Light is a form of electromagnetic wave
Light is a disturbance in electric and magnetic fields that travels through space. Unlike sound, it needs no material to carry it, which is why sunlight can cross the near-empty gap between the Sun and Earth. In a vacuum, all light travels at the same speed, about 300,000 kilometers per second. At that speed, sunlight takes a little over eight minutes to reach us.
Two measurements describe any wave of light. Wavelength is the distance between successive crests. Frequency is how many crests pass a point each second. They are tied together: the longer the wavelength, the lower the frequency, and the product of the two equals the speed of light.
The wave side of light
Many everyday effects only make sense if light behaves as a wave.
- Reflection: light bounces off a surface, as in a mirror, with the angle in equal to the angle out.
- Refraction: light changes direction when it passes from one material into another, such as from air into water or glass, because it travels at different speeds in each. This is how lenses focus light and why a straw looks bent in a glass of water.
- Diffraction and interference: light spreads around edges and overlapping waves can reinforce or cancel one another. The shimmering colors on a soap bubble or the back of a disc come from interference.
- Polarization: light waves can oscillate in a preferred direction, which is the principle behind polarized sunglasses that reduce glare.
The particle side of light
In the early twentieth century, experiments showed that light also comes in tiny, indivisible packets of energy called photons. The clearest evidence came from the photoelectric effect, in which shining light on certain metals knocks electrons loose. The results depended on the color of the light rather than just its brightness, which a pure wave picture could not easily explain. Albert Einstein’s explanation of this effect helped establish the photon idea.
Each photon carries an amount of energy proportional to its frequency. Blue light photons carry more energy than red light photons. Brightness, by contrast, reflects how many photons arrive.
So is light a wave or a particle? Modern physics says it is neither in the everyday sense. It is a quantum object that shows wave-like or particle-like behavior depending on how you set up the measurement. This is one of the central lessons of quantum physics, and it is a puzzle people still discuss, even though the mathematics predicts experiments extremely well.
Where visible light sits in the spectrum
The electromagnetic spectrum is the full family of light waves, ordered by wavelength. From longest to shortest, the usual groupings are:
- Radio waves
- Microwaves
- Infrared
- Visible light
- Ultraviolet
- X-rays
- Gamma rays
These are all the same kind of thing, differing only in wavelength and photon energy. Visible light is a very narrow slice in the middle. It runs from roughly 400 nanometers (violet) to roughly 700 nanometers (red), though the exact edges vary from person to person. A nanometer is one billionth of a meter.
Why do our eyes respond to this range? One common explanation is that the Sun emits a great deal of its energy there and that the atmosphere lets much of it through, so evolution favored sight tuned to it. That is a reasonable account rather than a proven certainty.
The colors of the visible spectrum
White light is a mixture of wavelengths. A glass prism separates them because shorter wavelengths bend slightly more than longer ones, producing the sequence red, orange, yellow, green, blue and violet. The traditional list of seven colors includes indigo, though the boundaries between colors are really a smooth gradient and the number of named bands is partly a matter of convention.
A rainbow is the same process happening in raindrops. Sunlight enters a drop, bends, reflects off the back and bends again on the way out, spreading into colors.
How we perceive color
Color is partly physics and partly biology. In the retina, there are three types of cone cells, each most sensitive to a different range of wavelengths, roughly corresponding to red, green and blue light. The brain compares the signals from these cones to construct the sensation of color. That is why a screen can display a huge range of colors using only red, green and blue pixels: it stimulates the cones in the right proportions. The details of color perception involve the brain as much as the eye, so two people may not experience a color identically.
Objects have color because of what they do with incoming light. A ripe tomato absorbs most wavelengths and reflects mostly red. A white sheet of paper reflects almost all wavelengths. A black surface absorbs nearly all of them, which is why it warms up quickly in sunlight.
Why the sky is blue and sunsets are red
Sunlight is scattered by gas molecules in the air, and shorter wavelengths scatter much more strongly than longer ones. Blue light is redirected across the sky and reaches your eyes from every direction, so the sky looks blue. Violet scatters even more, but the Sun emits less of it and our eyes are less sensitive to it, so blue wins out.
At sunrise and sunset, light passes through a much longer path of atmosphere. Much of the blue has been scattered away before it reaches you, leaving oranges and reds. Dust and haze can intensify the effect.
Light beyond what we can see
Other animals see a different slice of the spectrum, and many insects can detect ultraviolet light. Technology extends our own range. Infrared cameras detect warmth, and specialized optical telescopes collect faint light from distant stars, with other instruments covering radio waves and X-rays so astronomers can see features that visible light alone would hide.
Light is also how energy gets here: the same sunlight that lets us see powers photosynthesis and warms the planet. The atmosphere treats incoming visible light and outgoing infrared differently, which is the core of the greenhouse effect.
Key points to remember
- Light is an electromagnetic wave and also arrives in photons.
- Wavelength sets color: visible light runs from about 400 to 700 nanometers.
- Visible light is only a thin slice of the electromagnetic spectrum.
- The colors we see depend on reflection, absorption, scattering and how our eyes and brain process signals.