1.

(ßi) A 10 mm long pin is placed vertically in frontof a concave mirror. A 5 mm long image of thepin is formed at 30 cm in front of the mirror,find the focal length of this mirror[Ans.き-10 cm]

Answer»

We know that

m= h'/h= -v/u

5/10=-30/u

u=-300/5

u=-60cm

1/f=1/v+1/u

1/f = 1/-30 + 1/-60

1/f = -2-1/60

1/f=-3/60

1/f=-1/20

f=-20cm

Therefore the focal length of the concave mirror is 20 cm front of the mirror



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