If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Absolute & relative refractive index

Let's explore the refractive index in detail. We will see the difference between absolute and relative refractive index. Created by Mahesh Shenoy.

## Want to join the conversation?

• Why can’t we mention the refractive index with respect to air and call it “relative index” (with respect to air / vaccum ) ?
• Because defining refractive index with respect to air/vacuum is so useful that it got its own term.
• We write refractive index as “mew” ( the Greek alphabet) and not “n” ?
• Either one can be used. In most textbooks that I found, "n" was used, but "mu" was also used in some textbooks. I would advise you to go ahead with what is given in your textbook.
Happy if I was able to help. :)
• Isn’t speed of light in water more than the speed of light in oil? In the video, he says the opposite..
• he does! maybe it was purely as an example.
• Can our first medium be vacuum? I mean, can we say light travelling from vacuum to any another medium ? And will it be called as absolute refractive index ?
• yes and generally in most cases, we get our first medium as vacuum or air. And yes when we choose vacuum or air as a reference medium we call it Absolute Refractive Index.
(1 vote)
• In my book, there is some mistake. In one line, it is written that ¹n₂ is refractive index of medium 2 w.r.t. (with respect to) medium 1. But after a few lines, it is written than ¹n₂ is refractive index of medium 1 w.r.t. medium 2. I am totally confused. Which one is right?
(1 vote)
• The first one is right. _{1} n _{2} is the refractive index of medium 2 with respect to medium 1.