Question

find the focal length of a concave mirror with r=20 cm
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Answer 1

We know that :-

r = 2f

f = r ÷ 2

f = 20 ÷ 2

f = 10 cm

Answer 2

focal length of a spherical mirror is half of the radius of curvature

so f=10cm

but for a concave mirror the focal length is negative

so focal length of concave mirror having r=20cm is -10cm

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