There is more to light than what meets the eye.
An average human eye can see about one million colors. Some people can see 100 times more hues. However, the most intriguing question is how many colors are invisible. Yes, there are colors that human eyes cannot see.
There is a secret to color perception. Unlike most marine animals, humans are “trichromats.” or have the ability to see three colors: red, blue, and green. All of the other visible colors blend and resemble the three.
When light hits an object, the object absorbs specific wavelengths of light and reflects the rest of it, depending on its properties. For example, you are looking at a ripe banana. Wavelengths of 570 to 580 nanometers, also known as the yellow light, bounce back. It then hits the retina that is located behind your eyes.
Retinas have cones made of photoreceptor cells, that help the eyes respond to light. The three kinds of cones – red, blue, and green – are responsible for the perception of millions of colors. However, the cones do not equally respond to certain lightwaves. Only 2% of them respond to blue light, 34% to green light, and 64% to red light.
The yellow light from the ripe banana stimulates the cones, and the stimulated cones will carry nerve signals to the brain for color decoding. You might be surprised to know that the bright yellow you see is not a real yellow. Our eyes cannot directly see this color, but were instead a result of the mixed reflections of green and red.
Did you know that yellow-blue and red-green colors exist? Unfortunately, we cannot see them as our eyes automatically cancel out their light frequencies. This limitation is due to the way we perceive colors in the first place. Take red-green, for instance. Cones that respond to red light trigger nerve signals to tell the brain we are looking at something red. At the same time, the cones that respond to green will also be triggered, consequently damping out the nerve signals for red.
Most reflected lightwaves induce a mixture of effects to the cones, which will then be used by the brain to identify a color. However, red-light cancels green light as yellow light cancels blue light, making them almost impossible to see simultaneously. Therefore, not all lights are visible.
Additionally, contemporary science explains impossible colors through the electromagnetic spectrum. The EM spectrum is a continuum of all the electromagnetic energies radiated in frequencies and wavelengths by the things around us. It includes radio waves, microwaves, x-rays, and gamma rays.
The most essential EM spectrum is visible light. The visible light allows us to see through a spectrum that approximates to the ROYGBIV (red, orange, yellow, green, blue, indigo, violet) wavelengths. Outside this spectrum, there exists a whole range of unperceivable colors radiated by infrared and ultraviolet spectrums.
How can we see them?
Let’s use the TV and remote as an analogy. When we change the channel using our remote, we point it directly to that small red light dot. Looking closely, this red dot looks like a mini-light bulb, almost similar to the colorless bulb at the top of your remote. The red dot is an infrared sensor that receives information from the remote using colors that humans can’t see.
All objects reflect light, but not all emitted lights are visible. In the EM spectrum, infrared is located at the top of the visible light making biologically invisible to see with a naked eye. You will need an Infrared Camera to re-visualize invisible infrared wavelengths to visible colors. This is the reason why IR cameras have a green tint.
Moreover, genetic mutation allows one to see up to 100 times more colors than normal humans. It is called Tetrachromacy. Tetrachromats have four cone types allowing them to see invisible colors. A person with tetrachromatic vision will describe a leaf as something with violet, turquoise, and blue while we might dismiss it as dark green. In 2015, UK neuroscientists announced that they successfully found a tetrachromat woman after more than 25 years of searching.
Fun Fact: Mantis shrimps have 16 receptor cones enabling them to survive on burrows and seabed with little to none light visibility.
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