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-------------------- 2498 - MIRROR - how does it work, really?
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- Light is an electromagnetic wave. When electromagnetic energy hits a substance made of atoms the energy is either reflected or absorbed. The electromagnetic energy absorbed is going to put the electrons in the atoms at a higher level of energy. These electrons in the silver atoms have to stay in the silver because the glass is an insulator.
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- Because there is only a thin layer of metal in which the electrons can move, the electromagnetic energy is quickly reduced and there is only a small loss of energy. The wave amplitude decays very quickly in the silver, usually within a fraction of a wavelength.
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- A wavelength is about 600 nanometers. The small loss of energy in the silver does result in some heating of the metal. Very small. Most of the optical power is reflected at the air-metal surface. In other words the optical power is transferred to another optical wave traveling in the opposite direction.
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- This wave transfer occurs because the optical properties of the air are much, much different that the properties of the metal or the glass. Electromagnetic waves experience significant reflection at interfaces between media with substantially different propagation properties.
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- The optical power loss in a silver mirror is only a few percent.
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- These small losses in power, and slight heating of metal are no problem for bathroom mirrors. However, they become an engineering challenge for laser mirrors. Heating could cause the slightest bulging of the mirror surface which could in turn defocus a laser beam.
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- So, for lasers, engineers use dielectric mirrors. These consist only of non-conductive materials, that is insulators. A laser mirror is constructed of 15 pairs of silica, SiO2, and titanium dioxide, TiO2, layers only a few hundred nanometers thick.
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- The reflection of each layer is rather weak, but, after 15 layers these dielectric mirrors can reflect laser light with 99.9% efficiency. The trade-off is that the dielectric mirror is only effective at the very limited frequency of the laser light. Different mirrors must be constructed for different color lasers.
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- These dielectric mirrors , since they are frequency selective, can be designed to say reflect 80% of the green light, transmit 20% and transmit 100% of the red light.
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- Looking at this mirror from the quantum-mechanical perspective the photons reflected from the mirror are identical to the photons incident on the mirror, apart from the propagation direction.
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- The heating means that some fraction of the photons are lost. The ratio of reflection to absorption is all a matter of probabilities for each photon. If the photon is absorbed it becomes a “phonon” in the metal. A phonon is an oscillation or a type of vibration, or sound wave in the metal. Thermal energy is vibrating atoms. A phonon would be a wave of vibrating atoms.
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- One interesting aspect of engineering mirrors for telescopes is to shape the mirror into a parabolic surface so that all parallel light gets focused to a single point. Glass mirrors need to be ground and polished into their parabolic shape. But, an interesting idea is to simply slowly rotate a bowl of liquid mercury.
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- It is an interesting mathematical model that a rotating disk confined with sides dips in the center to produce a perfect parabolic surface.
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- Such telescopes already exist. There is a 6 meter telescope in Vancouver, Canada, and a 3 meter in NASA’s Orbital Debris Observatory in New Mexico.
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- Mercury freezes at Moon temperatures. An idea in the works is this may be a great way to construct a large telescope on the Moon. Once the mercury was warmed and spun into the right shape it could be frozen and become a permanent mirror.
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- Liquid mirrors usually have the limitation that they can only be pointed straight up. Unlike the standard telescope that can be pointed in many directions. However, engineers are working with a high viscosity hydrophilic liquid that can be tilted up to 10 degrees with out distortion.
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- Next they are working on metal-liquid-like films that are thin-reflective layers of self -assembling metallic nanoparticles. That is a mouth full. But, if successful these engineers could be designing large ultra-cheap telescopes that every one could use for backyard astronomy. You could build your own liquid mirrors right at home.
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- And, now you know how a mirror works.
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- November 27, 2019 2498 1161
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--------------------- Wednesday, November 27, 2019 --------------------
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