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Prism & Light Speed, Dispersion, etc.

Prism & Light Speed, Dispersion, etc. - Physics Forum

Prism & Light Speed, Dispersion, etc. - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 03-05-2005, 09:07 AM
phoenix
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Default Prism & Light Speed, Dispersion, etc.



Hi,

In a prism, light is said to slow down. Why does it slow
down? Did it become like 0.8C or something?

Red is refracted less than violet. Refractive index is
said to increase as wavelength decreases. violet has
wavelength of 360-440 nm while red has wavelength of
630-780nm.

Refractive index is normally greater than unity or 1.
Supposed. Just supposed (for sake of discussion) that
a wave faster than light enters the prism. Are we going
to have less than unity refractive index where instead
of the spectrum light going downwards in the right side
(with source light coming from lower side in the left of
the prism), the spectrum side goes upwards?? I know light
doesn't go faster than C. I'm just asking to understand
the prism better like does dispersion occurs because
of light getting slower.

Also why does it disperses downwards in the prism
and not upwards? Etc.

In a nutshell. What happens when superluminal photons
enter a prism? What would be the behavior of the spectrum
in the output side? Would it bend downward (as usual) or
would it bend upward?? I know there are no superluminal
photons like physics said. This is just to understand
the concepts better.

Many thanks.

Phoenix

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  #2  
Old 03-05-2005, 02:50 PM
bz
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Default Prism & Light Speed, Dispersion, etc.

"phoenix" <[Only registered users see links. ]> wrote in news:1110013649.441201.25680
@g14g2000cwa.googlegroups.com:


It obeys the 'speed limit' signs.


the index of refraction is the ratio of light speed in the media to light
speed in a vacuum.

[Only registered users see links. ]

....

lets consider that happens when a particle (remember, particles can not
travel faster than the speed of light) enters a media where they are NOW
traveling faster than light.

They slow down, rapidly. Of course to slow down they must give off energy.

bremsstrahlung radiation (braking radiation) is emitted.
Cerenkov Radiation is a form of such radiation as beta particles slow down
in the water moderator of a water moderated neuclear reactor.

[Only registered users see links. ]

--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+[Only registered users see links. ].lsu.edu
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  #3  
Old 03-05-2005, 03:12 PM
N:dlzc D:aol T:com \(dlzc\)
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Default Prism & Light Speed, Dispersion, etc.

Dear phoenix:

"phoenix" <[Only registered users see links. ]> wrote in message
news:1110013649.441201.25680@g14g2000cwa.googlegro ups.com...

"Whys" don't get answered in science. As a metaphor, why does
traffic slow down when cars get closer together? Matter has
charges, and charges start to interfere with the free passage of
EM radiation (light). It can only pass through if the matter
matrix doesn't absorb or scatter it... a pretty tall order.


It depends on the "index of refraction" of the material. With c
as the speed of light in a vacuum,
c_medium = n * c
some glasses have an index of refraction (n) of 1.4, so 71%.
Diamond has a higher index of refraction.


As a confusing sidenote, there are materials with a "negative
index of refraction". These are constructed for superior optical
characteristics.


The speed of light in a medium is controlled/modified by the
medium. It can have no knowledge of the initial speed the light
arrived at... only how fast light can pass through it.


Dispersion occurs because the higher frequency is more
"disturbing" to the matter matrix. The matrix has to "move
faster" to allow its passage.


See "Dispersion occurs because...".


They don't.


The same.


The output must be the same, since it depends only on the
frequency of the light and the dynamics of the matter the prism
is comprised of. Note that light is bent less when the interface
is water/glass/water, than air/glass/air or vacuum/glass/vacuum.
But it is not bent less if it comes from a very remote or
fast-moving source.

Things you will want to search on, for your own research:
Google Advanced
the exact phrase: index of refraction
the exact phrase: negative index of refraction
you might want to stick to the ".edu" sites...

David A. Smith


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  #4  
Old 03-05-2005, 04:29 PM
tadchem
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Default Prism & Light Speed, Dispersion, etc.


"phoenix" <[Only registered users see links. ]> wrote in message
news:1110013649.441201.25680@g14g2000cwa.googlegro ups.com...

Hi.


Light moves at c *in a vacuum*. Inside a prism light travels through a
volume of space that is filled with electromagnetic fields strong enough to
hold electrons close to atomic nuclei. The light is measurably slowed down
and the phasing of the electric and the magnetic fields of the light are
measurably retarded.

The classical explanation for this is that the light, as it moves through
such a medium, transfers some of its energy to the medium, inducing
temporary dipole moments in a *process* is called 'polarization' - not to be
confused with the *property* of light called polarization, which refers to
the orientation of the electric and magnetic fields of a photon. This is
intimately connected to 'molar refraction' R - a measure of the ability of
the material to refract light.

This transfer of energy to the medium (and back) reduces the average energy
E of the photon at any given time. The observed result of this lowering of
the average energy of the photon is an apparent reduction in the speed of
propagation of light.

The usual measure of refraction is the index of refraction n, defined as the
ratio of the speed of light in a vacuum to the speed of light in the medium.
For example in water n is about 1.333 for visible light, meaning that light
moves through water about 3/4ths as fast as it does through a vacuum.

For reasons too complex to describe here, the following equation holds:

R = (n^2 - 1)/(n^2 + 2) * M/rho

where M is the molecular weight of the material, and rho is the density
[M/rho is the molar volume - the volume occupied by one mole].

In the limit of long wavelengths, n^2 equals the dielectric constant D of
the material.


The real situation is more complex than that, but it requires a lot of
advanced math to understand Lorentz' theory of the complex refractive index.


The full Lorentz theory *does* account for resonances and those rare (but
real) cases when n < 1.

Normally the refractive index is less than unity close to a resonance where
the extinction coefficient is high - meaning a large fraction of the light
is absorbed rather than transmitted. These cases are very difficult to
demonstrate with a simple prism as the resonances are typically narrow, and
the relatively unrefracted light overwhelms the inversely refracted light.

There are also cases when particles move through a medium faster than the
speed of light *in that medium*. The result is a special kind of radiation
called Cherenkov radiation
[Only registered users see links. ]
(a special case of bremsstrahlung - German for 'braking' radiation)
[Only registered users see links. ]
The radiation is an eerie bluish glow that seems to have no source, and is
about the color of the light from an electric arc. Water-cooled nuclear
reactors (swimming pool reactors) are well-known sources of this radiation:
[Only registered users see links. ]
http://www.arrrr.com/photos/che.jpg


Snell's law: it depends on the relative angle of the light to the interface
and the *relative* indices of refraction:
[Only registered users see links. ]

If you made an 'anti-prism' - one with an index of refraction smaller than
that of the medium around it [easily done by putting a small empty aquarium
inside a larger full one(!)] - the light could be made to refract
'upwards'.

The word 'dispersion' has a very specific technical meaning in optics - it
refers to the variability of refractive index as a function of the
wavelength of the incident light. High dispersion media such as diamond
have what jewelers call 'fire'.


Can't say. Nobody's ever seen a 'superluminal' photon.


Try these sites:
[Only registered users see links. ]
and
[Only registered users see links. ]
specifically
[Only registered users see links. ]
for refraction and
[Only registered users see links. ]
for prisms.


HTH


Tom Davidson
Richmond, VA


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  #5  
Old 03-05-2005, 06:00 PM
Old Man
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Default Prism & Light Speed, Dispersion, etc.

"phoenix" <[Only registered users see links. ]> wrote in message
news:1110013649.441201.25680@g14g2000cwa.googlegro ups.com...


As light posses through matter, the photons interact with
the atoms of the medium, being repeatedly absorbed
(destroyed) and re-emitted (created).

In between interactions, the new photons travel at the
invariant speed of light, but each interaction introduces a
time lag and causes a cumulative phase-shift in the
composite electromagnetic wave.

When a charged particle, such as an electron or proton,
passes through matter at a speed greater than v = c / n,
Cherenkov radiation is emitted along it's path. This
process analogous to the "sonic-boom" created by an
aircraft traveling at a speed greater than that of sound in
air.

[Old Man]



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  #6  
Old 03-05-2005, 06:36 PM
Roy Culley
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Default Prism & Light Speed, Dispersion, etc.

begin <[Only registered users see links. ]>,
"tadchem" <[Only registered users see links. ]> writes:

Utter rubbish. The energy of a photon is hf (h - Planck's constant, f
- is the frequency). It has now't to do with its speed. Photons are
absorbed by the medium it is going through and new photons are
re-emitted by the medium. It is this absorption / re-emission that
appears to make photons slow down. In fact they always move at c.
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  #7  
Old 03-05-2005, 06:47 PM
Uncle Al
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Default Prism & Light Speed, Dispersion, etc.

phoenix wrote:

Refractive index, or something. Do the world a favor - fail the
course and free up scarce resources for somebody more deserving.
Don't read the links. Education will make you feel sad and
inadequate.

<http://www.schott.com/optics_devices/english/download/tie-29_refractive_index.pdf>
<http://www.containerless.com/IR%20product%20broch1.pdf>
<http://www.schott.com/optics_devices/english/products/precision_optical_glass.html>
<http://www.us.schott.com/optics_devices/english/download/>


Anomalous dispersion. Bzzzt.


Tell that to an x-ray mirror or to a totally reflecting binocular
prism.


Ain't ignorance grand? Are ya talkin' phase or group velocity, git?


"Left-handed" refractive materials.


Snell's Law, Etc.


You fail the course.

[snip]

--
Uncle Al
[Only registered users see links. ]
(Toxic URL! Unsafe for children and most mammals)
[Only registered users see links. ]
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  #8  
Old 03-05-2005, 07:27 PM
N:dlzc D:aol T:com \(dlzc\)
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Default Prism & Light Speed, Dispersion, etc.

Dear Roy Culley:

"Roy Culley" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

Please provide a citation. Please describe how this behavior is
different than what most people consider to be a propagating
photon. Your response is also "in sight" of the OP, since you
did not change the title. Do him/her a favor as well.

David A. Smith


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  #9  
Old 03-05-2005, 11:06 PM
Franz Heymann
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Default Prism & Light Speed, Dispersion, etc.


"bz" <bz+[Only registered users see links. ].lsu.edu> wrote in message
news:Xns961059EB332B8WQAHBGMXSZHVspammote@130.39.1 98.139...
light
not
NOW
energy.

No.
Bremsstrahlung has nothing to do with the motion of particles whose
speed exceeds the speed of light in the medium. Lead is a very
favourite target for producing bremsstrahlung, and it is totally
opaque to light.

slow down

Charged particles do not emit Cerenkov radiation because they are
slowing down. They emit Cerenkov light simply because they are moving
faster than the speed of light in the medium. The rate of energy loss
from that is utterly minute compared to the ionisation loss, or indeed
the energy loss by bremsstrahlung.

--
Franz
"A first-rate laboratory is one in which mediocre scientists can
produce outstanding work"
P.M.S. Blackett


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  #10  
Old 03-06-2005, 12:32 AM
bz
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Default Prism & Light Speed, Dispersion, etc.

"Franz Heymann" <[Only registered users see links. ]> wrote in news:d0de1l$aoj
$[Only registered users see links. ].ncs.ea.ibs-infra.bt.com:


Well, to slow down they must give off energy, right?


I thought Cerenkov Radiation was a special case of Bremsstrahlung
radiation. Is this incorrect? I realize that just changing the direction of
motion of a beam of particles will ALSO produce Bremsstrahlung. We have a
facility in Baton Rouge called CAMD that produces and uses such radiation.
I have seen the facility, but never been there when it was in operation.


Lead is not opaque to ALL Electromagnetic radiation, is it?


OK. Thanks. I always appreciate learning something.

--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+[Only registered users see links. ].lsu.edu
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