Question

focal length of concave lens is 20 cm .what is its Power​

Answer 1

Answer:

Focal length of concave lens = -20 cm = -20/100 m

Power = P₁ = 1/-20/ 100 = -5 D

Focal length of convex lens = +25 cm = 25/100 m power = P₂ = 1/25/100 = 4 D

So net power = P = P₁ + P₂ = -5D + 4D = -1 D (implies combination behave as concave lens).

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Answer 2

Power of the concave lens = -20D

The focal length of the concave lens = -20cm

Power is said to be the reciprocal of focal lenght length in m(meter).

Diopter (D) is the unit of power of the lens.

1 diopter is the power of a lens whose focal length is 1 meter.

Hence, 1D = 1m-¹

The power of concave lens is negative.

Therefore, P = 1

f

P = 1

-20

P = 1

- 0.2

P = -20D

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