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$15 for one month of unlimited tanning bed use ($30 value)

UV Rays: The Sun’s Invisible Touch

A nice tan might look great, but the thing that creates it isn’t visible at all. Check out Groupon’s guide to learn about the science of UV rays.

Whether they come from the bright, yellow sun burning in the sky or the white light spilling through the opening of a tanning bed, UV rays are impossible to see. Unlike the colorful patterns of the visible spectrum, ultraviolet light—like infrared light—shines invisible to the human eye. All kinds of light are forms of electromagnetic radiation, existing as energy moving in waves of varying frequency and length. Compared to visible light, ultraviolet light has a higher frequency—meaning our eyes aren’t tuned to it—and a shorter wavelength, which means more energy is packed into each second of radiation. UV rays themselves are divided into three levels of intensity: UVA, UVB, and, UVC. The light from the sun contains all three levels, but the atmosphere blocks UVC rays from making it to the earth at all, and clouds and windows block the majority of UVB rays. UVA rays can penetrate almost anything, however, which is why medieval knights wore suits of SPF 10,000 armor to protect their pale complexions.

UV light is responsible for tanning. Within the outer layer of the skin, cells known as melanocytes produce a brown pigment called melanin. As the epidermis absorbs UVA or UVB rays, these cells produce more melanin, leading to darker skin. UVB rays also encourage production of vitamin D, which is necessary for bone health and robust immune systems. Too much exposure to UV rays, however, can lead to skin damage, making it important to moderate tanning sessions. In addition, many tanning beds claim to emit only UVB light, thus avoiding the deeper-penetrating UVAs, but studies show neither ray is significantly safer than the other.

Because it’s invisible, UV light was unknown until 1801, a year after the discovery of infrared light. Johann Ritter heard about the mysterious light just beyond the red end of the visible spectrum, and he decided to test if there was anything beyond the opposite (violet) end. To do this, he separated white light into a spectrum using a prism and placed silver chloride—a chemical known to turn black when exposed to sunlight—within the different colors. Curiously, it reacted faster in violet light than in red light. He then placed silver chloride just beyond the violet light, in a place where there was apparently no light at all, and it quickly turned black. Ritter named his discovery “chemical rays,” but the moniker soon evolved into the more appealing term used today.