#1
Hi. In my written notes from class I have it written down that different wavelengths travel at different speeds through materials:

'In practice, low frequency, long wavelength waves travel more slowly through materials than high frequency, short wavelength waves.'

but later on I have:

'The higher the refractive index of the material, the lower the wavespeed and the shorter the wavelength. Therefore, the refractive index for a violet light is greater than the refractive index for a red, hence violet light refracts more on passing through a prism.'

It seems to me that these 2 quoted statements contradict one another. The second paragraph says that the lower the wavespeed, the shorter the wavelength, but the first paragraph states that short wavelengths travel faster through materials than long wavelengths. Am I making sense? Any help appreciated, cheers.
0
7 years ago
#2
(Original post by psychojunglecat)
Hi. In my written notes from class I have it written down that different wavelengths travel at different speeds through materials:

'In practice, low frequency, long wavelength waves travel more slowly through materials than high frequency, short wavelength waves.'
but later on I have:

'The higher the refractive index of the material, the lower the wavespeed and the shorter the wavelength. Therefore, the refractive index for a violet light is greater than the refractive index for a red, hence violet light refracts more on passing through a prism.'

It seems to me that these 2 quoted statements contradict one another. The second paragraph says that the lower the wavespeed, the shorter the wavelength, but the first paragraph states that short wavelengths travel faster through materials than long wavelengths. Am I making sense? Any help appreciated, cheers.
It's the statement in bold that is the problem.
It is a generalisation and is not true.
Water waves for example. In deep water, longer wavelength waves travel faster than short wavelength.
The speed of sound waves in air is very nearly independent of their wavelength.
Red light travels faster through glass than blue light. So longer wavelength light travels faster than shorter wavelength.
It's the fact that blue light is slowed down more than red that causes it to deflect more.
The "contradiction" is the result of the first statement being incorrect.
#3
(Original post by Stonebridge)
It's the statement in bold that is the problem.
It is a generalisation and is not true.
Water waves for example. In deep water, longer wavelength waves travel faster than short wavelength.
The speed of sound waves in air is very nearly independent of their wavelength.
Red light travels faster through glass than blue light. So longer wavelength light travels faster than shorter wavelength.
It's the fact that blue light is slowed down more than red that causes it to deflect more.
The "contradiction" is the result of the first statement being incorrect.
By deflect, do you mean refract?

In any case- thanks Stonebridge, that helps a lot. I was starting to get mentally tongue-tied thinking about it too much but you've cleared up my confusion. Cheers!
0
7 years ago
#4
(Original post by psychojunglecat)
By deflect, do you mean refract?

In any case- thanks Stonebridge, that helps a lot. I was starting to get mentally tongue-tied thinking about it too much but you've cleared up my confusion. Cheers!
Yes. Deflect means refract here. Just delete that first sentence from your notes and forget it.
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