# The Naked Scientists Forum

### Author Topic: Why does light refract?  (Read 13972 times)

#### qpan

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##### Why does light refract?
« on: 22/07/2004 21:11:35 »
Does anyone actually know the reason why light in fact refracts? It seems as though you are only ever taught how light refracts, but never why!

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

#### Quantumcat

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##### Re: Why does light refract?
« Reply #1 on: 26/07/2004 14:06:42 »
Is refracting where it bends in materials of different densities? If it is, then light refracts because going through different densities makes it slow down.Think of a wheelchair ... you go fast in a straight line by rolling the wheels at the same speed. Some stupid person has spilled honey mixed with gravel (a very strange person) and spread the mixture over an area of which the boundary you are coming dangerously close to. (are you feeling scared?)you don't enter it head on, but at an angle. So, one wheel hits it first. (slow time down a bit so we can examine what happens more easily) The other wheel hasn't hit it yet, since you're going in at an angle. So one wheel goes slow, one wheel goes fast. What happens? The wheel chair turns. It turns until the second whell enfonces in the honey. Then you'll keep pushing the wheels with equal force and it'll go straight again, at a slower speed than before. That's exactly what happens when Light meets a medium of another density. Notice that if you roll into the honey at right angles to the border (straight into it in other words) you slowdown but don't turn, since both wheels touch the honey at the same time.

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#### qpan

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##### Re: Why does light refract?
« Reply #2 on: 27/07/2004 12:50:01 »
Yeah- thats was pretty much the explanation my physics teacher gave to me when i asked him a few years ago. But what are the actual mechanisms involved? This is all fair and well if you take a photon to be a macroscopic object, but photons do not always adhere to the ways of classical physics.

In order for the photon to deflect at the interface of the two materials, a torque must be applied. In the case of the wheelchair, this is caused by the difference in friction between the two materials. What about for light? What is the equivalent of friction and how does it slow the light down?

What if you consider light to be a wave? How does one now explain refraction? Or does the wave theory of light not include an explanation of refraction?

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

#### tweener

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##### Re: Why does light refract?
« Reply #3 on: 28/07/2004 04:56:17 »
I can't remember studying this in any of my QM courses, and I don't know the answer.  I hope someone else can come up with something.

----
John - The Eternal Pessimist.

#### Quantumcat

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##### Re: Why does light refract?
« Reply #4 on: 29/07/2004 13:49:41 »
Okay so you were looking for a *higher* answer, sorry :) (thought you were a bit old for something year 9ish !)

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#### tweener

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##### Re: Why does light refract?
« Reply #5 on: 29/07/2004 22:02:59 »
QC, you gave a very good answer!  It is just not a quantum level explanation of what is happening to an individual photon.

I was thinking about this this morning (you don't want to know where) and came up with another question that is related and may lead to the answer to this one.  What causes the photon to slow down?  If I remember correctly, this is caused by the oscillating EM field in the photon interacting with the electrons of the material.  The material has very different transmission characteristics depending on how the electrons are configured.  Metals have lots of free electrons and insulators have bound electrons.  Transparent materials have combinations that result in a "band gap" where certain frequencies of radiation can propagate through the material while others are absorbed when the electrons oscillate and take away all the photon's energy.

In the case of a transparent material like glass, the electrons do oscillate, but they do not absorb the energy (how is this possible?).  Because of the interaction, the photons are slowed down - they have to "pause" for each electron to wiggle a little bit.

I made most of this up, so anyone who knows better should correct me while this topic is active!

----
John - The Eternal Pessimist.

#### cuso4

• Angel Delight
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##### Re: Why does light refract?
« Reply #6 on: 01/08/2004 20:53:17 »
There are no delocalised electrons in glass (SiO2) since all atoms are covalently bonded, so electrons cannot absorb the energy and become excited.

Angel

#### tweener

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##### Re: Why does light refract?
« Reply #7 on: 02/09/2004 16:13:45 »

I'm still interested in WHY the photons slow down as they pass through a material.  I took a graduate level course in solid state physics (long time ago) and don't remember that this question was answered even there.  Or maybe I was too dense to "get it", in which case I will have little hope now.

----
John - The Eternal Pessimist.

#### gsmollin

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##### Re: Why does light refract?
« Reply #8 on: 03/09/2004 17:37:11 »
It is easy to explain in clssical wave mechanics, although brevity prevents me from doing it here. Look for any good electromagnetics text, and find the chapter on plane waves in dielectric and conducting media. The upshot is that the wave propagation speed is v = 1/square root(ue), where u is the permeability of the space, and e is the permittivity.

In glass, the permittivity is greater than in vacuum, so the wave velocity decreases.

In free space, u and e obtain their smallest values, and v=c.

In QM, the photon in question still maintains its wave functions. Since the waves slow down, the photonic "energy packet" also slows down.

#### Peter_Boos

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##### Re: Why does light refract?
« Reply #9 on: 24/10/2004 00:30:49 »
Hi,

Light has intresting properties, but i believe light is not what we think of it. I've made a verry simple explanation about light which does answer why entangled photons can interact instantly over a distance. This might have some impact also on QM although i do not much about QM. You can read it here :

As far as i know no one else had ever thought about it.

#### The Naked Scientists Forum

##### Re: Why does light refract?
« Reply #9 on: 24/10/2004 00:30:49 »