# The power of lens is -2.5 D. identify the lens and find ‘f’.

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The power of lens is -2.5 D. identify the lens and find ‘f’.

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Here, P = -2.5 D

f (in metres) = $\frac{1}{P}$$\frac{1}{−2.5}$ = 0.4 m = -40 cm

Negative sign represents concave lens.

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Given ,

Power of lens =  -2.5 D

We know that ,

D = 1 / f

Therefore , f = 1 / D

= 1 / -2.5   =   - 0.4 m    =     - 40 cm

The mirror is a concave lens as the focal length is negative .