All our fossil fuels, such as coal and oil, owe their existence to the
solar energy stream that has engulfed the earth for billions of years.
The power in this stream amounts to about 1400 watts per square meter at
the earth, nearly enough to supply an average home if all the energy
were converted to electricity. The problem is to get the sun’s rays to
yield up their energy with high efficiency.
The sun’s visible surface has a temperature around 6000°K. Any object
heated to this temperature will radiate visible light mostly in the
yellow-green portion of the spectrum (5500 A[11]). Our energy conversion
device should be tuned to this wavelength.
The energy packets arriving from the sun are called photons. They
travel, of course, at the speed of light, and each carries an amount of
energy given by