Bouncing photons

[Margui]

Why, when I turn the lights off, the photons don't keep bouncing off the walls and keep the room illuminated?

 

Margui

[swansont]

Well, they do, but eventually they will be absorbed, where "eventually" is a really short time. Because the speed of light is 3x10^8 m/s, the light only takes of order 10 nanoseconds to traverse a normal-sized room. If the walls absorb 50% of the light an reflect the rest, in only 10 bounces (100 nanoseconds) you have 0.1% of the light left (0.5^10 = 0.001). After 100 bounces — a microsecond — the fraction left is about 10^-30, which means essentially no photons from the light bulb survive that long.