How Light Works


An illustration of a laser

An interesting application of the quantum nature of light is the laser. You can get the whole story on lasers in How Lasers Work, but we're going to cover some of the key concepts here. Laser is an acronym for "light amplification by stimulated emission of radiation," which is a tongue-tying way to describe light in which the photons are all at the same wavelength and have their crests and troughs in phase. Research physicist Theodore H. Maiman developed the world's first working laser, the ruby laser, in 1960. The ruby laser contained a ruby crystal, a quartz flash tube, reflecting mirrors and a power supply.

Let's review how Maiman used these components to create laser light, starting with the characteristics of ruby. Ruby is an aluminum oxide crystal in which some of the aluminum atoms have been replaced with chromium atoms. Chromium gives ruby its characteristic red color by absorbing green and blue light and emitting or reflecting only red light. Of course, Maiman couldn't use a ruby in its naturally occurring crystalline state. First, he had to form the ruby crystal into a cylinder. Next, he wrapped a high-intensity quartz lamp around the ruby cylinder to provide a flash of white light. The green and blue wavelengths in the flash excited electrons in the chromium atoms to a higher energy level. As these electrons returned to their normal state, they emitted their characteristic ruby-red light.


Here's where it got interesting. Maiman placed a fully reflecting mirror on one end of the crystal and a partially reflecting mirror on the other. The mirrors reflected some of the red-wavelength photons back and forth inside the ruby crystal. This, in turn, stimulated other excited chromium atoms to produce more photons, until a flood of precisely aligned photons bounced back and forth within the laser. At each bounce, some of the photons escaped, which allowed observers to perceive the beam itself.

Today, scientists make lasers out of many different materials. Some, like the ruby laser, emit short pulses of light. Others, like helium-neon gas lasers or liquid dye lasers, emit a continuous beam of light.

We're headed somewhere over the rainbow next.