Refractive Index...

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sps27

Full Member
5+ Year Member
In TBR - Physics book 2, if you take a look at the section which discusses Snell's law, page 240 - it says quite clearly that if n2 > n1, the ray bends more towards the normal and if n2 < n1, the ray bends away from the normal. n2 and n1 are refractive indexes. Assuming ray goes from medium 1, to medium 2.

Now take a look at example 10.6 a. The example talks about 2 rays, A and B, and A bends more than B. The conclusions I draw from that example is - 1) the ray that bends more i.e., away from the normal, has a higher refractive index, higher frequency, lower speed, lower wavelength. And I think all those conclusions are fine knowing that Violet refracts more than Red when a ray of light splits inside a medium.

Does anybody find this confusing? I mean, the first example says that the ray that bends more towards the normal has higher refractive index and the second example says the ray that bends away from the normal has a higher refractive index. Is there a distinction to be made between refractive index of medium v/s refractive index of ray? So in the first example they talk about refractive index of medium but in the second example they talk about 'dispersion' and refractive indexes of rays of light.

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DrknoSDN

Full Member
I think you might be misunderstanding refractive index. That has to do with the medium that the rays are traveling through. A more dense medium (usually) has a higher refractive index so if a ray enters a more dense medium it will bend towards normal.

The ray itself does not have a refractive index at all. The varying wavelengths of white light will cause rays to bend more or less when changing mediums.

sps27

Full Member
5+ Year Member
Thank you for your response, but From TBR......example 10.6a in the book....

When two rays bend differently upon entering a new medium, their indexes of refraction must be different. The ray that bends more has a higher refractive index; Ray A has the higher n. Now, consider how the wavelengths and speeds of the light rays are affected within the prism. Because v = c/n, Ray A must move more slowly than RayB. This rules out choices A and B. To determine the relative wavelengths, let's use the principal waveequation:

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DrknoSDN

Full Member
Thank you for your response, but From TBR......example 10.6a in the book....

When two rays bend differently upon entering a new medium, their indexes of refraction must be different. The ray that bends more has a higher refractive index; Ray A has the higher n. Now, consider how the wavelengths and speeds of the light rays are affected within the prism. Because v = c/n, Ray A must move more slowly than RayB. This rules out choices A and B. To determine the relative wavelengths, let's use the principal waveequation:
Sorry about that. I believe it is mostly just more common to talk about index of refraction of a material even though it is just a calculated ratio.
"Many materials have a well-characterized refractive index, but these indices depend strongly upon the frequency of light. Standard refractive index measurements are taken at yellow doublet sodium D line, with a wavelength of 589 nanometres."
http://en.wikipedia.org/wiki/List_of_refractive_indices

Regarding your original post, the first example I would say is a normal description that is accurate.
The second example where A bends more than B, if it is bending away from normal then the refractive index of the medium it is "entering" is lower than the material it was coming from. i.e. n2 < n1. That's what is described in the first example.
Or when example 2 says A 'bends more' than B, it might be saying A is bending more towards the normal than B. In that case n2 > n1 for index of refraction.

Not sure without looking at the diagrams but I believe both example are describing the same thing. Bending more towards normal is due to light entering a material with a higher index of refraction.

sps27

Full Member
5+ Year Member
Sorry about that. I believe it is mostly just more common to talk about index of refraction of a material even though it is just a calculated ratio.
"Many materials have a well-characterized refractive index, but these indices depend strongly upon the frequency of light. Standard refractive index measurements are taken at yellow doublet sodium D line, with a wavelength of 589 nanometres."
http://en.wikipedia.org/wiki/List_of_refractive_indices

Regarding your original post, the first example I would say is a normal description that is accurate.
The second example where A bends more than B, if it is bending away from normal then the refractive index of the medium it is "entering" is lower than the material it was coming from. i.e. n2 < n1. That's what is described in the first example.
Or when example 2 says A 'bends more' than B, it might be saying A is bending more towards the normal than B. In that case n2 > n1 for index of refraction.

Not sure without looking at the diagrams but I believe both example are describing the same thing. Bending more towards normal is due to light entering a material with a higher index of refraction.

Thank you for your response. I appreciate it. I think I understand this. And I agree with your assessment. That is now my understanding as well. Thanks for indulging me.