Sunshine 101

Posted Aug 28, 2011 by Nicole Yunger Halpern

Updated Aug 28, 2011 at 11:48 AM

Because I’m a Floridian, many assume, I suntan on the beach.

Because I study physics, I have no time to suntan on the beach. I spend too much time studying sunscreen.

Not that I slather sunscreen on lab rats, dump them on a beach, and measure their sunburns. Far be it from a theoretical physicist to perform such a practical experiment. I have, though, studied light. One sunny spring afternoon, I realized that my coursework illuminated my Floridian upbringing.

If you’ll pardon my puns, I’ll try to shed light on sunshine.

Light has properties that waves have. Imagine standing in the ocean, up to your waist in water. A foamy white crest appears yards away, whooshes up to you, and soaks your chest. Another wave appears, whooshes, and soaks. Then another. How many waves soak you in one minute? We call that number the “wave frequency.” Like ocean waves, light waves have frequencies. The higher a light wave’s frequency, the more energy the light carries.

Different species of light have different frequencies and energies. The set of all light species is called the electromagnetic spectrum. Radio waves and TV waves occupy the low-frequency end of the spectrum. Strolling from the low- toward the high-frequency end, we encounter infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

Let’s back up a few steps. The visible part of the spectrum contains the colors we can see. Remember meeting Roy G. Biv in elementary school? Mr. Biv’s name is an acronym for colors in the visible part of the spectrum: red, orange, yellow, green, blue, indigo, violet. The acronym helps us memorize the colors in order from lowest- to highest-frequency. (When we want to feel impressed with our knowledge, the acronym helps us remember “a segment of the electromagnetic spectrum.”)

More sunlight belongs to the visible species than to any other species. A small portion is ultraviolet (UV). Sunscreen bottles tout their contents’ protection against ultraviolet light. Ultraviolet light has higher frequencies than visible light; it carries more energy. UVC light carries more energy than UVB, which carries more energy than UVA.

Why should we screen ourselves from high-energy light?

Let’s return to the beach, where you’ve been counting wave frequencies. Only physicists would do something as boring as count them for more than ten minutes. You decide to build a sandcastle. Two hours later, a miniature Taj Mahal graces the beach. (You turn out to be quite the architect.) A miniature emperor and empress, and miniature servants, move in. The tide rises. Ocean waves creep higher and higher onto the beach. Soon, waves will hit your palace.

If the waves carry little energy, the waves might not destroy the palace. (Pretend that sand doesn’t dissolve in water.) The servants can continue serving, and the emperor can continue emper-ing. If water slams your palace like a lance in a joust, it’ll disturb the palace’s residents.

In place of ocean waves, imagine light waves; in place of the Taj Mahal, a skin cell; in place of rulers and servants, molecular machines that produce energy, signal other cells, and govern the cell’s life cycle. If low-energy visible light impinges on the cell, the cell can survive. If high-energy ultraviolet light slams the cell, the molecular machines might malfunction. Cancer could invade our cellular Taj Mahal.

How can we defend our castle? By imitating my friend John. Before discovering physics, I had time to play with John on the beach. Like you, John built a sandcastle. The tide rose. Fearing it would obliterate his masterpiece, he piled sand between the ocean and the sandcastle. This Great Wall of John absorbed the waves’ energy. It protected the castle (until we left the beach, I imagine, and a wind collapsed the castle or a seagull dumped on it).

Sunscreen behaves like the Great Wall of John. It defends Castle Skin Cell from the Dragon of Ultraviolet Light. By applying sunscreen, you up your chances of living happily ever after.

The storybook of sunlight contains tales other than that of sunscreen. A chapter by quantum physicist Max Planck sounds like a Michael Crichton novel: “The Ultraviolet Catastrophe.” “Dualism”—light’s exhibition of not only waves’ properties, but also particles’—flummoxed physicists for centuries. “Optics” governs those snapshots of galaxies by the Hubble Telescope.

Not typical beach reading. But don’t let my enthusiasm distract you from enjoying the sand. When faced with physics, I’m just a little ray of sunshine.