An object placed 20 cm in front of a mirror is found to have an image 15cm i) in front of it ii) behind the mirror. Find focal length and kind of mirror in each case?
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15
given
u=-20
1)v=-15(since image is form infront of mirror)
we know
1/v+1/u=1/f
-1/ 15 - 1/20 =1/f
-(7/60)=1/f
f=-60/7cm
it is a convex mirror
2)when image form behind the mirror)
then v=15
1/15 -1/20=1/f
1/60=1/f
f=60
therefore it is a convex mirror
u=-20
1)v=-15(since image is form infront of mirror)
we know
1/v+1/u=1/f
-1/ 15 - 1/20 =1/f
-(7/60)=1/f
f=-60/7cm
it is a convex mirror
2)when image form behind the mirror)
then v=15
1/15 -1/20=1/f
1/60=1/f
f=60
therefore it is a convex mirror
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