Difference between Optical and Geometrical length?

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1. Sep 30, 2014

deni

I'm searching for a week from now and can't find out difference between optical and geometrical length in optical path.
Can anyone explain or give me idea or how can I find it out?

2. Sep 30, 2014

Staff: Mentor

After looking around a bit, it appears that the "optical length" means the "optical path length", which is the product of the geometric length (the physical distance the light travels) and the refractive index of the medium.

For example, if light travels through 10 cm of water with a refractive index of 1.33 it will have traveled along a shorter optical path than light which travels through 10 cm of glass with a refractive index of 1.5, even though both have the same geometrical length.

Sources:
http://en.wikipedia.org/wiki/Optical_path_length
http://scienceworld.wolfram.com/physics/OpticalPathLength.html

3. Sep 30, 2014

Staff: Mentor

I agree that "optical path length" is probably what you're looking for. If you can give us a link to where you saw "optical length", or a brief quote if it wasn't on a web page, we can see the context and make sure about this.

4. Sep 30, 2014

deni

The content wasn't in English so this was the reason why I haven't posted reference.
I have learned Geometrical Optics but still I can't understand completely the question.

The question exactly was: What is difference between optical and geometrical length of optic path?

Now I'm really confused. So yes light speed depends on the medium but there really exist optical length or not!

How to calculate "optical path length" and how to calculate "geometrical path length"!

5. Sep 30, 2014

Staff: Mentor

The optical path length is simply the distance the light travels times the refractive index. For example, light traveling through 10 cm of water has an optical path length of 10 x 1.333 or 13.3 cm. The geometric length is simply the physical distance the light travels.

The optical path length is important because it allows us to find out what the phase of the light will be at any point, whereas the geometrical length doesn't.

6. Oct 1, 2014

deni

c1 = n * c2

where: c1 - optical path length , c2 - geometrical path length and n - refreactive index

Thank you a lot this is exactly what I was searching for.

Last edited: Oct 1, 2014