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Humans see only a narrow band of light wavelengths. Many animals see much deeper into infrared. Maybe one clue to explain why we don't see IR naturally, has to do with the light intensity as a function of wavelength? How would such a function look like on Earth in daylight and at night respectively?

I suppose that shorter wavelengths are less intense, because they are harmful. But what about IR, is it more or less intense than visible light in daylight outdoors?

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  • $\begingroup$ It has a black body distribution that does indeed peak within the visible spectrum. $\endgroup$ – lemon Feb 26 '15 at 10:59
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This is the plot of sunlight, red at ground level.

solar irradiance

Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.

As all light comes from the sun during daylight this should suffice. One can get the number of photons by dividing the energy by the energy of a photon at that wavelength, using the relation E=h*nu

It is more esoteric to find night irradiance spectra, here is a publication for a specific site. In fig 7, it has the ratio of the relation of day and night irradiance at that site. For most of the spectrum it is rather constant, and there is a loss at small wavelengths.

Number of photons: As in the plot there is a factor of five between small and high wavelengths, in number of photons the plot would gradually increase towards the right and show a distorted shape.I do not see the reason to be thinking of "number of photons" when it is the intensity in the energy of the electromagnetic wave that is important.

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