The Naked Scientists

The Naked Scientists Forum

Author Topic: When light leaves a prism, does it change speed?  (Read 22514 times)

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
When light leaves a prism, does it change speed?
« on: 10/08/2009 12:33:00 »
Dearest Prismologosts,

As a sheepy I of course luff prisms. From the moment I awake in the mornings till the last thing at night, all I think about are prisms !

Look here's one:


A Prism Doing Prism Stuff !

Nice eh ? A big one being delivered next Tuesday for me to climb upon and jump off of.

Prisms are great because they slow light down make rainbow stuff !



I have a few kweschuns about prisms.

When light leaves a prism does it speed back up to very fast ?..If so...what propels it ? (Hmm..I think I might have asked this once before..I expect 'embarrassment mode' to kick in soon when somone provides the link to my earlier attempt at asking that question)

When the light leaves the prism is there a way to mix it back up to make white light ?

Do the different coloured lights travel at different speeds ?

Why is it called a Prism ?..is it some kind of Latin stuff  or wot ?



Ewe see...it's Monday and it 12:33 and I just do not know !...do ewe know ?



Hugs & shmishes



mwah mwah mwah !






neil
Where Does Criminal Light Go To ?...it goes to Prism !!...(LOL....*le groan)*







« Last Edit: 08/02/2010 08:49:06 by chris »


 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #1 on: 10/08/2009 13:31:56 »
Light always travels at "the speed of light" in a vacuum, i.e. when it isn't hitting many particles.  When it enters the prism, it hits a lot of particles of glass and therefore gets effectively slowed down.  (The exact mechanism is complicated.)  When it gets back into the air, it isn't hitting nearly as many particles, so it goes fast again.

One of the effects of the glass is that is that different wavelengths of the light slow down different amounts.  Different wavelengths correspond to different colors, so different colors travel at different speeds in the prism.  The colors get separated because the way light bends at the surface depends on its speed, so different colors have different speeds in the glass, and get bent into different directions.
 

lyner

  • Guest
Re: When light leaves a prism, does it change speed?
« Reply #2 on: 10/08/2009 13:48:14 »
The apparent problem involved with "speeding up" is not really a problem. Light has no mass so you don't have to give it a push for it to speed up when it is back in air.
 

Offline krytie75

  • Full Member
  • ***
  • Posts: 57
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #3 on: 10/08/2009 19:09:13 »
I read somewhere that when light travels through a medium, it is infact being absorbed by the atoms of the medium and re-emitted by them right away, in effect passing the light beam down a chain of atoms until it reaches the end of the medium and re-enters the air/vacuum/whatever.  If this is the case, when the light is re-emitted by the final set of atoms, it will be re-emitted at the speed of light and hence continue at the speed of light.

That's my theory, if someone could confirm or deny this that' be great.

Jon
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Re: When light leaves a prism, does it change speed?
« Reply #4 on: 10/08/2009 22:53:15 »
Light is a wave motion in which proagating energy is oscill;ating between two different forms of energy storage.  the propagation velocity of all wave motions is deopenant on the "elasticity" of the medium only and changes in either direction as the medium changes.

It is possiblr to partially recombine the split light backinto white light with a prism in the reverse sense.  This is the way achromatic lenses are made.

The expression prism refers to the shape of a rod with a triangular cross section.

 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #5 on: 11/08/2009 12:44:48 »
Light always travels at "the speed of light" in a vacuum, i.e. when it isn't hitting many particles.  When it enters the prism, it hits a lot of particles of glass and therefore gets effectively slowed down.  (The exact mechanism is complicated.)  When it gets back into the air, it isn't hitting nearly as many particles, so it goes fast again.

One of the effects of the glass is that is that different wavelengths of the light slow down different amounts.  Different wavelengths correspond to different colors, so different colors travel at different speeds in the prism.  The colors get separated because the way light bends at the surface depends on its speed, so different colors have different speeds in the glass, and get bent into different directions.

Gosh..Thank ewe very much jpetruccelli Fascinating that the different coloured lights travel at different speeds.....and yet.....when blended they all travel at the same speed. Presumably there are numerous shades and hues that light seperates into.
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #6 on: 11/08/2009 12:47:52 »
The apparent problem involved with "speeding up" is not really a problem. Light has no mass so you don't have to give it a push for it to speed up when it is back in air.

Thank ewe Sophiecentaur .This is difficult for a sheepy to get his head around. So, because it has no mass this means it speeds up with no effort. Is there nothing propelling it ?...I gather it must go through an acceleration process !   Does this mean that all ' stuff ' like light- that has no mass - must always travel at light speed ?
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #7 on: 11/08/2009 12:50:57 »
I read somewhere that when light travels through a medium, it is infact being absorbed by the atoms of the medium and re-emitted by them right away, in effect passing the light beam down a chain of atoms until it reaches the end of the medium and re-enters the air/vacuum/whatever.  If this is the case, when the light is re-emitted by the final set of atoms, it will be re-emitted at the speed of light and hence continue at the speed of light.

That's my theory, if someone could confirm or deny this that' be great.

Jon

Thank ewe krytie75. That sounds like a great theory. I'm still a bit befuddles as to once light has been slowed why it must then return to it's original speed. What propels it in the first place ?
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #8 on: 11/08/2009 12:54:05 »
Light is a wave motion in which proagating energy is oscill;ating between two different forms of energy storage.  the propagation velocity of all wave motions is deopenant on the "elasticity" of the medium only and changes in either direction as the medium changes.

It is possiblr to partially recombine the split light backinto white light with a prism in the reverse sense.  This is the way achromatic lenses are made.

The expression prism refers to the shape of a rod with a triangular cross section.



Thank ewe Soul Surfer

This is all very interesting. Is it possible to elaborate as to why light can only be ' partially' recombined ? what does that mean ?..that's it's not light with all it's constituents ?
 

Offline krytie75

  • Full Member
  • ***
  • Posts: 57
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #9 on: 11/08/2009 17:09:18 »
Neilep, I will try to explain what I mean, and looking around on the internet sources point to me being correct.


When light hits a transparent medium such as glass, the energy from the light photons excite electrons in the medium which jump up to higher energy levels.  These electrons then jump back down to their original energy levels and re-emit the light as a photon with the same properties as first which carries on until it meets another electron. This will continue to happen until the light has seemingly passed right through the medium unobstructed.  The light infact travels between the atoms of the medium at the speed of light - it is the changing energy states of the electrons that introduce a delay and cause transmission to slow.  Hence, when the energy is released as a photon by the final atom in the chain, it will continue it's journey at the speed of light, as it did between atoms in the material.

It is worth noting that most transparent materials contain impurities so not all light shined at a piece of glass will pass through.  Some will either be reflected or completely absorbed.

Colour filters work because the mix of materials used to make them will absorb all frequencies of light other than the intended colour, which will be transmitted through the medium in the way mentioned above.

Hope this helps!

Jon
 

Offline krytie75

  • Full Member
  • ***
  • Posts: 57
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #10 on: 11/08/2009 17:13:28 »
Note: My guess is that different frequencies (colours) of light take different amounts of time to cause the electrons to change energy levels.  This would explain why different colours travel at different speeds in a dense medium.
 

Offline neilep

  • Withdrawnmist
  • Naked Science Forum GOD!
  • *******
  • Posts: 20602
  • Thanked: 8 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #11 on: 11/08/2009 18:15:45 »
Neilep, I will try to explain what I mean, and looking around on the internet sources point to me being correct.


When light hits a transparent medium such as glass, the energy from the light photons excite electrons in the medium which jump up to higher energy levels.  These electrons then jump back down to their original energy levels and re-emit the light as a photon with the same properties as first which carries on until it meets another electron. This will continue to happen until the light has seemingly passed right through the medium unobstructed.  The light infact travels between the atoms of the medium at the speed of light - it is the changing energy states of the electrons that introduce a delay and cause transmission to slow.  Hence, when the energy is released as a photon by the final atom in the chain, it will continue it's journey at the speed of light, as it did between atoms in the material.

It is worth noting that most transparent materials contain impurities so not all light shined at a piece of glass will pass through.  Some will either be reflected or completely absorbed.

Colour filters work because the mix of materials used to make them will absorb all frequencies of light other than the intended colour, which will be transmitted through the medium in the way mentioned above.

Hope this helps!

Jon

Hi Jon and thank ewe again for your great explanation.

I am beginning to understand....would an analogy be the fact that light that is emitted from the sun has already taken about 100,000 years to get there because of all the molecules it bounces off of on it's way to the perimeter ?


Neil
 

Offline labview1958

  • Sr. Member
  • ****
  • Posts: 104
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #12 on: 13/08/2009 10:42:36 »
Can a glass prism separate radiowaves as it separates light into seven colours? Is it true that a glass prism separates sunlight into 9 colours (7 visible and 2 invisible}?
 

lyner

  • Guest
Re: When light leaves a prism, does it change speed?
« Reply #13 on: 13/08/2009 17:21:17 »
There aren't just "seven" colours. There is a continuum of wavelengths which merge into one another via as many other colours as you care to describe.  We just catagorise them in the coarsest possible divisions.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #14 on: 13/08/2009 21:22:10 »
Can a glass prism separate radiowaves as it separates light into seven colours? Is it true that a glass prism separates sunlight into 9 colours (7 visible and 2 invisible}?
A glass can separate radiowaves extremely little, because glass' refractive index at those frequencies is extremely low.
 

Offline krytie75

  • Full Member
  • ***
  • Posts: 57
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #15 on: 16/08/2009 00:00:49 »
Can a glass prism separate radiowaves as it separates light into seven colours? Is it true that a glass prism separates sunlight into 9 colours (7 visible and 2 invisible}?
A glass can separate radiowaves extremely little, because glass' refractive index at those frequencies is extremely low.

Presumably there are substances which act on other areas of the Electromagnetic Spectrum like glass does on light? If so, what are these materials?  If not, why not?
 

lyner

  • Guest
Re: When light leaves a prism, does it change speed?
« Reply #16 on: 16/08/2009 00:19:25 »
The ionosphere, consisting of a very rarified gas whch has been ionised by cosmic rays and other radiations from the Sum, does the same to radio waves as glass does to light.
The speed of the wave depends upon the frequency and also on the density of ions etc, etc. You get dispersion, bending and total internal reflection.

Similar effects are observed in the troposphere (the part of the atmosphere where we live) when layers of warm and cold air can cause 'ducting' of radio waves and cause them to travel way beyond their normal range.

The above effects are all very slight and are only seen when the paths traveled are many km / hundreds of km but they still occur. In terms of wavelength, there is not so great a difference, however.
 

Offline Laura_Kelly

  • Full Member
  • ***
  • Posts: 66
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #17 on: 17/08/2009 08:53:16 »
Just a little aside as well, the seven traditional colours of the rainbow were invented by our friend Newton as he believed that this was the ultimate number. He even made up indigo to fit his idea of seven colours.
 

Offline Madidus_Scientia

  • Neilep Level Member
  • ******
  • Posts: 1451
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #18 on: 17/08/2009 09:46:47 »
If the light is absorbed by the atoms in the prism, shifting electrons to a higher energy level, then being re-emitted as the electron returns to its original energy level, why does the light continue in the same direction? Why isn't it re-emitted in a completely random direction?
 

lyner

  • Guest
Re: When light leaves a prism, does it change speed?
« Reply #19 on: 17/08/2009 10:55:45 »
An isolated Hydrogen Atom (as I've said more than you've had hot dinners) is not a relevant model in solid state Physics - or, indeed anywhere but a theoretical, isolated, H atom.
If you don't use the right model then you can't rely on the answer.
If you have a wave, then these photon 'packets' (they don't have to be particles) will have some kind of phase, associated with them - related to the conventional phase of the traveling wave front. If you really need to talk in terms of photon interaction with each atom then each photon will be interacting and be re-emitted , maintaining its original phase relation with the others. The vector sum of them all will be a wave traveling on one direction (modified by what we would normally call diffraction effects, if we were talking waves.
There can't really be any doubt about the em interaction idea - it's just the details which are important.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #20 on: 18/08/2009 08:35:49 »
If the light is absorbed by the atoms in the prism, shifting electrons to a higher energy level, then being re-emitted as the electron returns to its original energy level, why does the light continue in the same direction? Why isn't it re-emitted in a completely random direction?
Light IS NOT absorbed by the glass' atoms. Glass is transparent in the visible for that reason...
 

Offline Madidus_Scientia

  • Neilep Level Member
  • ******
  • Posts: 1451
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #21 on: 18/08/2009 10:34:23 »
I see. So what krytie75 was saying is incorrect?
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #22 on: 18/08/2009 12:49:33 »
I see. So what krytie75 was saying is incorrect?
Yes.
Look at the post I wrote in this recent thread:
http://www.thenakedscientists.com/forum/index.php?topic=24826.0
« Last Edit: 18/08/2009 12:53:06 by lightarrow »
 

Offline labview1958

  • Sr. Member
  • ****
  • Posts: 104
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #23 on: 19/08/2009 12:45:07 »
Is it possible that light becomes heavier thus slows down in glass? E=mc2. Thus increase in mass slows the speed to keep E constant?
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: When light leaves a prism, does it change speed?
« Reply #24 on: 19/08/2009 21:13:47 »
Is it possible that light becomes heavier thus slows down in glass? E=mc2. Thus increase in mass slows the speed to keep E constant?
You can interpret that m in the formula in two ways:
1. m = proper = invariant mass. Nowadays most physicists call it simply mass. Then that formula is valid *only* at zero momentum, that is, when the object is stationary. A beam of light is not stationary, so that formula is not valid for that case.
2. m = relativistic mass. Then the formula is valid even if the object is moving. But then mass is just another name for energy and so one changes (or not) ecactly as the other, so your reasoning is wrong.
 

The Naked Scientists Forum

Re: When light leaves a prism, does it change speed?
« Reply #24 on: 19/08/2009 21:13:47 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums