in time. The light reaching our eyes from the Moon left it about 1.3 seconds ago. The light from the Sun left it about eight minutes ago. The light from the nearest star, Proxima Centauri, left it four years ago. The light from the Andromeda Galaxy left it 2.5 million years ago.
The most distant light we can detect — the cosmic microwave background radiation — was emitted approximately 380,000 years after the Big Bang, when the universe first became transparent to light. It is a photograph, of sorts, of the early universe — light that has been travelling for nearly 13.8 billion years to reach us. Beyond that distance, no light has yet had time to arrive.