Question

What is focal power of a spherical mirror or a lens? What may be the
reason for using P = 1/f as its expression?
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Answer 1

Answer:

Explanation:

Focal power is the deviation of incident light ray produced by a lens on refraction through it.

NOW on to the second part that why we use P=1/f as its expression then consider a thick and a thin lens. The thick lens will have less focal length and so deviate the light ray more, while the thin lens will have more focal length and so will deviate the light ray less..Hence, the two are inversely proportional so that is why the expression P=1/f(in metres) is used

HOPE IT HELPS.