Just like visible light, infrared light, and radio waves, ultraviolet light is electromagnetic radiation. On the spectrum, ultraviolet light lies between violet light and x rays, with wavelengths ranging from four to 400 nanometers. Although it is undetectable to the naked eye, anyone who has been exposed to too much sunlight has probably noted the effects of ultraviolet light, for it is this radiation that causes tanning, sunburn, and can lead to skin cancer.

The man credited with the discovery of ultraviolet light is the German physicist Johann Ritter. Ritter had been experimenting with silver chloride, a chemical known to break down when exposed to sunlight. He found that the light at the blue end of the visible spectrum—blue, indigo, violet—was a much more efficient catalyst for this reaction. Experimenting further, he discovered that silver chloride broke down most efficiently when...