Refraction of Light

Refraction of Light

  • Refraction is the change of direction a light ray undergoes when it enters a medium with a different optical density
  • Light travels at different speeds in materials with different optical densities, and the change in direction occurs due to the change in speed of light
  • Speed of light for any electromagnetic wave is 2.98x10^8ms-1
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Equations

(n1/n2) = (sinθ2/sinθ1) = (v2/v1) = (λ2/λ1)

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Medium 1 refractive index = n1

Medium 2 refractive index = n2

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Snell's law of refraction states:

n1Sinθ1 = n2Sinθ2

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For a light ray travelling from air into a transparent substance:

refractive index (n) = sini/sinr (i = incidence, r = reflected)

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Refraction of Light by Glass

Use a ray box to direct a light ray into a rectangular glass block at different angles of incidence at the midpoint P of one of the longer sides. Note that the angle of incidence is the angle between the incidence light ray and the normal at the point of incidence.

For each angle of incidence where the light enters the block (P), mark the point where the light leaves the block (Q). The angle of refraction is the angle between the normal and the line PQ. Measurements of the angles of incidence and refraction for different incidence rays show that:

  • the angle of refraction, r, at P is always less than the angle of incidence, i.
  • The ratio of sini/sinr is the same for each light ray. This is known as snell's law (see equations card)

Notice that partial reflection also occurs when a light ray in air enters glass (or any other refractive substance).

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Comparing glass to air refraction with air to glas

The angle of refraction of the light ray emerging from the block is the same as the angle of incidence of the light ray entering the block. This is because the two sides of the block at which refraction occurs are parallel to each other.

  • If i1 and r1 are the angles of incidence and refraction at the point where the light ray enters the block, then the refractive index of the glass n = sini1/sinr1
  • At the point  where the light ray leaves the block, i2 = r1 and r2= i1, so sini2/sinr2 = 1/n
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Refraction of a light ray by a triangular prism

This image shows the path of a monochromatic light ray through a triangular prism. The light ray refracts towards the normal where it enters the glass prism then refracts away from the normal where it leaves the prism.

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