Refraction and the Refractive Index

Refraction and the Refractive
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”.
What about the Bending ?
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
 How rainbows
 Why lenses
........and many other phenomena

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