A week ago, who among us would have guessed that light, the universe's ultimate speed demon, would be observed getting outpaced by a bunch of reckless neutrinos? Yes, these observations will obviously need to be checked and rechecked, but it just goes to show that you rarely know as much about something as you think you do.
So in the interest of keeping you all as educated on light as possible, here are ten little-known historical and scientific facts about everyone's favorite source of illumination.
10) Light can make some people sneeze
Between 18% and 35% of the human population is estimated to be affected by a so-called "photic sneeze reflex," a heritable condition that results in sneezing when the person is exposed to bright light.
The exact cause of the reflex is poorly understood, but people have been kicking around possible explanations for millennia; Aristotle, for example, chalked the reflex up to the heat of the sun on one's nose, while most modern-day scientists posit that a cranial nerve responsible for facial sensation and motor control (that is in close proximity to the optic nerve) picks up on electrical signals intended for the optic nerve and tells the brain that there is an irritant in the nose that needs to be cleared out.
9) Plato thought that human vision was dependent upon light, but not in the way you're imagining
In the 4th Century BC, Plato conceived of a so-called "extramission theory" of sight, wherein visual perception depends on light that emanates from the eyes and "seizes objects with its rays."
Plato's student, Aristotle, was among the first to reject the extramission theory and the idea of a so-called "active eye," advocating instead a passive, "intromission" theory of vision, whereby the eyes receive information via rays of light as opposed to generating these rays on their own. (Image via.)
8) Einstein was not the first one to come up with a theory of relativity
Many people associate "the speed of light" with Einstein's theory of relativity, but the concept of relativity did not originate with Einstein. Props for relativity actually go to none other than Galileo, who was the first to propose formally that you cannot tell if a room is at rest, or moving at a constant speed in one direction, by simply observing the motion of objects in the room.
What Einstein did do was bring Galileo's conception of relativity up to speed by combining it with Newton's work with gravity, and James Clerk Maxwell's equations addressing electricity and magnetism (equations, it bears mentioning, that predicted that waves of electromagnetic fields move at 299 792 458 meters per second — i.e. the speed of light).
7) E=mc^2 was once m=(4/3)E/c^2
Einstein was not the first person to relate energy with mass. Between 1881 and 1905, several scientists — most notably phycisist J.J. Thomson and Friedrich Hasenohrl — derived numerous equations relating the apparent mass of radiation with its energy, concluding, for example, that m=(4/3)E/c^2. What Einstein did was recognize the equivalence of mass and energy, along with the importance of that relevance in light of relativity, which gave rise to the famous equation we all recognized today.
6)The light from the aurorae is the result of solar wind
When solar winds from cosmic events like solar flares reach Earth's atmosphere, they interact with particles of oxygen atoms, causing them to emit stunning green lights like the ones captured by the International Space Station last week (featured here).
These waves of light — termed the aurora borealis and aurora australis (or northern lights and southern lights, respectively) — are typically green, but hues of blue and red can be emitted from atmospheric nitrogen atoms, as well.
5) Neutrinos aren't the first things to apparently outpace the speed of light
The Hubble telescope has detected the existence of countless galaxies receding from our point in space at speeds in excess of the speed of light. However, this still does not violate Einstein's theories on relativity because it is space — not the galaxies themselves — that is expanding away (a symptom of the Big Bang), and "carrying" the aforementioned galaxies along with it.
4) This expansion means there are some galaxies whose light we'll never see
As far as we can tell, the Universe is expanding at an accelerating rate. On account of this, there are some who predict that many of the Universe's galaxies will eventually be carried along by expanding space at a rate that will prevent their light from reaching us at any time in the infinite future.
3) Bioluminescence lights the ocean deep
More than half of the visible light spectrum is absorbed within three feet of the ocean's surface; at a depth of 10 meters, less than 20% of the light that entered at the surface is still visible; by 100 meters, this percentage drops to 0.5%.
In fact, at depths of over 1000 meters — a region of the ocean dubbed the "aphotic zone" — there is no detectable light whatsoever. As a result, the largest source of light in the Earth's oceans actually emanates from animals residing in its depths; marine biologists estimate that between 80 and 90 percent of deep-sea creatures are bioluminescent (image via).
2) Bioluminescence: also in humans!
Bioluminescene isn't just for jellyfish and the notorious, nightmare-inducing Anglerfish; in fact, humans emit light, too.
All living creatures produce some amount of light as a result of metabolic biochemical reactions, even if this light is not readily visible. Back in 2009, a team of Japanese researchers reported that "the human body literally glimmers," after using incredibly sensitive cameras (the light is a thousand times weaker than the human eye can perceive) to capture the first evidence of human bioluminescence, pictured here. It's worth mentioning that images C, D, E, F, and G, are not thermal images, but actually pictures of emitted photon intensity over the course of an average day.
This time-dependent photon emission is illustrated in the chart shown in figure H. Figure I shows the thermal image you're more accustomed to seeing.
1) It's possible to trick your brain into seeing imaginary (and "impossible") colors
Your brain uses what are known as "opponent channels" to receive and process light. On one hand, these opponent channels allow you to process visual information more efficiently (more on this here), but they also prevent you from seeing, for example, an object that is simultaneously emitting wavelengths that could be interpreted as blue and yellow — even if such a simultaneous, "impossible" color could potentially exist.
In theory, you can train yourself to see these and other so-called "imaginary" colors with a few simple tricks, which you can check out in our quick, how-to guide on seeing impossible and imaginary colors.