Refraction
Refraction occurs when light passes from one medium to another, for example from air to glass or water to air or the reverse. The reason is that the light slows down when it enters a denser medium (and speeds up when it leaves). In the diagram we can see that this makes the direction of the waves alter.
Waves slow down and become bunched up as they enter the glass the glass and then speed up and spread out as they leave (note that the speed and the wavelength change but the frequency remains the same).
When we pass light through a shape where the sides are not parallel, as in this prism, we get a spectrum. Because the speed of the different waves, which we see as colour, travel at different speeds in dense material. (They all travel at the same speed in a vacuum and almost the same in air). As a consequence the different waves/ colours in white light refract differently. Blue slows the most and is refracted the most, red refracts the least and so the white light is split up.
Snell’s Law
The explanation of the connection between the angles as the light bends is called “Snell’s Law”.
Snell's Law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for any pair of materials. It is called the refractive index, the symbols used are sometimes μ but usually n. Most refractive indexes are quoted between the material and air.
The refractive index is inversely proportional to the speed of light in that material, so:
sin i/sin r = ng /na= speed of light in air/speed of light in glass
where ng is the refractive index for glass. The refractive index of air na, as for a vacuum is 1 for glass the values vary between are between 1.45 and 1.8. The denser the glass, the slower the light travels and the higher the refractive index. For water nw is 1.33 meaning that light travels 1.33 times faster in a vacuum or air as it does in water.
Measuring the refractive index
We can make a measurement of the refractive index by measuring the angle of incidence and refraction as light passes from one medium to another. The images below show two rays entering the same block and then a drawing which was made by marking the paths of the rays and measuring the angles.
For the ray entering the block from air to perspex
The speed of light in air = np = sin i
The speed of light in perspex na sin r
Using the second part of this equation and remembering that the refractive index of air is (the same as a vacuum) 1
np /1 =sin 35/ sin 24 = 0.574/0.399 = 1.44
using the figure from the second ray:
np /1 =sin 54/sin 34 = 0.809/0.559 = 1.45
these are not exactly the same but the angles were only measured to the nearest 0.5 degrees.
We could have used the results from the exiting rays which are (expectedly) almost the same except the equation would be inverted.