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Why are red beams less visible (compared to blue at same distance from 555nm)?

julianthedragon

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Here is an illustration of what I mean:

Take a 445nm and 650nm of the same power.
|445 - 555| = 110nm while |650 - 555| = 95nm, so one would expect the one closer to the 555nm peak in visibility to be brighter as seen below.
Screen Shot 2021-11-18 at 9.36.51 AM.png

However, checking off beam brightness with the same settings shows the following:
Screen Shot 2021-11-18 at 9.36.57 AM.png

Is there a reason for this? Is it because of human perception? Rayleigh scattering? Red light carrying "less energy" due to its longer wavelength?
I've noticed in person too that the beam from 30mw 450nm is more visible than 50mw at 635nm.

P.S. I did google this question and search on LPF before I asked believe it or not
 



CurtisOliver

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Longer wavelengths don't scatter as much as shorter wavelengths, therefore less photons enter your eye making it appear not as bright. Red itself is brighter than blue during daylight conditions, hence dot brightness is greater.
 

julianthedragon

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Longer wavelengths don't scatter as much as shorter wavelengths, therefore less photons enter your eye making it appear not as bright. Red itself is brighter than blue during daylight conditions, hence dot brightness is greater.
Thank you, I read it had to do with scattering but the dots didn’t fully connect for me at first. Makes sense
 

bostjan

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CurtisOliver already answered perfectly.

I just wanted to add that this is the reason why the sun is red as it sets, and partially the reason the sky is blue. Blue light scatters. A white light (the sun) looks red when it passes through a thicker layer of the atmosphere (like the low angle of sunset).
 




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