# If photons bounce off of surfaces (get reflected), why does it immediately go dark after we switch off the lights?

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If photons bounce off of surfaces (get reflected), why does it immediately go dark after we switch off the lights?

In: Physics

>ELI5: If photons bounce off of surfaces (get reflected), why does it immediately go dark after we switch off the lights?

Because only a fraction gets reflected. The rest gets absorbed by the material and turned into thermal energy. Since that absorption happens on every bounce eventually 100% of all light that does not go on an infinite journey into outer space gets absorbed at some point (sooner rather than later, since it moves at light speed).

Light travels incredibly fast. So while there are definitely light particle still bouncing around after the light is switched off it’s simply not noticeable for the brain.

Because even if they bounce the speed with which they travel makes the phenomena for us in deterministic time.

The speed of light is incredibly fast – nearly 300,000,000 metres per second.

For all intents and purposes, everything that happens with light on Earth is instantaneous. Say you have a lamp, and a mirror 5 m away from the lamp. The amount of time it takes the light to travel the 5 m between lamp and mirror is measured in nanoseconds.

The light is still travelling at the speed of light (sort of) so could bounce of 20 surfaces and we still wouldn’t be able to measure any time gap.

Each time they bounce a bit of energy is absorbed by the surface. But since they travel so fast, they can bounce an incredible amount of time in what seems instantaneous to us, and all the “light” is absorbed into the walls of the room as heat.

Think about a ball bouncing, and with each bounce the ball doesn’t come fully back to the height it was previously. Eventually the ball will barely be bouncing at all. The height of the bounce is comparable to the brightness of the room and since they bounce so quickly it appears that the light disappears instantly.

Because first of all surfaces do not reflect 100% of light. Secondly, light is FAST. Light is moving at 299.792.458 meters per second.

Now why is that important?

Because the faster something is, the more frequently it could bounce right?

For simplicity imagine photons as a collection of balls, now imagine them to be in a room where the longest distance they could travel before bouncing was 3 meters. This would mean they could do at least roughly 100.000 bounces per second.

Ok so it bounces often, now what?

Imagine the surface could reflect 99% of light:

before the bounce intensity is 1.

1 bounce -> 0.99

2 bounces -> 0.99^2 = 0.9801

3 bounces -> 0.99^3 = 0.970299

10. bounces -> 0.99^3 = 0.904382075

1000 bounces -> 0.99^1000 = 0.00004317124

So after 1000 bounces that reflect 99% of the light, only 0.004% of the light is left.
But the light is able to do 100.000 bounces per second, which means it only needs 10 milliseconds to do the 1000 bounces. That is the timespan a single frame is shown on a 100hz monitor!

On top of that, you do only ever see light that hits your eyes in the first place, this not only reduces the amount of possible light to see, but you yourself remove light with your eyes because the light you see doesn’t bounce back!