Physics Help Forum refraction

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 Oct 23rd 2012, 11:47 AM #1 Junior Member     Join Date: Oct 2012 Location: India (Kashipur, Uttarakhand) Posts: 10 refraction hello, I have a little question about refraction of light. As refraction is the bending of light when it passes from lower density to higher density or vice-verse, but what's the reason which makes light bend. It should just pass into it, in the same direction but with a reduction in it's speed (from low to high density medium). In fact that doesn't happen, light get bend along with a speed reduction. I know Snell's law gives the expression about how much angle light would get bend during refraction, but I'm missing the reason why light is doing so. Thanks __________________ German^longstrike
 Oct 23rd 2012, 02:01 PM #2 Physics Team     Join Date: Jun 2010 Location: Naperville, IL USA Posts: 2,271 The simplest explanation is that Fermat's Principle posits that light takes the fastest possible path. Since light travels slower in a denser medium, the fastest route between a point in the dense medium and a point in the adjacent lighter medium is a path that travels further through the lighter medium and less through the dense medium than a straight line would indicate. But this still begs the question of "why" refraction obeys Snell's Law, or why light seemingly takes the fastest route. It's possible to derive Snell's Law using boundary conditions of Maxwell's equations, but I won't pretend to try that here. Another method is to use the fact that the component of the light's velocity parallel to the boundary is the same on both sides of the boundary, so therefore the perpendicular component must be different, and the result is Snell's law. Last edited by ChipB; May 27th 2014 at 06:39 AM.
 Oct 24th 2012, 12:17 AM #3 Junior Member     Join Date: Oct 2012 Location: India (Kashipur, Uttarakhand) Posts: 10 Very very thanks Chip B for your help __________________ German^longstrike
 Oct 31st 2012, 09:44 PM #4 Physics Team   Join Date: Feb 2009 Posts: 1,425 Thanks Chip. Could you explain "the component of velocity parallel to the boundary"part in more detail? Have also heard that the question of why normally incident light does not bend can also be explained thru' boundary conditions of Maxwell'equations.
May 26th 2014, 09:26 PM   #5
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 Originally Posted by physicsquest Thanks Chip. Could you explain "the component of velocity parallel to the boundary"part in more detail? Have also heard that the question of why normally incident light does not bend can also be explained thru' boundary conditions of Maxwell'equations.
And also related to the boundary: What about the component of the Poynting vector, S = energy flow per unit area, dU/dtA = 1/u0 ExB
Since when light is oblique at boundary of medium the area is greater than incident light. So then would value of S decrease and if so, would this effect refraction ? And also explain why normal incident light does not bend.

May 26th 2014, 11:21 PM   #6

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 Originally Posted by morrobay And also related to the boundary: What about the component of the Poynting vector, S = energy flow per unit area, dU/dtA = 1/u0 ExB Since when light is oblique at boundary of medium the area is greater than incident light. So then would value of S decrease and if so, would this effect refraction ? And also explain why normal incident light does not bend.
The "flow" is unimpeded since we still have the same energy transfer per unit time. Thus the only change in the Poynting vector is due to the change in the permeability constant.

Normally incident light does refract, in a way. It's just that the angle of refraction is 0.

-Dan
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May 28th 2014, 06:52 AM   #7
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 Originally Posted by Germanlongstrike As refraction is the bending of light when it passes from lower density to higher density or vice-verse, but what's the reason which makes light bend.
The reason is that light consists of waves. It's comprised of photons, but photons are waves, not specks.

So when light meets glass at an angle, one part of the wave is slowed down but another part isn't. So the beam changes direction. Think tank tracks, and take a look at Professor Ned Wright's deflection and delay of light.

 Nov 24th 2014, 04:09 AM #8 Junior Member   Join Date: Jan 2013 Posts: 24 refraction index Why different colors have different refractive indexes? Last edited by harve; May 25th 2017 at 06:40 AM.
 Nov 24th 2014, 05:14 PM #9 Junior Member   Join Date: Nov 2014 Posts: 16 Because they are different wavelengths.
Nov 26th 2014, 04:35 AM   #10
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 Originally Posted by Jerromyjon Because they are different wavelengths.
Please don't play with me.I already know this.This is my quest in other words.Would you like me to repeat it?Change the word "colors" to "wavelengths".Can anybody answear please?

Last edited by harve; May 25th 2017 at 06:41 AM.

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