Why is light bright?
Brightness is your eye counting photons; more photons per second on your retina reads as brighter.
Light arrives as countable packets
Light is not a smooth fluid. It comes in discrete packets of energy called photons. A dim glow and a blinding flash differ mostly in one thing: how many photons land on you each second.
Brightness, in the everyday sense, tracks that flux. Crank up the rate of arriving photons and a surface looks brighter, even though each individual photon carries the same energy.
Your eye turns photons into a signal
This is why brightness feels relative. The eye does not report a raw photon count; it compresses an enormous range, from starlight to noon sun, into a manageable signal. Double the light and you do not feel twice as dazzled.
Color versus brightness
A single photon has a color (its wavelength) but no brightness of its own. Brightness is a crowd phenomenon. One red photon and one blue photon look like nothing; a torrent of either looks bright. Color is about the individual, brightness is about the count.