What light is
Light is an electromagnetic wave: oscillating electric and magnetic fields that travel through empty space at the speed of light, about 300,000 kilometers (186,000 miles) per second. Visible light covers wavelengths from roughly 400 nanometers (violet) to 700 nanometers (red). Beyond those ends lie ultraviolet and infrared, and further out, X-rays and radio waves.
Light also comes as photons, packets of energy tied to the wave’s frequency. Both descriptions are needed. The wave picture explains bending, interference and color; the photon picture explains what happens when light is absorbed or emitted by atoms. Both matter for understanding crystals and lasers.
What happens when light enters a crystal
A crystal is a solid whose atoms are arranged in a regular repeating pattern. When light reaches one, three things can happen, and often more than one at once: it can be reflected, absorbed, or transmitted.
Refraction. Light travels more slowly inside a transparent material than in a vacuum. The ratio is the material’s refractive index. When a beam enters at an angle, the change of speed bends its path. Diamond has a high refractive index, about 2.4, compared with about 1.5 for ordinary glass and 1.33 for water. A high index, together with careful cutting, helps a diamond bend and reflect light inside it, which gives its sparkle.
Dispersion. The refractive index varies slightly with wavelength, so a crystal can spread white light into colors, the same effect behind a prism.
Double refraction. In some crystals, such as calcite, atoms are arranged differently along different directions. Light polarized in different ways travels at different speeds, so a single beam splits into two. Looking through a clear calcite crystal at a line of text shows a doubled image. Polarizing filters and some optical instruments exploit this kind of property.
Why some crystals are clear and others colored
Whether a crystal absorbs a photon depends on its electrons. In a solid, electrons occupy ranges of allowed energies called bands, separated by a gap. A photon can be absorbed only if its energy is enough to lift an electron across that gap or to another available state.
- If the gap is larger than the energy of visible photons, as in quartz, visible light passes through and the crystal looks clear.
- If the gap is small, as in silicon, visible light is absorbed and the material looks opaque and metallic.
- Small amounts of impurities or defects can create new energy levels that absorb particular colors. Rubies are red largely because of a small amount of chromium in aluminum oxide, and the same crystal host without that impurity is colorless sapphire.
So much of a crystal’s appearance comes directly from quantum energy levels.
Spontaneous and stimulated emission
An atom that has absorbed energy sits in an excited state. It can return to a lower state by emitting a photon. If it does so on its own at a random moment, that is spontaneous emission, and it is how ordinary lamps and glowing screens produce light. Each atom emits independently, so the light has mixed phases, directions and often many colors.
In 1917, Albert Einstein pointed out that a passing photon of the right energy can also trigger an excited atom to emit a second photon. This is stimulated emission. The new photon has the same wavelength, travels in the same direction and is in step with the first one. Each stimulated emission adds a matching photon to the beam.
How a laser works
The word laser stands for light amplification by stimulated emission of radiation. A laser has three basic parts:
- A gain medium, such as a crystal, a gas, or a semiconductor, containing atoms that can be excited.
- A pump, an energy source such as a flash lamp, an electric current or another light, which excites many atoms into a higher state. For amplification, more atoms must be excited than are in the lower state, a condition called population inversion.
- An optical cavity, typically two mirrors facing each other, one partly transparent. Photons bounce back and forth through the medium, triggering more stimulated emission each pass. Some light leaks out through the partly transparent mirror as the beam.
The first working laser was built in 1960 by Theodore Maiman, who used a synthetic ruby crystal pumped with a flash lamp, producing red light. That brings the story from crystals full circle: the same chromium energy levels that color a ruby made it a gain medium.
What makes laser light different
Laser light is usually described by three properties:
- Nearly single wavelength, so it is close to one pure color.
- Coherent, meaning its waves are in step, which allows precise interference effects.
- Highly directional, so the beam spreads very little over distance.
A flashlight beam widens quickly and mixes many wavelengths, while a laser pointer stays narrow. Lasers are not necessarily powerful. The pointer is weak, but the properties above are what make it useful.
Where lasers are used
Common uses include barcode scanners, fiber-optic communication, laser printers, surveying, and various medical and industrial procedures. Semiconductor lasers, the kind in small devices, are tiny crystals in which electric current produces the inversion, so they depend on band-gap physics much like the ones described above.
Even low-power lasers can injure eyes, and pointing one at a person, aircraft or vehicle is dangerous and often illegal. Safety guidance from manufacturers should be followed for any laser device.
The connecting thread
Crystals and lasers are two sides of the same physics: light interacting with the energy levels of atoms in a solid. Refraction and color show what happens when light passes through such a material. A laser shows what happens when that interaction is arranged so atoms add to a beam instead of just absorbing from it.