The power of lens is 2.5 D. What is its focal length ?
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Answered by
16
The power of lens is given by :
P=1/f
2.5=1/f
f=1/2.5
f= 0.4 m
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1
Answer:
1. Power P =1/f = 2.5D
f= 1/p= 100/2.5= 40 cm
Here f= 40 cm which is positive , it means the lens is a convex lense .
2. In case of a concave f is negative . So in the question f = 30 cm
Magnification m = -v/u =3
It means v=-3u
Mirror formula =1/f =1/v+1/u
1/(-30)= 1/(-3u)+1/u
-1/30 =1/u-1/3u
-1/30=3/(3u)
u=-20cms
It means the object is placed 20 cms in front of the mirror. Since focal length is -30 cm , it means the object is placed between focus and optical lenses .
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