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Author Topic: Refraction of Light  (Read 11763 times)
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Puzzled
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on: March 22, 2009, 11:40:47 am »

Hi there! Sorry if this is dumb question, but i learnt in school the other day that when light enters a medium with a higher refractive index it refracts. Since light is an EM wave, it can be expressed in the following equation: v = f λ. It will most definitely slow down, meaning that its velocity will decrease. However, since its frequency will not change, its wavelength will have to compensate the decrease in velocity by decreasing in magnitude itself. However, it is well known that the refracted light will not change colour.

For example, if the laser used was red, the refracted red beam would have a smaller wavelength that might fall within yellow's band of wavelengths. But the refracted beam will always remain as red, apparently. Does this mean that the wavelength of visible light gives little indication on its colour? Is frequency the sole deciding factor then?
« Last Edit: March 22, 2009, 11:50:37 am by Puzzled » Logged
Fu-Kwun Hwang
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Reply #1 on: March 22, 2009, 04:44:56 pm » posted from:Taipei,T\'ai-pei,Taiwan

When light travel from one media to another media, part of the light will reflected back to the original media.
Some of the light will enter another media and change it direction (due to change in velocity).
Yes, the wavelength of the light will change when enter different media.(only frequency will not change).
The wavelength of light in the water is different from wavelength in air.

You forgot that light must enter into our eye in oder to see it.
The path of the refracted light you saw, must be back to the air and enter you eye for you to see it.
So the wavelength is the same when light enter your eye again!


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lookang
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Reply #2 on: March 22, 2009, 06:01:21 pm »

Hi there! Sorry if this is dumb question, but i learnt in school the other day that when light enters a medium with a higher refractive index it refracts. Since light is an EM wave, it can be expressed in the following equation: v = f λ. It will most definitely slow down, meaning that its velocity will decrease. However, since its frequency will not change, its wavelength will have to compensate the decrease in velocity by decreasing in magnitude itself. However, it is well known that the refracted light will not change colour.

agreed!



For example, if the laser used was red, the refracted red beam would have a smaller wavelength that might fall within yellow's band of wavelengths. But the refracted beam will always remain as red, apparently.
i agree with prof hwang that the light you mentioned has to exit the medium and enter air again thus it will appear to be red light again when it enter our eyes.

Does this mean that the wavelength of visible light gives little indication on its colour?

i disagree here with Puzzled
wavelength of visible light is indeed ~400 to 700 nm as mention in http://en.wikipedia.org/wiki/Visible_spectrum
http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Linear_visible_spectrum.svg/200px-Linear_visible_spectrum.svg.png
Is frequency the sole deciding factor then?
I disagree here with Puzzled
I understand that wavelength determines the Spectral colors.

by the way, Puzzled you are my ex-student rite? LOL
hope you find the forum useful in your uni studies.
there are many applets you can learn from by doing. Enjoy Smiley
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Fu-Kwun Hwang
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Reply #3 on: March 22, 2009, 07:26:52 pm » posted from:Taipei,T\'ai-pei,Taiwan

Quote
wavelength of visible light is indeed ~400 to 700 nm

The correct statement should be :
The wavelength of visible light in vacuum is between 400-700 nm.
From my personal point of view: wavelength is not a good indicator for light
because it has different value when travel in different meterial.
Frequency should be a better parameter to specify light/electromagnetic wave.

However, people used to refer to vialble light with wavelength (wavelength of light in vacuum.)
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Puzzled
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Reply #4 on: March 22, 2009, 10:47:19 pm »

by the way, Puzzled you are my ex-student rite? LOL
hope you find the forum useful in your uni studies.
there are many applets you can learn from by doing. Enjoy Smiley

Lol you've guessed right, Mr Wee ><;

Thanks for the heads up, this forum here is a goldmine of information! The applets here are wonderful! Cheesy

When light travel from one media to another media, part of the light will reflected back to the original media.
Some of the light will enter another media and change it direction (due to change in velocity).
Yes, the wavelength of the light will change when enter different media.(only frequency will not change).
The wavelength of light in the water is different from wavelength in air.

You forgot that light must enter into our eye in oder to see it.
The path of the refracted light you saw, must be back to the air and enter you eye for you to see it.
So the wavelength is the same when light enter your eye again!

Wow, of course! Thanks very much!

I must apologise for dragging this question out, but just to clarify, would the refracted beam within the material with a higher refractive index than air (meaning that before it emerges back into air) still be red (theoretically), if say a red laser was used?

EDIT: But I also understand from what you said that the wavelengths of visible light are different in different mediums, so i guess that means that the laser would still retain its colour...
« Last Edit: March 22, 2009, 10:51:59 pm by Puzzled » Logged
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Reply #5 on: March 23, 2009, 06:05:07 pm » posted from:Taipei,T\'ai-pei,Taiwan

We only sense color of light when it enter our eye.
We saw RED laser light path in the water (or other material ) is because some of the light is reflected back to our eye.
We would not see it if there is not reflection (Just like we do not see laser light path in air normally (Unless there are a lot of dust around).
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Puzzled
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Reply #6 on: March 23, 2009, 07:47:43 pm »

Ah yes...I just remembered also that our eye functions as a lens, and refracts the light beam so it would appear as the same colour anyway. Thanks for the help!
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