Mary Louise Riale, Age 11, Of Bala Cynwyd, Pa., for her question:
What are ultraviolet rays?
In ordinary light you may see a sample of calcite as a bumpy lump of dirty white rock. The ultraviolet rays from a quartz lamp can transform this drab mineral with glowing jewel tones of pinks and purples, blues and greens. However, the ultraviolet rays themselves are invisible to human eyes.
Our starry sun is a powerhouse of electromagnetic energy. All forms of this radiant energy travel at the fantastic speed of about 186,000 miles a second. If a low flying sate11ite could circle the earth at this speed, it could pass overhead 300 or 400 times in a single minute. Radio, infrared, ultraviolet and ordinary white light are different parts of this fabulous electromagnetic energy. They differ because they travel with different wave lengths.
Wave lengths are like pulses of energy with crests and dips somewhat like perfectly even waves of water. Each form of radiant energy pulses along on its own wave length, which is measured by the distance from crest to crest. Some wave lengths are yards and some even miles long. But the wave lengths of visible white light are measured in angstrom units and there are 254 million angstrom units in an inch.
The wave lengths of the rainbow spectrum of light range from 3800 to 7800 angstroms. The wave lengths of ultraviolet rays are shorter than the shortest violet rays of visible light. They range from 3800 to 100 angstroms and are invisible to the human eye. Ultraviolet is ca11ed black light or invisible light, and though we cannot see it directly we sometimes see and feel the results of this form of electromagnetic energy.
Ultraviolet radiation can be both good and bad for the human body. It can give us a gentle sun tan or a dangerous sun burn. It can destroy germ cells and may be used to purify food and water and to sterilize the air in an operating room.
But too much ultraviolet can damage skin and flesh, and it is a good thing that much of this radiation from the sun is filtered out by the atmosphere before it reaches the earth.
A glass prism absorbs most of the ultraviolet rays in white light, but a prism of glassy quartz lets many of them through. Though invisible, they can be detected on film and they leave their marks just beyond the shortest violet rays of the rainbow colored spectrum.
F1uorescent lamps can change the invisible rays of ultraviolet into a variety of colored lights. The ultraviolet is created when an electric current passes through mercury vapor sealed inside the glass bulb. A chemical coating on the inside of the bulb changes this invisible energy into visible light. The wonderful array of colored fluorescent lights is created by different chemical mixtures on the inside of the bulbs.