Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Yair Doza on 13/12/2016 22:05:40
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I found a web site claiming that the sun emits 10 to the power of 45 photons per second. I wonder how many of them hit Earth each second? How many photons enter our eyes each second in a sunny day?
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From memory the Sun emits energy with a mass equivalent of 4.26*10^6 tons per second assuming the photons average 2eV the number comes out at approximately 10^45
But I am prepared to have my arithmetic corrected.
The eagle eyed will note I made several corrections my memory of the Suns output and the mass equivalent energy of a 2eV photon were both wrong but at least my arithmetic was OK
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If we use the 1e45 figure for photons/sec emitted by the Sun, then you get ~1.8e36 photons striking the Earth per sec and 4.4e16 photons per sec hitting the eye (assuming a pupil diameter of 2mm.)
However, the eye figure doesn't take attenuation by the Earth's atmosphere, accounting for that leaves us with ~3.4e16 photons per sec hitting the eye. This works out to 0.013 watts entering your eye if you are staring directly into the Sun (Not a good idea.)
Using the second, smaller figure would decrease the photon count by a factor of 2000 in each case (but not the wattage)
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The figure of interest is not what quantity is reaching the eye looking directly at the Sun but the quantity received when looking at typical objects illuminated by sunlight.
It is recommended that when looking directly at the Sun one should use a density 4 filter giving 40db of attenuation, is reflected Sunlight reduced by that amount ?