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LEARNING TARGET
Solve problems involving refraction and refractive index
When light is incident on a boundary it can either be reflected or transmitted across the boundary. If the light is transmitted then often it changes direction as it crosses from one medium to another. We call this refraction.
When light enters a new medium its speed changes. Light travels fastest in substances that have a low refractive index (this generally means they have a low density). If light speeds up as it crosses the boundary (e.g glass to air) then it will bend away from the normal line. If light slows down when it crosses the boundary (e.g. air to glass) it will bend towards the normal line.
The bigger the change in refractive index the bigger the change in speed. The bigger the change in speed the bigger the change in direction.
The refractive index tells us how fast light travels in that medium compared to in a vacuum. The bigger the refractive index the slower the speed of light in that medium. You can calculate the refractive index of a material using Snell's law:
In one special case the light does not refract as it crosses a boundary and this is when it crosses along the normal. The light will still slow down (if travelling from air to glass) and this will cause the wavelength to reduce, but the wave will not change direction. The frequency of the wave is determined at the source and will not change after the wave has been produced. Therefore if the wave speed goes down then the wavelength must too. The wave-fronts on the diagram below represent the crests of each wave, so the distance between them is a wavelength.
One important application of refraction is in the use of lenses in optical devises such as glasses, microscopes and telescopes. The lenses redirect the light to a new position and this allows us to magnify an image or to correct vision problems. Convex shaped lenses cause the light from an object to converge, whilst concave shaped lenses cause the light to diverge.
Different wavelengths of light are refracted by different amounts. Violet light is refracted through a greater angle than red light and so a glass prism can be used to refract the light at both boundaries and split white light into a spectrum. This process is called dispersion. It is a similar process to how rainbows are formed as light passes through water droplets in the sky.
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