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A candle flame 3 cm high is placed at a ...

A candle flame 3 cm high is placed at a distance of 3m from a wall. How far from the wall must a concave mirror be placed so that it may form 9 cm high image of the flame on the same wall ? Also find the focal length of the mirror.

A

225 cm

B

300 cm

C

450 cm

D

150 cm

Text Solution

Verified by Experts

The correct Answer is:
C

The candle of height AB = 3 cm is placed at a distance u from the concave mirror.
` therefore v=u+3` and it is -ve.
`m=(I)/(O)=-(9)/(3)=-3` for a real image
`therefore -(v)/(u)= -((u+3))/(u) therefore (u+3)/(u)=3`
`therefore 3u =u+3 " " therefore 2u=3 " " therefore u=(3)/(2) m`
`therefore` The distance of the wall from the mirror
`=(u+3)m=4.5 m=450 cms.`
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