# Refractive index- will it travel back along the same way?

1. Dec 2, 2013

### Outrageous

Refractive index----- will it travel back along the same way?

From the picture
http://en.wikipedia.org/wiki/File:RefractionReflextion.svg
If I travel a light ray from n2 to n1 , I think the light ray will travel in the opposite direction shown in the first picture.
But what about the second? The light that 90 degree will find the same way back? I think it should go straight? How should the light travel and why?
Thanks

2. Dec 2, 2013

### Drakkith

Staff Emeritus
Treating it is a ray, it would simply go straight since it isn't hitting the boundary and trying to move from one medium to another.

3. Dec 2, 2013

### sophiecentaur

I agree. There is no refraction for any angle of incidence greater than the critical angle - and that includes 90 degrees. (It goes without saying that we are starting in the more dense medium.)

4. Dec 2, 2013

### Outrageous

Then is this picture correct?
This is an experiment to find the region n2 refractive index

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5. Dec 2, 2013

### sophiecentaur

That equipment is based on the Abbé refractometer(?) which can be used to analyse tiny samples of liquids. So, yes, you could use it.

6. Dec 2, 2013

### Outrageous

No, I am using spectroscopy and prism to detect the refraction index.

7. Dec 2, 2013

### Drakkith

Staff Emeritus
Are you using total internal reflection to find the refractive index, or what?

8. Dec 2, 2013

### Outrageous

Sorry, it is spectrometer.
Yup it is use to find the refractive index of the glass cell.
My book says : a sharp boundary distinguishes the bright from the dark region because no rays are refracted with an angle of refraction larger than critical angle.
So ? I think the refraction will occur along the liquid glass interface. But I feel I still don't understand how the monochromatic light ray is seen.

9. Dec 2, 2013

### Drakkith

Staff Emeritus
From wiki: http://en.wikipedia.org/wiki/Abbe_refractometer

Is your light source illuminating the entire face of the prism, or is it a single thin beam? I'd expect it to be the former, as I don't see how a thin beam at a 90 degree angle will be refracted or reflected anywhere.

10. Dec 3, 2013

### sophiecentaur

Looking at the diagram again, it would appear that it is not using 'reflection' but just the limiting case of refraction as the critical angle. (Reflection must obey the laws of reflection and that ray is not reflecting) The incident ray must be travelling along the 'inside' of the liquid and just grazing the inside surface, to be refracted to just under the critical angle and into the prism. If you look at Abbe refractometers, the setup is pretty much the same, looking at the extinction angle. Of course you need a monochromatic light source or the critical angle is not so easy to find (refractive index is wavelength sensitive).