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Study Notes: Lenses

Lenses, made from ground glass or quartz, can be described as convex or concave. A convex lens (roughly oval shaped) and a concave lens (thinner in the middle, like an apple core).

Light is refracted as it passes through a lens. A convex lens will bend the light rays so that they are focused at a single point.

These lenses are used to concentrate a light beam into an intense spot called the focus. A concave lens can collect weak (low intensity) light and make it more intense. A beam of light striking and passing through a convex lens causing the emergent rays to converge and focus at a single point.

A concave lens will bend the light so that it spreads out (diverges). A beam of light striking and passing through a concave mirror causing the emergent rays to diverge.

Some optical instruments will use a combination of lenses to make a wide, low intensity beam of light much narrower and more intense. Intense light is much easier for an optical instrument to detect and measure.

Lenses suffer from two common defects.

‘Spherical aberration’ occurs when light rays near the edge of a lens are focused at a point that is different to light rays near the middle of the lens. The individual rays of a beam of light emerging from a convex lens do not converge to exactly the same point.

This defect may be corrected by placing a barrier in front of the lens to block any incident light from reaching the edges of the lens. A barrier in front of a convex lens causes the outside rays of a light beam to be blocked.  This results in the emergent rays to converge to a single defined point.

‘Chromatic aberration’ occurs when the different colours in white light are refracted by different amounts. The image will appear to have coloured edges and is not clear. One method of preventing this defect is to place another lens in the light path to combine the colours again.

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