What is light? Waves, photons and how we see

Light is electromagnetic radiation, a traveling wave of electric and magnetic fields, in the narrow range of wavelengths the human eye can detect, roughly 380 to 700 nanometers. It travels through empty space at about 300,000 km a second. It behaves as a wave when it spreads and interferes, and as a stream of particles called photons when it is absorbed or emitted, each carrying a fixed amount of energy.

In short

• Light is a wave of electric and magnetic fields that needs no material to travel through. Visible light is one small band of a much larger spectrum.
• It also comes in photons, packets of energy. A photon's energy depends only on its wavelength: shorter, bluer light carries more per photon.
• Color is made by the eye and brain: three kinds of cone cell respond to overlapping ranges of wavelength, and the brain compares them.

Is light a wave or a particle?

It is both, and neither in the everyday sense. Light shows wave behavior when it travels: it spreads around edges, and two beams can add up or cancel. It shows particle behavior when it meets matter: it is absorbed and emitted in whole photons, never in fractions of one. Physics describes it with one theory, quantum electrodynamics, in which light is a field whose energy comes in lumps. The wave tells you where photons are likely to land; the photons are what land.

The evidence for the wave came first. Around 1801 to 1803, Thomas Young shone light through two narrow slits close together. On the screen beyond, he saw not two bright lines but a row of bright and dark stripes. That pattern is interference: where the crests of the waves from the two slits arrive together, they reinforce and make a bright band; where a crest meets a trough, they cancel and leave darkness. Particles flying in straight lines can't do that.

The evidence for particles came a century later. Shine light on a metal and it can knock electrons out, but only if each photon carries enough energy; making dim light brighter adds more photons, not stronger ones. Albert Einstein explained this photoelectric effect in 1905, and the Nobel Foundation cites it for his 1921 prize. How that led to quantum mechanics is a story of its own. Even the double-slit stripes build up dot by dot when the light is made dim enough to send one photon at a time.

What is light made of?

In its wave form, light is a pair of linked fields. A changing electric field creates a magnetic field, and a changing magnetic field creates an electric field, so the two keep regenerating each other as they travel, at right angles to each other and to the direction of travel. James Clerk Maxwell worked this out in the 1860s from the laws of electricity and magnetism and found that such waves would move at exactly the measured speed of light. He concluded that light is an electromagnetic wave.

That is why light, unlike sound, crosses the vacuum of space. Its speed in a vacuum is exactly 299,792,458 meters a second, a number whose history and meaning are in the note on the speed of light. Visible light is only a narrow band of a vast family that runs from radio waves kilometers long to gamma rays smaller than an atom, all the same thing at different wavelengths: the electromagnetic spectrum.

A worked example: the energy of one photon of green light

A photon's energy is E = h × c ÷ λ. Here E is the energy in joules, h is the Planck constant, about 6.626 × 10⁻³⁴ joule-seconds, c is the speed of light, 2.998 × 10⁸ m/s, and λ is the wavelength in meters. Take green light at 550 nanometers, near the middle of the visible range: 550 × 10⁻⁹ m.

1. Multiply h by c: 6.626 × 10⁻³⁴ × 2.998 × 10⁸ = 1.986 × 10⁻²⁵ joule-meters.
2. Divide by the wavelength: 1.986 × 10⁻²⁵ ÷ 5.50 × 10⁻⁷ = 3.61 × 10⁻¹⁹ joules.

That is about 2.25 electronvolts, the unit physicists use at this scale. One photon is an absurdly small amount of energy, which is why light seems smooth. A 1-milliwatt green laser pointer delivers 0.001 joules a second, so it sends out about 0.001 ÷ 3.61 × 10⁻¹⁹ ≈ 2.8 × 10¹⁵ photons, nearly three thousand trillion, every second. Halve the wavelength and each photon carries twice the energy, which is why ultraviolet light can damage skin and red light cannot.

How do we see color?

Seeing starts when photons strike the retina at the back of the eye and are absorbed by light-sensitive molecules in two kinds of cell. Rods work in dim light and see no color. Cones need brighter light and come in three types, which respond most strongly to different wavelengths: short cones near 420 nm, medium near 530 nm and long near 560 nm, according to the standard figures summarized in Wikipedia's article on cone cells. Their ranges overlap widely.

Color is not a property of light alone but the brain's comparison of how strongly each cone type is responding. Light of 580 nm excites the long cones a lot and the medium cones somewhat, and the brain calls that yellow. A mix of red and green light can excite the cones in the same proportions, and the brain sees exactly the same yellow. That is how every phone and TV screen works: it makes the whole range of colors from tiny red, green and blue dots. Some colors, magenta among them, match no single wavelength at all; they exist only as a mixture.

An object's color is the light it doesn't absorb. A leaf absorbs red and blue light for photosynthesis and returns more green. White is a mix of all visible wavelengths, as Isaac Newton showed by splitting sunlight with a prism and recombining it, published in his Opticks of 1704. Why the daytime sky picks out blue is the subject of why the sky is blue.

Why does light bend?

When light passes from one transparent material into another at an angle, it changes direction, which is called refraction. It happens because light travels more slowly in glass or water than in air. Each material has a refractive index, n, the speed of light in a vacuum divided by its speed in the material: about 1.00 for air, 1.33 for water and about 1.5 for glass. Snell's law gives the new angle: n₁ × sin θ₁ = n₂ × sin θ₂, with each angle measured from a line perpendicular to the surface.

Light going from air into water at 45°: sin 45° = 0.707, so sin θ₂ = 1.00 × 0.707 ÷ 1.33 = 0.532, and θ₂ = 32°. The light bends toward the perpendicular. Run the path backward and you see why a straw looks bent in a glass and a fish is deeper than it appears. Lenses in eyes, glasses and cameras shape surfaces so that refraction brings light to a focus.

The index varies slightly with wavelength, so blue light bends a little more than red. A prism spreads white light into a rainbow band this way, and raindrops do it in the sky. Reflection, by contrast, follows a simpler rule: light bounces off a smooth surface at the same angle it arrived.

How did we come to understand light?

Year	Step	Who
1678–1690	Light is a wave in a medium, a mathematical theory published in his Treatise on Light	Christiaan Huygens
1704	White light is a mixture of colors; light is made of particles	Isaac Newton, Opticks
1801–1803	Interference stripes show that light is a wave	Thomas Young
1860s	Light is an electromagnetic wave	James Clerk Maxwell
1887–1888	Radio waves made in a laboratory behave just like light	Heinrich Hertz
1905	Light comes in quanta of energy	Albert Einstein
1926	The quanta are named photons	Gilbert Lewis

Newton's particle theory held sway through the 1700s largely on his reputation. Young's experiment revived waves, Maxwell explained what was waving, and Einstein brought particles back without removing the waves. Each step kept what the last one got right.

Check yourself

• Without looking back: what does the double-slit experiment show about light, and why can't particles in straight lines explain it?
• Why can ultraviolet light burn skin while far brighter red light cannot?
• How can a mix of red and green light look exactly like pure yellow?

Sources

• Visible Light — NASA Science
• Albert Einstein, Facts, the Nobel Prize in Physics 1921 — Nobel Foundation
• 1.3 Refraction, in University Physics Volume 3 — OpenStax, Rice University
• Light — Wikipedia
• Cone cell — Wikipedia
• Young's interference experiment — Wikipedia