Question

An eye doctor advised to use 2.5D lens. what is it's focal length?​

Answer 1

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P = 1 / F

so , F = 10/25 m

= 2/5 = 0.4m

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Answer 2

Answer:

power of lens is reciprocal of its focal length

power of lens = 2.5 D

therefore focal length = 1/ power of lens

so focal length is 1/2.5 = 0.4 m