Where does the light go when the switch is turned off?

Adam Murphy · « **on:** 12/05/2020 17:46:09 »

Norman got in touch to ask:

"I am sitting in the room with the light on. I switch off the light. Where does the light go?"

Any thoughts?

alancalverd · « **Reply #1 on:** 12/05/2020 17:52:10 »

Out.

Bored chemist · « **Reply #2 on:** 12/05/2020 18:18:39 »

The light is absorbed by the surfaces in the room (and, to a tiny extent, by the air).
It warms them up slightly while the light is on.

yor_on · « **Reply #3 on:** 12/05/2020 19:49:53 »

Slightly inebriated, but I do agree with you. Where does it go?
All of what we do 'release' energy', doesn't it?

yor_on · « **Reply #4 on:** 12/05/2020 19:53:23 »

One way to see it is about transformations and entropy. In that case nothing is lost, it just transforms. Which bring us to a question. What does photons transform into?

Bored chemist · « **Reply #5 on:** 12/05/2020 20:09:52 »

Quote from: yor_on on 12/05/2020 19:53:23

One way to see it is about transformations and entropy. In that case nothing is lost, it just transforms. Which bring us to a question. What does photons transform into?

Typically phonons.

yor_on · « **Reply #6 on:** 12/05/2020 20:14:36 »

Heh, maybe?
I don't know

What is 'energy'?

yor_on · « **Reply #7 on:** 12/05/2020 20:16:40 »

Yeah, maybe BC. Heat?
But that's just a different frequency, isn't it?

Bill S · « **Reply #8 on:** 12/05/2020 20:44:09 »

Am I right in thinking that a phonon is a quantum of vibrational energy?

If so, is it; like the photon; a particle with no rest mass, but having relativistic mass when in relative motion?

Bored chemist · « **Reply #9 on:** 12/05/2020 21:03:06 »

Quote from: yor_on on 12/05/2020 20:14:36

What is 'energy'?

The capacity to do work.

yor_on · « **Reply #10 on:** 12/05/2020 22:13:26 »

You can do better BC

Bored chemist · « **Reply #11 on:** 12/05/2020 22:18:15 »

Quote from: yor_on on 12/05/2020 22:13:26

You can do better BC

It rather depends on the audience I'm addressing.
What's your guess of the OP's understanding of phonons?

yor_on · « **Reply #12 on:** 12/05/2020 22:50:06 »

Actually BC, phonon''s are intriguing :)
but if you're going to be serious, its also make you question everything you think you know.

evan_au · « **Reply #13 on:** 12/05/2020 22:51:50 »

Quote from: OP

"I am sitting in the room with the light on. I switch off the light. Where does the light go?"

If your window is open, some of the light from your room will reach space, and in 1 second reach the Moon - or even travel for millions of years before it hits something and is absorbed.

yor_on · « **Reply #14 on:** 12/05/2020 22:56:36 »

The point here, is that I need your ideas

alancalverd · « **Reply #15 on:** 12/05/2020 23:17:43 »

Energy is a parameter that is conserved.

The energy of visible photons is generally absorbed by matter and degraded to heat - the kinetic energy of the constituent atoms and molecules of the absorber. Some photons initiate chemical reactions (initially supplying energy to the binding electrons of molecules) or eject photoelectrons.

Colin2B · « **Reply #16 on:** 12/05/2020 23:27:45 »

Quote from: Adam Murphy on 12/05/2020 17:46:09

Norman got in touch to ask:

"I am sitting in the room with the light on. I switch off the light. Where does the light go?"

Any thoughts?

One way to look at it is to think of a hosepipe. When you turn off the tap (light bulb) the water (light) stops coming out. The water that has just left the pipe hits the ground and either gets absorbed or bounces off.

alancalverd · « **Reply #17 on:** 13/05/2020 12:40:26 »

Or you could simplify it further and say "the light doesn't go. It just stops coming".

Mention of phonons is not strictly sufficient. A phonon is a quantised collective oscillation of condensed matter, whereas much photon energy is absorbed into random movement of atoms and molecules - white noise rather than pure tones.

Bill S · « **Reply #18 on:** 13/05/2020 13:11:39 »

Quote from: Alan

A phonon is a quantised collective oscillation of condensed matter,

Is that consistent with being a quantum of vibrational energy?

alancalverd · « **Reply #19 on:** 13/05/2020 13:24:27 »

Yes, but it doesn't imply that vibrational energy is always quantised!

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Where does the light go when the switch is turned off?

Adam Murphy (OP)

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