Why violet bends to a greater extent through prism

[SaintJude]

Topic: dispersion's poster child example of light passing through a prism.

EK & Kaplan both say that violet bends to a greater extent than the other visible colors, b/c it has a greater frequency. Cool. But they don't say WHY?!

P.S. Whenever, one sees a "rainbow" of colors in a prism, rainbow, soap bubble, that's due to "dispersion", correct?

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[milski]

You can safely skip the why in this case. All you need to know for MCAT purposes is that dispersion exists - in some materials the speed will depend on the wavelength.

Prism and rainbow in the sky are dispersion based, soap bubbles - thin film interference.

 

[SaintJude]

Ok, thanks, I guess that's the danger in dragging out content review. You start to go overboard. I guess it's sufficient to know that violet light "sees" a greater index of refraction that red does and so is bent to a greater extent.

Anyway, does anyone have to add to this? AAMC topic says "dispersion (change of index of refraction with wavelength)"

 

[MedPR]

You could skip the why, or you could know and understand a very simple (and relevent) reason why.

If you consider the equation n*lambda=n*lambda, where n is the refractive index and lambda is the wavelength, you see that higher wavelength means lower refractive index. Lower refractive index means bends least. So lower wavelength (violet is a lower wavelength than red, for example) means higher index of refraction, which means more bending.

Although rays of light don't really have a refractive index, thinking about it like that makes it easy to remember. Plus it helps you to remember the relationship between n, lambda, frequency, etc.

 

[milski]

That's just saying that it bends more because it has higher refraction index, it does not explain why the refraction index changes when wavelength changes.


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[MedPR]

That's just saying that it bends more because it has higher refraction index, it does not explain why the refraction index changes when wavelength changes.


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Oh, well in that case. Light travels more slowly in a media with a higher index of refraction. Since v=f*lambda, and the frequency is dependent on the source, only the wavelength changes when a wave changes media. So if the wave is to move slower, the wavelength must decrease.

Well, I guess I'm just talking myself in circles now. Nevermind.

 

[milski]

You can explain wavelength differences that way but not the fact that n is different for the same media and different wavelengths. That is ok - it is not a single reason and it's not an intro physics subject.

Btw, if SJ was the why question with your interpretation, that's something to be known. It's the why with my interpretation that can be ignored.


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