What exactly is light?

Short answer

Light is an electromagnetic wave that travels through space as electric and magnetic fields feed each other. Of the vast family stretching from radio waves to gamma rays — the electromagnetic spectrum — our eyes perceive only a very narrow slice, and we call that slice "visible light." It's no coincidence that our eyes are tuned to exactly this slice: the sun's most abundant light, the window that the atmosphere and water let through, and the eye's own protective mechanism all meet in this range.

Light is an electromagnetic wave

Until the middle of the nineteenth century, electricity, magnetism, and light were thought to be separate phenomena. When the Scottish physicist James Clerk Maxwell published his equations unifying electricity and magnetism into a single theory in 1865, a surprising result followed from them: a changing electric field gives rise to a magnetic field, and a changing magnetic field gives rise to an electric field in turn, and this mutual cycle can travel through space as a wave. When Maxwell calculated the speed of these waves, he found a value very close to the experimentally measured speed of light, and concluded that light was a wave of this very kind.

It was the German physicist Heinrich Hertz who put the theory to the experimental test. In 1887, in his laboratory, he used electric sparks to produce invisible waves and caught them with a receiver at the other end of the room. These waves reflected, refracted, and interfered with one another — in other words, they behaved just like light. Today's radio, television, and cell phone technology are the descendants of this experiment.

The only thing that distinguishes these waves from one another is wavelength: the distance from one crest of the wave to the next. A radio wave kilometers long and a gamma ray smaller than an atom are things of the same kind. How wavelength relates to frequency and to the energy it carries is the subject of BI-02.

Spectrum: from radio to gamma

When we arrange electromagnetic waves by wavelength, we get a spectrum. Radio waves lie at the longest end; then come microwaves, infrared, visible light, ultraviolet, X-rays, and, at the shortest end, gamma rays. This arrangement is called the electromagnetic spectrum.

You can picture this spectrum as a piano keyboard: imagine a keyboard stretching for kilometers, where our eyes can hear only a single octave. Visible light is exactly that narrow a slice compared to the rest of the spectrum. The rest of the keyboard keeps playing, but we don't hear it directly.

The discovery of the visible slice's two neighbors was the first experimental proof that the spectrum isn't bounded by what the eye can see. In 1800, the astronomer William Herschel, while measuring the temperature of the colors a prism split apart, placed a thermometer just beyond the red, in a region where no light was visible, and noticed that the temperature rose there too: this was infrared. A year later, Johann Wilhelm Ritter placed silver chloride, which darkens in light, just beyond the violet end and saw an even stronger darkening there: this was ultraviolet.

The visible slice

The range the human eye perceives stretches from roughly 380 to 780 nanometers — a band a few hundred units wide, at the scale of a billionth of a meter. The limits aren't sharp: with a very strong light source, you can see a little beyond this range too, and sensitivity drops off quickly toward the edges. The short end of the range appears to us as violet and blue, the middle as green and yellow, and the long end as orange and red.

At this point it's worth recalling KE-01's important warning: the waves themselves aren't colored. This is also the answer to the question "why don't infrared and ultraviolet count as colors?" Color is an experience that arises when the brain compares the responses of the cone cells in the eye. Infrared and ultraviolet waves, even when they reach the retina, don't produce a color experience, either because they can't stimulate our cones or because they never reach the retina at all. They are physically light, but for us they aren't color.

Why is the eye tuned to this range?

Several reasons work together to explain why our eyes see exactly this slice. Most of the energy the sun emits falls in the visible and near-infrared region; Earth's atmosphere also lets a large part of this region through to the ground, while ozone and other gases filter out the very short wavelengths. Another window is water itself. Water transmits visible light relatively well while strongly absorbing infrared. Because the first eyes evolved in water, it was an advantage to use light that could pass through water.

The eye itself also carries a filter. The cornea, the eye's front surface, absorbs very short-wavelength ultraviolet, while the eye's lens absorbs most of the ultraviolet between 300 and 400 nanometers and protects the retina. It has been reported that some people who had their natural lens removed in cataract surgery and replaced with a lens that doesn't filter ultraviolet can see near-ultraviolet as a pale bluish light. In other words, part of the violet boundary of the visible range comes not from our cones but from our lens.

Example — faces in a thermal camera

Think of the thermal cameras that measure fever at an airport. These cameras don't see visible light; they see the infrared the skin emits. In visible light, skin tone varies across a wide range, from light to dark, depending on how much the melanin absorbs light. In infrared, the situation is entirely different: regardless of its color, skin radiates at close to a perfect rate, and in a thermal image the difference between skin tones disappears, leaving only a difference in temperature.

This example shows that different slices of the spectrum tell us different things about the same object. Visible light tells us the skin's color, infrared tells us its temperature. Color is information written only in the slice of the spectrum that our eyes can read.

Common misconception

Summary

1. Light is an electromagnetic wave that travels as electric and magnetic fields feed each other; Maxwell showed this with theory, Hertz with experiment.
2. The electromagnetic spectrum stretches from radio to gamma; our eyes see the narrow slice roughly between 380–780 nm, and infrared and ultraviolet are this slice's invisible neighbors.
3. The eye's being tuned to this range is explained by sunlight, the transmittance of the atmosphere and water, and the lens's filtering of ultraviolet.

Test yourself

1. (Easy) Who was the first scientist to show theoretically that light is an electromagnetic wave?
2. (Easy) Who discovered infrared in 1800, and with what experiment?
3. (Medium) Why don't infrared and ultraviolet count as "color"?
4. (Medium) Why is the eye's lens one of the factors that determines the violet boundary of the range our eyes can see?
5. (Hard) What hypothesis about the evolution of vision systems does water's strong absorption of infrared support?
6. (Hard) What does the fact that different skin tones look alike in a thermal camera tell us about the information carried by visible light versus infrared?

Answers

1. James Clerk Maxwell; in 1865 he derived that electromagnetic waves travel at the speed of light ("Light is an electromagnetic wave").
2. William Herschel; he saw that a thermometer he placed beyond the red end of light split by a prism grew warmer ("Spectrum: from radio to gamma").
3. Color is an experience that arises when the brain interprets the cones' response in the eye. Infrared and ultraviolet are physically light, but they don't produce a color experience because they can't stimulate our cones or can't reach the retina ("The visible slice").
4. The eye's lens absorbs most of the ultraviolet between 300–400 nm; when the lens is removed, some people can perceive near-ultraviolet ("Why is the eye tuned to this range?").
5. Because the first eyes evolved in water, it was advantageous to use the visible region that water could transmit; the range of vision overlaps with this transmittance window ("Why is the eye tuned to this range?").
6. Visible light carries skin color based on melanin's absorption; in infrared, skin has nearly the same emissivity regardless of its color, so only temperature information remains. Each slice of the spectrum carries different information about the same object ("Example — faces in a thermal camera").

Sources

• Maxwell, J. C., "A Dynamical Theory of the Electromagnetic Field," Philosophical Transactions of the Royal Society of London 155, 1865.
• Caltech IPAC, Cool Cosmos, "The Herschel Experiment" and "The Ritter Experiment."
• Delgado-Bonal, A. and Martín-Torres, J., "Human vision is determined based on information theory," Scientific Reports 6, 2016.
• Steketee, J., "Spectral emissivity of skin and pericardium," Physics in Medicine and Biology 18(5): 686–694, 1973.
• Stark Lab, Saint Louis University, a research page on human ultraviolet sensitivity.

Next step

BI-02 goes down into this lesson's units of measurement: it explains the inverse relationship between wavelength and frequency, why a blue photon is more energetic than a red one, and why magenta is seen even though it doesn't appear in the spectrum. BI-04 moves on to the moment light meets an object: it explains selective absorption, reflectance curves, and transparent versus opaque materials through why a tomato looks red.