1.

A concave lens has focal length 15 cm. At what distance should the object from the lens be placed , so as to form an image at 10 cm., from the lens . Also find the magnification of the lens.

Answer»

Solution :Given : F = -15 (concave LENS )
v = -10 (virtual image )
Solution :`1/f = 1/v - 1/u`
`1/-15 = 1/-10 - 1/u`
`1/u = -1/10 + 1/15`
`1/u =(-3 + 2)/30`
`1/u = -1/30`
u = -30
`THEREFORE` OBJECT distance = 30 cm
(negative SIGN INDICATES virtual image)
M = `v/u`
`M=-10/-30`
M = `1/3`


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