Question:
at what distance the object having hight 2cm should be placed in front of a concave mirror of focal length 10 cm to get an erect image of size 6cm
Answer:
Focal length of concave mirror f = -10 cm
Height of the object ho = 2 cm
Height of the image hi = 6 cm
Magnification m = hi/ho = 6/2 = 3 (erect image)
m = -v/u
3 = -v/u ⇒ v = -3u
Mirror formula: 1/v + 1/u = 1/f
1/(-3u) + 1/u = 1/(-10)
⇒ u = -20/3 = -6.7 cm
So the object should be placed at a distance of 6.7 cm from the pole of the concave mirror (on the left, in front of the mirror).
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