### Refraction and the Refractive Index

```Refraction and the Refractive
Index

Why does light bend when it goes from one
substance into another ?
Refractive Index

Because different substances
have different Refractive
Indices !
So what’s that ?
Light travels at different speeds through
different substances.
 The property of a substance which causes
this is called the REFRACTIVE INDEX

It is a Number
Refractive index is a number.
 It is the ratio of the speed of light in a
vacuum to its speed in a substance


By convention, we give it the symbol “n”.

Light will bend at the junction of two
different substances because of the Principle
of Least Time.
Fermat Thought of It back
around 1650
Pierre de Fermat was a
French mathematician.
 Fermat’s principle of least time
states that:
 Light will travel in whatever
direction will minimise the time
taken to complete its journey.

......and this means

The shortest journey for light passing
through several different substances is
Never a straight line.
Snell’s Law

This gives rise to Snell’s Law
 n1.sinA
= n2.sinB
It also Explains......
Why the sky is
blue
 How rainbows
happen
 Why lenses
work


........and many other phenomena
```