# Does light refract like they say it does

1. Mar 13, 2005

### sts107a

Does light work like they say it does

Hey guys!!!! I was wondering if you could help me. I’m fascinated how light can be separated when it passes threw a medium. However I’m not really convinced of the current interpretation on how it happens. I was hopping every one could give me a clear understanding of how it is suppose to work and how they think it works. How it works for a wave interpretation and how it works for a particle interpretation. Plus some lesser known theories, if there are any, on how light works. Thanks!!!!

Last edited: Mar 13, 2005
2. Mar 13, 2005

Staff Emeritus
May I suggest you download this package http://www.cs.man.ac.uk/~pt/proofs/QED.sty, and run it. The first example IIRC, is the reflection of a beam of light off a panel of finite thickness, done in terms of Feynmann's sum over paths. It should raise your consciousness a bit. If anything in it confuses you, check back here.

3. Mar 13, 2005

### sts107a

Thanks for the site!!!! but I can't get it to work.

4. Mar 14, 2005

### Beyond-Numbers

Can't get it to work? I don't even know the peck that gobbledygook was!

5. Mar 15, 2005

### zeta101

selfadjoint, not everyone knows what latex is you know...

6. Mar 16, 2005

### Antiphon

How they think it works is how it really work. Here's how it works.

Waves that travel at different speeds in different materials will change the
angle they move once they pass through a material boundary. This is
because the peaks and troughs of the waves still have to line up at the
interface, and the only way to make that happen is to have a different
angle on either side of the material.

There is no good particle picture for this, its a wave phenomenon.

7. Mar 16, 2005

### sts107a

Ok but what happens to all that momentum when it hits the boundary. I mean if its traveling at C and then strikes different medium wouldn’t it slow down? And my biggest question is why it is so uniform when it comes out the other side I.E. a rainbow. And if light has duality isn’t it safe to say their is a model out there that would incorporate the particle pic?

8. Mar 16, 2005

### Antiphon

If there are no reflections at the boundary and no losses in the medium, the
momentum sails right through the object. If some is absorbed or reflected, then momentum is transferred to the object.

When it slows down, that's not the same as losing momentum. The mometum
must be computed from the fields and the dielectric constant of the material.

It looks uniform to you on the output of the prism because there are many photons.
Each photon (according to it's energy/momentum will come out of a prism at a
different angle (assuming they all go in at the same angle.) This IS the particle
picture.

The thing you have to remeber is that Maxwell's equations ARE the (first
quantization now) quantum mechanical euqations for the movement of photons.
That is, the magnitude of the electromagnetic 4-vector IS the probability of finding a photon at that point in space time.

9. Mar 17, 2005

### Crosson

Probabilities are numbers between 0 and 1. How is the "electromagnetic 4-vector" scaled to this range?

What do you mean by "electromagnetic 4-vector"? Do you mean the source vector, or the potential vector? or the Field tensor?

10. Mar 17, 2005

### Antiphon

The units of the EM field are not probability but it's easy to make them
be so. You simply normalize them by the volume integral over the
universe of the square of the fields.

11. Mar 17, 2005

### Edgardo

Last edited by a moderator: May 1, 2017
12. Mar 17, 2005

### Claude Bile

One can derive Snell's Law using the boundary conditions imposed by Maxwell's Equations. You can also derive Snell's Law using conservation of momentum arguments (i.e. using the particle nature of the photons).

Claude.

13. Mar 17, 2005

### marlon

You mean the group velocity of the EM-wave. The speed of a photon is always c...

marlon

14. Mar 17, 2005

### Edgardo

Nope, I am talking about the speed of light.
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html
"Does the speed of light change in air or water?
Yes. Light is slowed down in transparent media such as air, water and glass. The ratio by which it is slowed is called the refractive index of the medium and is always greater than one"

With speed of light I mean how fast the wavefront (no photons here) moves (as can be seen in the animation).

From http://en.wikipedia.org/wiki/Speed_of_light: [Broken]
"The speed of light through a medium (that is, not in vacuum) is less than c (defining the refractive index of the medium)"

From http://encyclopedia.laborlawtalk.com/Speed_of_light: [Broken]
"In passing through materials, light is slowed to less than c by the ratio called the refractive index of the material. The speed of light in air is only slightly less than c. Denser media, such as water and glass, can slow light much more, to fractions such as 3/4 and 2/3 of c. This reduction in speed is also responsible for bending of light at an interface between two materials with different indices, a phenomenon known as refraction."

Last edited by a moderator: May 1, 2017
15. Mar 18, 2005

### Antiphon

Sorry Edgardo, Marlon is correct.

In a material like glass only the group velocity slows down.
The individual original quanta (which don't make it through the glass alive)
always travel at c.

Molecules in the glass get stretched by the light and abosorb the photon.
A little while later the molecules release another photon just like the one
that got caught but it's behind where it would have been in free space.

The math that describes this is called "dispersion relations" and the details
are different for different kinds of materials depending on how their molecules
react.

Last edited: Mar 18, 2005
16. Mar 18, 2005

### marlon

http://encyclopedia.laborlawtalk.com/Group_velocity [Broken]

This states the problem quite clearly

marlon

Last edited by a moderator: May 1, 2017
17. Mar 19, 2005

### Edgardo

Ok, thx for correcting me. And with respect to the photon always travelling at c, I found this one interesting:
https://www.physicsforums.com/archive/topic/t-9995_Speed_of_Light???.html [Broken]

Last edited by a moderator: May 1, 2017