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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Adam Murphy on 12/05/2020 17:46:09

Title: Where does the light go when the switch is turned off?
Post by: 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?
Title: Re: Where does the light go when the switch is turned off?
Post by: alancalverd on 12/05/2020 17:52:10
Out.

Title: Re: Where does the light go when the switch is turned off?
Post by: Bored chemist 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.
Title: Re: Where does the light go when the switch is turned off?
Post by: yor_on 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?
Title: Re: Where does the light go when the switch is turned off?
Post by: 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?
Title: Re: Where does the light go when the switch is turned off?
Post by: Bored chemist on 12/05/2020 20:09:52
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.
Title: Re: Where does the light go when the switch is turned off?
Post by: yor_on on 12/05/2020 20:14:36
Heh, maybe?
I don't know

What is 'energy'?
Title: Re: Where does the light go when the switch is turned off?
Post by: yor_on on 12/05/2020 20:16:40
Yeah, maybe BC. Heat?
But that's just a different frequency, isn't it?
Title: Re: Where does the light go when the switch is turned off?
Post by: Bill S 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?
Title: Re: Where does the light go when the switch is turned off?
Post by: Bored chemist on 12/05/2020 21:03:06
What is 'energy'?
The capacity to do work.
Title: Re: Where does the light go when the switch is turned off?
Post by: yor_on on 12/05/2020 22:13:26
You can do better BC
Title: Re: Where does the light go when the switch is turned off?
Post by: Bored chemist on 12/05/2020 22:18:15
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?
Title: Re: Where does the light go when the switch is turned off?
Post by: yor_on 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.
Title: Re: Where does the light go when the switch is turned off?
Post by: evan_au 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.
Title: Re: Where does the light go when the switch is turned off?
Post by: yor_on on 12/05/2020 22:56:36
The point here, is that I need  your ideas
Title: Re: Where does the light go when the switch is turned off?
Post by: alancalverd 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. 
Title: Re: Where does the light go when the switch is turned off?
Post by: Colin2B on 12/05/2020 23:27:45
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.
Title: Re: Where does the light go when the switch is turned off?
Post by: alancalverd 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.   
Title: Re: Where does the light go when the switch is turned off?
Post by: Bill S 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?
Title: Re: Where does the light go when the switch is turned off?
Post by: alancalverd on 13/05/2020 13:24:27
Yes, but it doesn't imply that vibrational energy is always quantised!
Title: Re: Where does the light go when the switch is turned off?
Post by: Bill S on 13/05/2020 13:47:01
What might be an example of non-quantised vibrational energy?
Title: Re: Where does the light go when the switch is turned off?
Post by: alancalverd on 13/05/2020 14:32:58
The motion of molecules in air or water. They are substantially independent and interacting with one another at random, so any molecule can have any value of kinetic energy at any time. The phonon concept has some validity in liquids, particularly those that demonstrate a degree of order ("liquid crystals") but is principally used to model the behavior of solids.

The term "vibrational energy" is a bit misleading. We generally model phonons as superimposed simple harmonic waves propagating in a solid, so at any instant each atom or molecule has some kinetic and some potential energy associated with each wave element.
Title: Re: Where does the light go when the switch is turned off?
Post by: Bill S on 13/05/2020 18:45:32
Thanks Alan. Weve been in this general area before; but its always good to add to existing notes. 

https://www.thenakedscientists.com/forum/index.php?topic=71490.msg524077#msg524077    #5

Quote
Unlike the atomic orbitals which are discrete, the phonon spectrum can be broad and continuous over a large frequency range.