Most of us enjoy nature due to the various colors we see—from the hues of flowers to the color-changing camouflage of animals. What many of us do not realize is that the visible colors that we see are just a tiny fraction of the overall light spectrum. One such spectrum component is ultraviolet, which can be perceived by animals but appears invisible to humans.
How Does Human Vision Work?
In the back of our eye, there is a layer of cells called the retina, which contains photoreceptors that detect light and send signals through the optic nerve. These signals carry information about the wavelength of light, which the brain then interprets as colors.
Surprisingly, a type of receptor in the retina known as cones can detect some forms of ultraviolet light. However, the eye's clear, curved structure, called the lens, focuses light onto the retina to help a person see more clearly. This involves filtering out ultraviolet light so that the high-energy wavelength would not reach the cones.
According to biologist Michael Bok from Lund University in Sweden, most people's lenses filter out most UV wavelengths. This ability of the lens helps protect the human eye from damage by ultraviolet light, known to age eye structures and increase the risk of cancer.
Despite the ability of the lens to filter out most ultraviolet light, most young people can perceive some of it. In a 2018 research entitled "Individual variation in the transmission of UVB radiation in the young adult eye," all college-aged subjects at the University of Georgia were found to perceive ultraviolet light at approximately 315 nanometers. The entire range of ultraviolet light is about 10-380 nanometers, with violet starting at the latter. During the study, the participants reported that the light appeared desaturated violet-blue. However, this ability seems to decline at around age 30, indicating that aging reduces the ability of the human eye to see ultraviolet wavelengths.
UV Light in Nature
Ultraviolet light is a component of the electromagnetic spectrum that has a shorter frequency than visible light. While most adult humans cannot see ultraviolet light, that is not the case in the animal world. According to the study "The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals," a lot of mammals, such as cats, dogs, reindeer and ferrets, can perceive some ultraviolet wavelengths throughout their lifetimes. The researchers also noted that the ability to see ultraviolet light is prevalent in amphibians, reptiles, birds, fish, and invertebrates. They possess cones in their eyes specifically designed to detect ultraviolet light, and lenses that let it through.
So why do many animals see in the ultraviolet range? According to Bok, it has something to do with the ability to improve contrast for detecting objects or essential objects in the environment.
There are many ways ultraviolet light helps animals achieve this. For instance, a lot of predatory sea creatures use ultraviolet light to help perceive the silhouettes of their prey. This is true for fish larvae and plankton because there is a lot of ultraviolet light in shallow water. Meanwhile, many insects use this type of vision to detect patterns in flowers, and so; somelarized ultraviolet light in the sky to help them in navigate are bird species which signal to each other through their plumage in colors in the ultraviolet range and use it to spot ripe berries.
Bok also added that this ability seems pretty common the more we look into it. The ancestors of vertebrates are believed to have the ability to see ultraviolet light and even had a photoreceptor specifically designed for it. Somewhere in humans' evolutionary history, these photoreceptors shifted more toward the detection of violet than ultraviolet wavelengths.
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