Definition of focal length in English:

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focal length

noun

1The distance between the centre of a lens or curved mirror and its focus.
Example sentences
  • Yet Stork determined the probable focal length of a concave mirror made by inverting and silvering the convex mirror shown in the painting.
  • This produces a rounded lens with a small focal length, so the drop can focus light only to a point near itself.
  • This will give a negative focal length for a convex lens of air inside water.
1.1The equivalent distance in a compound lens or telescope.
Example sentences
  • He developed a general rule for the focal length of lenses and described a reflecting telescope.
  • Huygens later went back to the study of optics and developed long focal length lenses used in ‘aerial telescopes.’
  • One evening he was observing with a rather crude refractor (a lens telescope), with a focal length of 24 feet.

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