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 ?

Waves

Waves are a means by which energy travels.

There are many different types of waves. For example the waves on an ocean are physical waves caused mainly by wind. Light is an electromagnetic wave caused by excited electrons moving. Sound also moves in waves with the air particles moving backwards and forwards creating areas of different pressure.

Last week we looked at:

Reflection - when waves, whether physical or electromagnetic, bounce from a surface back toward the source. A mirror reflects the image of the observer.

and

Refraction - when waves, whether physical or electromagnetic, pass from one material into another they are deflected. The wave generally changes the angle of its direction because it's velocity changes. As it moves from a less dense to a more dense material it it bends towards the normal.

This week we are looking at a third property of waves:

Diffraction - when a wave goes through a small hole it's edges are bent and it flares out. Diffraction is a characteristic of waves of all types. We can hear around a corner because of the diffraction of sound waves. For instance, if a wall is next to you when you yell, the sound will parallel the wall. The wall may stop, but the voice doesn't; sound will almost turn the corner of the wall. This is diffraction.