Истории и востоковедения
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- Бұл бет үшін навигация:
- Here is a puzzle
| C. Activities
Read, discuss this text (in pairs) and try to explain this puzzle: Mirrors exist in many forms. An important mirror for radio waves is the ionosphere; especially during the night, when certain absorbing layers disappear, the ionosphere allows receiving radio stations from far away. When the weather is favorable, it is possible to receive radio stations sending from the antipodes. Another radio mirror is the Moon; with modern receivers it is possible to receive radio signals and, since a few years, even television signals reflected by the Moon. Why do metals provide good mirrors? Metals are strong absorbers of light. Any strong absorber has a metallic shine. This is true for metals, if they are thick enough, but also for dye or ink crystals. Any material that strongly absorbs a light wavelength also reflects it efficiently. The cause of the strong absorption of a metal are the electrons inside it; they can move almost freely and thus absorb most visible light frequencies; this leads to evanescent waves in the material and strong reflection. Strong reflection appears as soon as the absorption length is as low as about one wavelength. This is the reason that, for example, strong coffee, strong tea and dense alkali vapor work as mirrors. (However, strong reflection is also possible without strong absorption, as the ubiquitous dielectric multilayers show.) 84 Here is a puzzle: a concave mirror shows an inverted image; so does a plane mirror if it is partly folded along the horizontal. What happens if this mirror is rotated around the line of sight? (Adopted from www. motionmountain.net ) |