• Question: How do laser beams work?

    Asked by lumiereclair to Matthew on 17 Mar 2014.
    • Photo: Matthew Malek

      Matthew Malek answered on 17 Mar 2014:

      In principle, this does just what it says on the tin; the word “laser” was originally an acronym for: “light amplification by stimulated emission of radiation.”

      Of course, maybe this particular tin isn’t so easy to read! 🙂

      I’ll try to elaborate a little: The difference between a laser beam and normal light is that the laser light is what we call “coherent”, which means that the spot is narrowly focused. Over distances, this is rather significant, as the light doesn’t spread out (unless it hits something, of course!). Light from an ordinary source, like a light bulb or an LED or the Sun, disperses. If you go twice as far from the source, you see only 1/4 as much light because it has spread out. If you go twice as far from a laser beam, you see just as much light.

      So how do we make a laser beam? You supply energy to a material (called the “gain medium”) that amplifies light of a specific wavelength. The energy is usually electrical, but can also be in the form of light. Most lasers have an optical cavity consisting of mirrors on each end of the optical cavity. Light bounces between these mirrors, passing through the gain medium over and over again. Each time it passes through, the light is amplified.

      That should give you some idea of how the laser light is produced. But if the cavity is bounded by mirrors, how do we ever get the light out? Generally, one of the two mirrors is a partial mirror. Rather than reflecting all the light that strikes it, some of the light is reflected and some is transmitted through. Thus, some of the light escapes, providing the laser beam that we see.

      There’s a lot more that can be said about lasers, like the difference between a dye laser and a diode laser. Some lasers, like the red dot in your laser pointer, give a continuous beam, whilst some (like the Nd:YAG laser I used in Argentina) provide pulses of light. I hope this answer gives you a general overview… and if you have more questions, please feel free to ask!

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