Does light refract like they say it does

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In summary, light can be separated when it passes through a medium due to the change in speed and angle of the waves. This is known as refraction and is explained by both the wave and particle interpretations of light. Maxwell's equations and conservation of momentum can be used to derive Snell's Law, which describes the relationship between the angle of refraction and the speed of light in different materials. The speed of light in a medium is always slower than c, the speed of light in a vacuum. However, the individual photons still travel at c while the group velocity is slowed down. This phenomenon is known as dispersion and is described by dispersion relations.
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sts107a
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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!
 
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  • #2
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
Thanks for the site! but I can't get it to work.
 
  • #4
Can't get it to work? I don't even know the peck that gobbledygook was!
 
  • #5
selfadjoint, not everyone knows what latex is you know...
 
  • #6
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
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
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
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.

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
Crosson said:
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?



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.
 
  • #12
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
Edgardo said:
Hello sts107a,

you can explain refraction by saying that the speed of light changes in the

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

marlon
 
  • #14
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:
"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:
"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."
 
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  • #15
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.
 
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  • #16
http://encyclopedia.laborlawtalk.com/Group_velocity

This states the problem quite clearly

marlon
 
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  • #17
Ok, thanks for correcting me. And with respect to the photon always traveling at c, I found this one interesting:
https://www.physicsforums.com/archive/topic/t-9995_Speed_of_Light?.html
 
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1. How does light refract?

Light refracts when it passes through a medium with a different density, causing the light to change direction. This is due to the change in speed of light as it travels through the medium.

2. Why does light refract?

Light refracts due to the change in speed as it travels through a medium. This change in speed causes the light to bend, resulting in the phenomenon of refraction.

3. What is the law of refraction?

The law of refraction states that the incident ray, the refracted ray, and the normal (a line perpendicular to the surface of the medium) all lie in the same plane. Additionally, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media.

4. How does the angle of incidence affect refraction?

The angle of incidence, which is the angle between the incident ray and the normal, affects the amount of refraction. When the angle of incidence is larger, the angle of refraction also becomes larger, resulting in a more significant change in direction of the light. When the angle of incidence is smaller, the angle of refraction is also smaller, resulting in a less noticeable change in direction of the light.

5. Does light always refract?

Light will always refract when it passes through a medium with a different density. However, if the angle of incidence is zero (the light ray is perpendicular to the surface of the medium), there will be no change in direction and therefore no refraction will occur.

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