Dispersion/Refraction, but WHY?

  • Thread starter mjcguest
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In summary, the interaction between light and a medium causes a change in the speed of the light, which in turn changes the wavelength. This change in wavelength is what causes the bending of light, known as refraction. The nature of this interaction is due to the way light interacts with the molecules in the medium, particularly the bound electrons. The wavelength of light is a determining factor because it is closer to the natural resonant frequency of the bound electrons, making it more effective in causing them to oscillate. This change in speed and direction of the light wave can be explained using Huygen's principle and the concept of wavefronts. For a more detailed explanation, refer to sources such as the Physics Forums FAQ and the article linked above
  • #1
mjcguest
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I understand and have seen light disperse as it shines through a prism, crysal or other medium; I have seen snells law, I have searched this forum for answers..!

But I can not find an explanation I can understand of *why* a shorter wavelength of light will bend differently to a longer wavelength? What is the nature of the interaction or the nature of the wave that causes the bend, and why would the wavelength be the determining factor?

Thanks in advance for the newb-busting answer!
Matt
 
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  • #2
Hi Matt! :smile:

It's the speed of the light that determines it …

the index of refraction is the ratio of the speeds before and after …

and the speed happens to depend on the wavelength.

(why? well, that's quantum mechanics for you! … something to do with the way the light interacts with the molecules. :redface:)
 
  • #3
Thanks Tim. You've sort of highlighted the question... I have access to plenty of text that tells me what will happen, but nothing I've found that tells me why?
 
  • #4
mjcguest said:
Thanks Tim. You've sort of highlighted the question... I have access to plenty of text that tells me what will happen, but nothing I've found that tells me why?

The why is usually harder than the what.

EDIT: I was looking for some link to give you a better intuitive idea of what is going on when a wave interacts with the 'boundary' but I can't find anything!
 
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  • #5
Ultimately, Physics cannot answer 'why'. We sometimes describe observations and sometimes make predictions. We observe dispersion, we have reasonably good models, and we can predict the dispersion for some simple materials. That's as good as it gets.
 
  • #6
At the microscale an electromagnetic wave is slowed in a material because the electric field
creates a disturbance in the charges of each atom - primarily the electron proportional to the permittivity.
The oscillation of charges itself causes the radiation of an electromagnetic wave that is
slightly out of phase with the original wave. The sum of the two waves creates a wave of
the same frequency but shorter wavelength than the original leading to slowing of the waves travel.
* An electron in an atom or molecule is bound there by strong restoring forces.It has a
definite natural frequency.For electrons in atoms it is usually in a region corresponding to
violet or U.V. light. In mechanical systems it is possible to "drive" the system most
effectively if we impress on it an external force whose frequency is as close as possible to that of the natural resonate frequency.In the case of light the blue is closer to the natural
resonate frequency of the bound electrons than red light. Therefore we would expect the blue light to be more effective in causing the electrons to oscillate.

last paragraph referenced from Physics, Halliday-Resnick
 
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  • #7
mjcguest said:
nothing I've found that tells me why?

I guess you haven't checked out the Physics Forums FAQ at the top of this very forum? :smile:

(in particular post #4)
 
  • #8
jtbell said:
I guess you haven't checked out the Physics Forums FAQ at the top of this very forum? :smile:

(in particular post #4)

I have - and it's gives a superb explanation of the change in speed; but I can't see anything that explains why the angle of refraction changes in relation to the wavelength

Any pointers to layman-friendly sources would be gratefully receieved!

Thanks
Matt
 
  • #9
mjcguest said:
I have - and it's gives a superb explanation of the change in speed; but I can't see anything that explains why the angle of refraction changes in relation to the wavelength
What matters is the change in speed, which of course changes the wavelength. When a wave encounters a medium (of higher index of refraction) it slows down. If it meets the boundary at an angle, the change in speed will cause different parts of the wave to slow down at different times, thus changing the direction of the wave. See: http://www.physicsclassroom.com/Class/refrn/U14L1e.cfm"
 
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  • #10
Doc Al said:
What matters is the change in speed, which of course changes the wavelength. When a wave encounters a medium (of higher index of refraction) it slows down. If it meets the boundary at an angle, the change in speed will cause different parts of the wave to slow down at different times, thus changing the direction of the wave. See: http://www.physicsclassroom.com/Class/refrn/U14L1e.cfm"

I relaize this is a stock answer, but possibly confusing to the student- there is a steady stream of 'how does one part of the wave know what the other part is doing?' type questions here.

Better (IMO) to simply say that the momentum changes, so the direction changes.
 
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  • #11
Andy Resnick said:
I relaize this is a stock answer, but possibly confusing to the student- there is a steady stream of 'how does one part of the wave know what the other part is doing?' type questions here.
You're right, there's a good bit of handwaving here. But I think the argument can be made at least semi-rigorous by invoking Huygen's principle and detailing how wavefronts are defined.
Better (IMO) to simply say that the momentum changes, so the direction changes.
Can you expand on this?
 
  • #12
Morrowbay's post #6, including the quote from Halladay & Resnik, describes the dispersion caused by the atomic electric dipole resonances and anomalous dispersion (see http://en.wikipedia.org/wiki/Dispersion_(optics )
usually in the UV region, where the index of refraction, which normally increases with decreasing wavelength, suddenly increases very fast, and then decreases very fast (the anomalous part), diving below n=1. The phase shift described by morrowbay is identical to the phase shift observed in RLC electric circuits, when driven off-resonance.
Bob S
[Edit] See also discussion in Section 7.5 in Jackson "Classical Electrodynamics" (second edition) on frequency dispersion.
 
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  • #13
Doc Al said:
You're right, there's a good bit of handwaving here. But I think the argument can be made at least semi-rigorous by invoking Huygen's principle and detailing how wavefronts are defined.

Can you expand on this?


Sure-
http://www.iop.org/EJ/article/0031-9120/35/5/310/pe0510.pdf?request-id=229de04f-40ae-4082-8189-f4e9ffae82b1

The article mentions use of this model for photons, but it works for rays as well.
 
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  • #14
Hey, I like it. Thanks, Andy!
 

1. What is dispersion and refraction?

Dispersion and refraction are two optical phenomena that occur when light passes through a medium, such as air or water. Dispersion is the splitting of white light into its component colors, while refraction is the bending of light as it passes from one medium to another.

2. Why does dispersion occur?

Dispersion occurs because different colors of light have different wavelengths. When white light enters a medium, such as a prism, the different wavelengths are refracted at slightly different angles, causing the colors to separate.

3. How does refraction work?

Refraction occurs because light travels at different speeds through different materials. When light enters a medium, such as water, it slows down and changes direction. This change in direction is what causes the bending of light.

4. Why do different colors of light refract at different angles?

Different colors of light have different wavelengths, which means they have different frequencies. When light enters a medium, the speed of each color is affected differently, causing them to bend at different angles.

5. How is refraction used in everyday life?

Refraction has many practical applications in everyday life. For example, it is used in the design of eyeglasses and contact lenses to correct vision. It is also used in the lenses of cameras and telescopes to focus light. Refraction is also responsible for the creation of rainbows, mirages, and other optical illusions.

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