Multiple choice

A lens with +1 dioptre must have its focal length equal to _______.

  1. 1 cm

  2. 10 cm

  3. 100 cm

  4. 1000 cm
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The power of a lens in dioptres (D) is the reciprocal of its focal length in metres (f = 1/P). Since 1/1 = 1 metre, and 1 metre equals 100 cm, the focal length is 100 cm.

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