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Refraction

Lenses rely on bending light in order to form an image.  When a ray of light passes from one clear material to another (say, from air to glass or air to water), it gets bent at the boundary towards a different direction.  This bending (or refraction [1]) can be seen in Figure 4a, where a beam of light is bent as it passes to and from a glass slab.  Refraction also takes place in Figure 4b, causing an ordinary pencil to seem like it is being bent at the surface of the water.

Figure 4a: Refraction through a glass slab [2]		Figure 4b: An example of refraction [3]

The amount of bending is determined by the angle at which the light rays strike the surface and by the bending power (or refractive index [4]) of the two materials.  Figure 6 shows a ray of light passing from air (whose refractive index is n₁) to water (whose refractive index is n₂).   The direction of the refracted light ray is given by Snell’s law [5].   According to the law, a ray coming towards the surface at an angle θ₁ will be refracted at an angle θ₂ such that:

n₁ × sin(θ₁) = n₂ × sin(θ₂).

Figure 5: Geometry of refraction

 

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