Refraction of light
Try placing a straw in a glass full of water. What do you observe? You may observe that the straw appears to be broken or twisted just at the point where it meets the surface of the water.
This is caused by the refraction of light and this effect is also called as the broken stick effect.
Refraction of light occurs because:
- Light enters a different substance or material
- Light suddenly slows down when it enters the material
Facts on refraction
A ray of light is bent towards the normal when it enters an optically denser medium. Here the angle of refraction r is less than the angle of incidence i.
A ray of light is bent away from the normal when it enters an optically less dense medium. For example from glass to air.
A ray emerging from a parallel-sided block is parallel to the ray entering, but is displaced sideways.
A ray travelling perpendicular along the normal at 900 is not refracted and the ray of light simply passes through the material.
Note! ‘Optically denser’ means having a greater refraction effect; the actual density may or may not be greater.
Refractive index and Snell’s law
When a ray of light enters a glass block, it slows down and bends towards the normal. Refractive index is the quantity that describes how much light is slowed down.
For example, if the speed of light is halved when it enters a particular material, its refractive index of that material would be 2.
Hence, an equation can be written to find the refractive index n of a material:
The more the light is slowed down when it enters a medium from air, the greater is the refractive index. A diamond has a refractive index (n) of 2.4; hence we can say that light slows down by a refractive index (n) of 2.4 when it enters a diamond.
Snell’s law is the law that relates the size of an angle of refraction (r) to the angle of incidence (i). It is interesting to note that Snell’s law also relates to refractive index as well!
Hence we can write an equation for Snell’s law:
Dispersion of light and refraction
The splitting up of white light into a spectrum is known as dispersion.
White light splits up into a spectrum of colours because it is a mixture of all the different colours of the spectrum.
Ever wondered what happens in a prism to produce a spectrum? Let’s see:
- White light enters the prism.
- It gets refracted (as its speed slows down and its direction changes).
- Dispersion occurs as each colour in the spectrum is refracted by a different amount.
- Violet light slows down the most; it is refracted the most.
- Red light slows down the least; it is refracted the least.
Note: Laser light is not dispersed by a prism. It is refracted, so that it changes its direction. However, it does not split up into a spectrum. This is because it is monochromatic.
A monochromatic light is a light of a single colour and wavelength.
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