Image of the sun
|Lenses and mirrors can be used
to concentrate sunlight. All schoolboys know that a magnifying glass
can focus sunlight into a small spot to create very high temperatures.
Concentration is not creation, of course. A 2-inch diameter magnifying
glass concentrates the sunlight into a small spot, but it also leaves a
2-inch diameter "shadow" where sunlight is not present.
The small spot is nothing more and nothing less than an image of the sun. The larger the lens, the more sunlight it gathers, and presumably the hotter the spot. However, large lenses usually have longer focal lengths, thereby making larger images of the sun. The more spread out the image, the cooler it is.
But temperature and heating power are two different things. A small lens may cause as high temperatures as a large one, but the large one invariably gathers more sunlight. Usually, solar applications require lots of power, and that means the ability to gather lots of light. For high-temperature applications, concentrators are necessary, but for low-temperature applications (such as house heating or water pre-heating), the concentrators may be an unnecessary expense.
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