Spectrum produced by a fluorescent lamp (left) and by a HeNe laser (right). Specifically, Blu-rays use 405 nm light, while CDs and DVDs use 780 and 650 nm respectively. The reason Blu-rays can store more data than CDs or DVDs is because the laser used to read the disc is blue, hence the name. Different applications have different wavelength requirements – for example, optical disc drives use lasers to read information off of CDs, DVDs, and now Blu-ray discs. Wavelengths of visible light range from about 400 nm to 750 nm – see Andy Berger’s post for an idea of how small that is.) Lasers can be made with a wide variety of wavelengths, ranging from 150 nm to over 1 mm. (The color of light is determined by its wavelength. Unlike, say, a light bulb, a laser produces light that is almost completely the same color. The feature that distinguishes a laser from other light sources is the laser’s coherence. This process results in a steady stream of highly coherent photons.īasic design of a laser – in this case, the flashlamp provides the energy source, the Nd:YAG crystal is the gain medium, and the two mirrors form the optical resonator. Finally, the optical resonator – at its simplest a pair of mirrors at either end of the laser – passes the photons back and forth through the gain medium, so that each photon produces many additional photons before escaping the resonator. This is why the word “gain” is used, because you start with one photon and end up with two. The photons pass through the gain medium, and each photon interacts with the medium and has a chance to produce another photon traveling in the same direction as the original. First, the energy source generates the first few photons to get the laser started. Lasers can be made with many different gain mediums, including dyes, gasses, and solids. In that case, the gain medium is a mixture of helium (He) and neon (Ne) gas, which produces a red beam. The gain medium is what makes it possible to generate the laser beam and determines the color of the light produced this is what people are referring to when they say, for example, “ HeNe laser” (say hee-knee). Essentially, lasers consist of an energy source, a gain medium, and an optical resonator. The word “laser” originated as an acronym, standing for light amplification by stimulated emission of radiation. Light behaves both like a wave and like a particle, and the individual light particles are called photons (the idea of light as particles originated with Einstein, and is actually the work for which he won the Nobel Prize). Physicists use the term “light” to refer to all electromagnetic radiation, which includes the visible light that we see as color, but also things like radio waves, microwaves, and x-rays. Before I talk about lasers, I need to talk a little about light.
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