Most of us were introduced to lasers by science fiction: “phasers” in Star Trek or “blasters” in Star Wars - but how familiar are most of us with these spectacular beams beyond the scope of fantasy?

Simply put, a laser beam is emitted light, but it is very different than the light provided by other sources such as a reading lamp. Rather than being emitted as scattered light, laser light is organized into tightly focused beams by a process called stimulated emission. Three components are responsible for this phenomenon — an optical resonator, a pump source to provide energy (e.g. a short burst of intense light), and a laser medium that’s either a gas, solid or liquid material. The pump source excites the laser medium and causes its atoms to release energy in the form of light (photons). This light then bounces back and forth between a pair of mirrors in the optical resonator, causing even more photons to be released (i.e. we get amplification of the light). A partially-reflective mirror at one end allows some of these photons to escape as a concentrated beam of laser light.

Did you know? The term “Laser” is an acronym. L = Light, A = Amplification, by, S = Stimulated, E = Emission, of, R = Radiation

So why do lasers come in different colours? And are some laser beams visible, while others aren't?

To answer these questions we need to take a look at the electromagnetic spectrum, which displays all possible wavelengths of light. Light with wavelengths between 200 and 10 600 nm is capable of becoming a laser. The human eye is only responsive to light in the visible spectrum (400 to 750 nm), so in fact, we can see BOTH green (495-570 nm), and red (620-750 nm) lasers. However, our eyes are more sensitive to light in the yellow/green (555 nm) range, so green laser beams appear brighter than red ones. But not all laser beams are visible. Since the laser spectrum is more broad than the visible spectrum, there are some lasers that we cannot see, such as ultraviolet (UV) lasers (200-400 nm) and infrared (IR lasers) (750-10,600 nm).

Did you know? In 1964 Townes, Basov and Prokhorov shared the Nobel Prize in Physics for their work which led to the construction of lasers.

The wavelength of emitted laser light is based on the laser medium. For example, helium-neon gas lasers generate radiation at 633 nm (visible and green). These are weak lasers and used in today’s laser pointers. On the other hand, lasers that employ CO2 as a lasing medium emit light that is in the IR spectrum, producing very powerful and dangerous lasers that can cut through steel!

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Article first published February 25, 2011.

Chelsea Nimmo

I completed my BSc at McGill University, majoring in biochemistry, and am currently a graduate student at the University of Toronto in the Department of Chemistry working on tissue engineering of the retina using stem cells. When I am not in the lab, I enjoy going to yoga and cardio classes, reading chick literature, and shopping!

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