# The focal length of a concave mirror is 30cm, find the position of the object in front of the mirror so that the size of the image is three times the size of the object.. please help me out with this one

2
by richiegooner

2015-03-23T22:43:09+05:30

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F = -30 cm
let position of object = u cm
position of image = v cm
magnification = -3
magnification is taken as negative as real image is formed.

m = -v/u
⇒ -3 = -v/u
⇒ 3 = v/u
⇒ 3u = v
⇒ v = 3u

we know that

The object is at a distance 40cm in front of the mirror.
2015-03-24T01:49:18+05:30

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Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
F = -20 cm
magnification m = +3 (erect image)  or  -3 (inverted image)

case 1)  erect virtual image
m = 3 = -v/u  => v = -3 u
1/f = 1/v + 1/u =>    -1/30 = -1/3u  + 1/u = 2/3u
u = -20 cm

case 2)  inverted real image.
m = -3 = -v/u  => v = 3u
-1/30 = 1/3u + 1/u = 4/3u
=>  u = -40 cm