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Physics vocabulary · Light and Optics

Focal length

Meaning in physics, with examples

nounCommon term

In physics, focal length means: The distance from the centre of a lens or mirror to its focal point, where parallel rays of light meet or appear to come from. It is usually given in centimetres or metres.

In Turkish: odak uzaklığı

How it is used

Symbol f. The shorter the focal length, the more powerful the lens. Lens power is measured in dioptres (D): power = 1/f, with f in metres. It appears in the lens equation, which in one common sign convention is 1/f = 1/u + 1/v.

Example sentences

  1. A lens with a focal length of 0.5 m has a power of 2 dioptres.

  2. Focus the image of a distant window on the screen to estimate the focal length of the lens.

Common collocations

  • short focal length
  • long focal length
  • focal point
  • principal focus
  • lens equation
  • lensA curved piece of glass or plastic that bends light by refraction to form an image. A converging lens brings light rays together; a diverging lens spreads them out.
  • refractionThe change in direction of a wave as it passes from one material into another, caused by a change in its speed. It is why a straw looks bent in a glass of water.
  • reflectionThe bouncing back of light, sound or another wave when it hits a surface. For a flat mirror, the angle of reflection equals the angle of incidence.

This word in other fields

The same English word can have a different meaning in another field:

More terms: Light and Optics

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