# Is a monochromatic, or non-monochromatic light visible at greater distance?

Just what the title states.

Assuming identical conditions, excellent visibility - If a 1W monochromatic light source, and a 1W non-monochromatic light source were viewed at a location in deep space, which of the two would be visible from a greater distance?

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Are you asking about sensitivity or scattering? –  Ron Maimon Aug 31 '12 at 3:23
Not sure I understand what is asked. Could you put it in a layman's terms please? –  Everyone Aug 31 '12 at 17:52

Actually, that all depends on the wavelength frequency of your monochromatic light source.

Obviously, the $1/r^2$ falloff of the intensity is the same for both cases. What is more important here is the sensitivity of the human eye to specific colors. As you can read here, the human eye is most sensitive for light around 550nm, which is yellowish (makes sense, since the Sun is also yellow).

Somewhat simplified: your 1W panchromatic light source will have less energy in the neighbourhood of the 550nm wavelength, if the 1W monochromatic light source will be emitting closely to or on this specific wavelength.

Therefore:

• if the monochromatic light source is yellow, you will be able to see it further out than a panchromatic source.
• If the monochromatic one is deep red, the panchromatic source will win.
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