Unseen Passage: Mirrors

Mirrors have been used since ages. It is said that Archimedes used huge mirrors to focus the heat of the sun’s rays on enemy ships to burn them. The same principle is used in solar cookers, where mirrors are used to reflect and focus the sun’s rays on to cooking containers. Astronomers use huge concave mirrors to focus starlight. The mirrors used in telescopes are polished by giving a thin coat of silver or aluminium to the glass. With the telescopes, astronomers can see the heavenly bodies, but even without the telescopes one can see the moon. A full moon shining overhead is a beautiful sight. But it is also a puzzle, because if the sun and the moon both give the same light, then why is the moonlight more pleasant?

The reason is that the moon reflects very little sunlight, in fact, only seven percent of the light it receives. The earth reflects about half of the radiation that it receives from the sun. Imagine how bright the earth must be looking from the moon! One can even read a book on the moon in the earthlight. However, you do not have to travel all the way to the moon to realize how strong the earth light is. Observe the crescent moon soon after all the new moon. The bright crescent is illuminated by the sun. The rest of the moon is illuminated by earthlight. In other words, the light reflected by the earth is falling on the moon. This is being reflected back to us and is strong enough to show the moon faintly. As the crescent increases in size, this effect disappears.

Fresh snow is the best reflector of sunlight in nature. As a highly reflective substance, snow dramatically increases UV-B exposure near the Earth’s surface, as it reflects most of the radiation back into the atmosphere, where it is then scattered back toward the surface by aerosols and molecules. Fresh snow can reflect as much as 9.4 percent of the incoming UV radiation. The bright light reflected by it can dazzle and harm your eyes. Mountaineers, therefore, use dark glasses.

Q. Answer the following questions:

1. Mirrors used in telescopes are polished by which coating?
2. Mountaineers wear _________ while climbing mountains.
3. The bright light reflected by fresh snow can _________ .
4. Why did Archimedes used mirrors to focus the heat of the sun rays?
5. The earth reflects _________ of the radiation received from the sun.
   1. about one-fourth
   2. about three-fourth
   3. about two-third
   4. about half
6. The principle of focusing the sun rays is used in:
   1. Solar cooker
   2. Pressure cooker
   3. Inverter
   4. None of the above
7. The dim part of the crescent moon is lit by:
   1. sunlight
   2. satellite
   3. stars light
   4. earth light
8. With the _________, astronomers can see the heavenly bodies.
   1. telegraph
   2. telephone
   3. telescope
   4. television

Answers

• Mirrors used in telescopes are polished by Silver or Aluminium coating.
• dark glasses
• harm our eyes
• Archimedes used mirrors to focus the heat of the sun’s rays on enemy ships to burn them.
• about half
• Solar cooker
• earth light
• telescope