It’s obvious the Sun is the source of both almost all the light by which we see, and most of heat that makes the Earth a warm oasis in the deep cold of interstellar space. In recent centuries we’ve learned the Sun releases other sorts of energy: high frequency radiation such as ultraviolet light and X-rays, as well as the kinetic energy carried on the solar wind.
Perhaps you have wondered how these compare energetically? Is most of the Sun’s radiation at visible wavelengths? If not, is there more at wavelengths too high, or too low, for our eyes? If so, this article is for you.
First, a quick background on electromagnetic radiation
Light is composed of photons, the carrier of the electromagnetic force. Under the wave/particle duality, photons are both, and their wave aspect means every photon has a wavelength. The shorter the wavelength, the more energy a photon carries.
Photons exist over a vast range of wavelengths, and therefore energies. Radio waves are composed of photons with very little energy. Even the shortest radio waves have a wavelength of a millimeter (0.04 inches) and many are measured in meters.
At the other end of the spectrum, gamma rays have wavelengths of less than a hundred billionth of a meter, and the shortest we have detected is millions of times shorter still.
Humans can only see a tiny segment of the spectrum, from 380 to 700 nanometers, give or take a little depending on one’s eyes.
The energy of each photon is inversely proportional to its wavelength, but even the most powerful gamma rays from the Sun are millions of times shorter still. Consequently, the energy released in a particular band is very dependent on the number of photons produced at those wavelengths.
Peak output
Given we see an exceptionally narrow window of the spectrum, it’s easy to assume that this also represents an almost infinitesimal share of the Sun’s output.
However, every object has a peak wavelength at which it radiates, which is usually dictated by its temperature. Objects’ everyday temperatures radiate primarily in the infrared, which is why some night hunters, such as snakes, can see at wavelengths much longer than our eyes can capture. Night vision goggles are based on the same principle.
If something gets a little hotter, however, it starts to emit substantial amounts of red light, as seen in bar heaters or electric oven coils. As temperature rises the peak moves to shorter wavelengths. The temperature of the Sun (5,776 K, 5,500°C, 9,900°F) means its peak output is at 500 nm, which is in the green part of the spectrum.
The Sun’s temperature is higher than 5,000 K, so its energy is even more peaked than this line in the part of the spectrum we can see.
We don’t see sunlight as green because the peak isn’t particularly sharp – in addition to green, the Sun is also putting out some shorter radiation and plenty of longer wavelengths as well, and the mix looks yellow to us.
It’s probably not a coincidence that we have evolved to see at the wavelength near our star’s peak. We won’t really know, however, until we encounter aliens who evolved around a red dwarf whose output peaks at wavelengths too long for our eyes.
So, two factors compete, between the narrowness of the window we can see, and the fact that this is where the energy peak lies. It’s like if you added up the height of everyone in the world, and compared the cumulative height of those within a few centimeters of the most common height with those taller and shorter.
Complication corner
Above we’ve described the energy of what physicists call a “blackbody radiator”, something of a confusing name that refers to a source of radiation based entirely on its heat. However, stars are not perfect black bodies. They emit more light at specific wavelengths associated with the elements that make them up. Some of these elements radiate at visible wavelengths, such as the famous twin orange bars that indicate the presence of sodium, but many others are beyond our capacity to see. Part of the reason the JWST operates in the infrared, rather than at optical wavelengths is that so many of the molecules it is hunting for in the atmospheres of planets radiate at wavelengths too long for our eyes.
Nevertheless, the extra radiation from these spectral lines is small enough not to change the overall picture.
The solar spectrum
The Sun radiates about 3.8 x 1026 watts of energy into space.
Very little of this is at the extreme ends of the spectrum. Solar storms may make radios crackle, but the radio share is barely noticeable as a share of the Sun’s vast output. The same goes for X-rays. You might think ultraviolet radiation would be different, because there’s enough of it to give us sunburn and skin cancer, even with the ozone layer filtering most of it out. Nevertheless, these three combined add up to only a small proportion of solar output. (Normally reliable sources differ on how small, quoting figures of less than 1 percent, 2-3 percent and – more often – 8 percent just for ultraviolet.)
Gamma rays, despite their individual energy, are barely a rounding error when it comes to solar energy release.
Consequently, most the Sun’s energy is released either as visible light, or infrared.
That’s why solar cells are built to capture light at visible wavelengths along with infrared – there’s just no point chasing the tiny amount UV offers.
All the light we cannot see
That doesn’t mean the majority of the Sun’s energy is available to guide our eyes. At Earth’s distance from the Sun, 1,400 Joules falls on every square meter (10 square feet) every second, more frequently described as 1,400 watts/m2.
About 40 percent of this is visible light, with infrared about half the total.
However, this is the energy that reaches the top of our atmosphere, or the Moon. Clouds filter a lot out, but even on clear days the air blocks some of that light, and it doesn’t do it evenly.
Of the light that makes it to ground level, visible light is a slightly higher proportion (about 42 percent) because the nitrogen, oxygen, and trace gases in our atmosphere are particularly transparent at visible wavelengths. Yet even at ground level over the course of the year the Sun is providing more heat than light, albeit narrowly.
So the light we can see is a minority of the Sun’s energy, but only just. This isn’t a tip of an iceberg situation, which would be the case if we found ourself on a planet orbiting a deep red star.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.
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