B Why does the Sun seem so bright?

[thetexan]

TL;DR Summary

Brightness of the sun

I would like to know why the sun seems so bright.

I know why the sun is a great source of light but that’s not what I’m asking. Why do my eyes perceive the brightness. Is it based on the number of photons that enter my eyeball? The more photons the brighter I perceive the light to be? Therefore a source that produces a lot of photons will provide that large number?

So, I’d guess the question is, is the difference between looking at the sun and looking at a flashlight and the corresponding difference in perceived brightness simply because of numbers or is something else involved?

Tex

 

[anorlunda]

Summary:: Brightness of the sun

The more photons the brighter I perceive the light to be? Therefore a source that produces a lot of photons will provide that large number?

Yes.

 

[hutchphd]

Pretty much it is just the number of photons. Of course thanks to evolution your eye is particularly responsive to just the mix of colors put out by the sun (which is a blackbody spectrum for 6000 degrees Kelvin)

 

[kuruman]

Your body also perceives a full bucket of water as “heavier“ when it contains more water molecules than a full cup. It’s the same idea.

 

[Ibix]

So, I’d guess the question is, is the difference between looking at the sun and looking at a flashlight and the corresponding difference in perceived brightness simply because of numbers or is something else involved?

Perceived brightness is essentially a measure of the power per unit area your retina receives (it's not quite that simple because wavelength matters and your eye's response is non-linear, but power is the heart of it). Thus the Sun is very bright because even the tiny fraction of its power that reaches your eye is still a lot of power. The lamp in a lighthouse is very bright because it uses a lot of electrical power to produce a lot of light power, but easily tolerable at distance because the beam spreads and only a small fraction enters your eye.

Lasers don't spread very much, and their coherent nature means that they focus down to a smaller spot on your retina than other light sources. Those two characteristics make them really good at delivering power to your retina at distance, which is why even quite low powered lasers can be quite dangerous to your eyesight.

 

[sysprog]

Therefore a source that produces a lot of photons will provide that large number?

As @Ibix indicated, perceived brightness depends on how much light enters the eye $-$ it doesn't depend only on how many photons the perceived object emits $-$ Sirius is the brightest-to-us star in the night sky $-$ inherently much brighter than the Sun (about 25.4 times the luminosity) $-$ it emits a far great number of photons than the Sun does, but Sirus is so much further away (about 1.7 light-years away), the Sun is so much closer, that in our perception the Sun is so much brighter as to make the difference between night and day.