It shapes our daylight hours, powers the planet’s biosphere, and can even provide clean electricity to your community. But what is sunlight, really?
You already know that it is, most obviously, illumination generated by the sun, the closest star to Earth. But there’s more to sunlight than you know! From its physical nature to the journey it takes to reach the surface of our planet, there’s more to sunlight than meets the eye.
Sunlight: A beam or a wave?
We typically refer to sunlight as a “sunbeam” or a “sunray,” suggesting that a continuous stream of solar units must flow from the sun to us in order to provide constant illumination.
This is not far off the mark. Indeed, light is composed of discrete units, known as photons. Technically speaking, photons are elementary particles that convey electromagnetic force — one of the four fundamental forces of nature governing how the physical world works. (For the record, the other forces are gravity, with which we’re all basically familiar, plus the strong and weak nuclear forces, which only operate on the Lilliputian scale of atoms.)
And while light can, therefore, be thought of as individual “drops” or “streaks,” like falling rain, sunlight’s true nature is paradoxically more complicated. This is because elementary particles behave according to a set of laws known as quantum mechanics. Depending on how light is observed and measured, it can either act like a discrete “sun drop,” or act like a wave of energy that can combine with or cancel out other such waves, creating so-called diffraction patterns.
Sunlight is “a continuous stream and a deluge of particle-like photons, both at the same time,” said Dana Longcope, a professor of physics at Montana State University. “Due to the weirdness of quantum mechanics, things are neither always particles or always waves. They can be either.”
The delayed feed of the Sun
If that’s not perplexing, let’s share an interesting idea by pointing out that sunlight is, in a manner of speaking, a time machine.
Light travels, as you might expect, at the speed of light. Lightspeed is the maximum velocity physically permissible in the universe. In a perfectly matter-free vacuum, light clocks in at a blistering 670,616,629 miles per hour. When passing through transparent materials, such as air and water, it moves a touch slower.
Now consider that the sun, on average, is located about 93 million miles away from Earth. That’s a difficult distance to imagine. To provide some perspective, consider how far a person might cumulatively walk over a typical lifetime of 80 years: various back-of-the-envelope calculations put that walking distance figure on the order of 110,000 miles. Accordingly, you would need over 800 lifetimes before ever racking up enough footsteps to reach the sun.
Naturally, light can cross this span much, much faster. But it still takes a photon about 8 minutes, 26 seconds to complete the trip. In other words, the sun we see in the sky is actually the sun as it was more than 8 minutes ago.
(To extend this concept to the rest of the observable universe, astronomers likewise view things the way they once were, based on how far away these things are. The most distant galaxies on record faintly glow from more than 13 billion light-years away. That means we are fathoming these galaxies from when the 13.8-billion-year-old universe was in its early childhood.)
Let there be light
How does the sun generate light in the first place? Two numbers tell the tale:
- 28,000,000: the number of degrees Fahrenheit of the sun.
- 250,000,000,000: the number of bars of atmospheric pressure of the sun. (For reference, Earth’s atmospheric pressure at sea level is about 1 bar.)
Those immense figures describe the extreme conditions in the sun’s core, sufficient for squeezing and heating the lightest element, hydrogen until it fuses with other hydrogen particles eventually on into the second lightest element, helium. These thermonuclear fusion reactions release copious energy, mostly in the form of photons (light), to the tune of 3.8 x 1033 ergs per second. Expressed another way, the sun perpetually generates an astonishing 5 x 1023 horsepower.
The released light travels less than half an inch, though, before it is absorbed in the melee of fusing hydrogen. The light is then re-emitted as a new photon and again, just as quickly, re-absorbed. “No single photon inside the sun lives longer than a nanosecond. Most live only a picosecond or so,” said Longcope, referring to a billionth and a trillionth of a second, respectively.
Yet half-inch by half-inch, a packet of energy eventually crosses a few hundred thousand miles, making its way to sun’s surface. The journey is a considerable one. “The totality of all these many, many steps, made by all the different photons,” said Longcope, “requires a million years.”
The takeaway: The sunlight you feel now had its ultimate origin a good 800,000 years before Homo sapiens.
The colors of sunlight, seen and unseen
Despite its common origins, all sunlight is not the same. And in the grand scheme of things, humans are only visually privy to a minority portion of it.
The photons that escape the sun come in a range of wavelengths, or energies. The total range of energies that light can possess is known as the electromagnetic spectrum. The most energetic form of light on this spectrum is gamma rays—a dangerous kind of radiation to us because they damage our tissues and break the bonds in our DNA. The lowest energy form of light is a friendlier sort, called radio waves. We rely on radio waves for much of modern technology, including mobile communications, Wi-Fi, and the Global Positioning System (GPS).
In the middle of the spectrum, proteins in our eyes can register but a teensy sliver of energies as visible light. Slightly less energetic light, dubbed infrared, we can’t see but instead feel as heat. Most of the sun’s output is in these energies, with about a tenth emitted in the form of ultraviolet (UV) rays, which are more energetic than visible light. The Earth’s atmosphere filters out most of that dangerous UV, though what gets through triggers the protective tanning response in our skin.
Overall, around 53 percent of the sunlight that reaches us on the ground is in the infrared range. Visible light, meanwhile, tallies up to about 44 percent. The sun emits most strongly in the green portion of the visible spectrum, but the overall mix of sunlight energies combine to create white light. The sun only happens to look yellow-orangey toward the horizon, where its light passes through more atmosphere than when straight overhead. This is because the bluer light is more easily scattered by the molecules in our air than lower energy, redder light. The same scattering phenomenon makes blue light seem to come to our eyes from all over the whole sky—hence rendering the firmament “sky blue.”
With all this in mind, go ahead, soak in some sunlight and ponder its hidden magnitudes.