Refractive Index

The refractive index of a substance is a comparision of the speed of light in that material to the speed of light in a vacuum.

This will be slightly different for different frequencies (colours) of light, and are normally quoted using an orange sodium light.

Light is bent towards the normal as it passes into a more dense material (eg air into water) and away from the normal when it speeds up by entering a less dense material (eg glass into air)






Every substance has its own index of refraction.
Here are a few examples:-
Water 1.33 Glass 1.50
Quartz crystal 1.54 Glycerin 1.47
Diamond 2.42



The angle of incidence θ1 is related to the angle of refraction θ2 by the formula (left) where n1 and n2 the refractive indices.





In class we bent light towards the normal by passing it from air through a perspex block, and then away from the nnormal as it came out the other side. In my case the angle of incidence θ1 was 43 degrees, and the angle of refraction θ2 was 28 degrees.


Now we know that Sin 43 = 0.681 and that Sin 28 = 0.469,
also that the RI air = approx 1 (same as a vacuum)

so using the formula above 0.681 / 0/469 = RI perspex 1.45

Compare this to your results.

Can we now use our knowledge of refraction to understand a rainbow ?