A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Can you tan through glass?

September 2, 1983

Dear Cecil:

Sitting here on the sun deck, we've been involved in a heated debate for the past several weeks. It seems these midwesterners and New Yorkers have some distorted perceptions about the capacities of the sun's rays. Those of us from the south and southeast, having far more experience and innate wisdom, know better.

You see, they insist that absolutely no ultraviolet rays can pass through the plain clear glass of a car window. I have apparently hallucinated the tan or burn I've noticed after driving across the desert. And, according to them, sun lamps either don't give off ultraviolet rays or the bulb is not glass. Please enlighten my poor misguided friends. In return, I'll be happy to share with you the lunch I win from this wager.

Cecil replies:

Good thing I'm on a diet this week, Alice, because you lose. For what it's worth, those other characters don't know what they're talking about either.

To summarize briefly: some ultraviolet rays can pass through car window glass, although most of the burning (as opposed to tanning) rays can't. On the other hand, most sunlamps aren't made of ordinary glass--they're made of quartz or special UV-transparent glass.

Having thus confronted our ignorance, let us humbly endeavor to learn. Ultraviolet light, which causes both erythema (sunburn) and tanning, ranges in wavelength from 4,000 angstrom units (A) down to about 100 A. (Light with wavelength greater than 4,000 A lies in the visible spectrum.) The most potent rays for burning and tanning lie in the 2,900-3,050-A range, with radiation of 2,967 A supposedly being most effective of all.

Ordinary window glass, however, is pretty much opaque to wavelengths below 3,000 A. From this we deduce that the intervention of a window will significantly reduce but not halt the burning/tanning process.

In addition, UV rays above 3,200 A will cause tanning (but usually not burning) if administered in sufficiently massive doses, such as you get when you're driving across the desert. Your tan, therefore, was no hallucination. This business about your innate wisdom I'm not so sure about.

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