Question
Thu November 17, 2011 By:

The expression for the magnification of a spherical mirror in terms of focal length (f) and the distance of the object from mirror (u) is : a) -f/ (u-f) b) f/ (u+f) c) -f/ (u+f) d) f /(u-f) THE ANSWER I CHOSE WAS OPTION A.BUT TOPPPER SAYS ITS INCORRECT. Incorrect :( Correct Answer: D Magnification, m = - v/u A/C to mirror formula, 1/f = 1/v + 1/u or, 1/v = 1/f - 1/u or, v = uf / (u -f) Therefore, m = v/u = f / u -f IN THE LAST STEP ITS GIVEN THAT m = v/u BUT IN THE FIRST STEP ITS GIVEN m = - v/u CAN YOU JUST TELL ME WHATS GOING ON..??

Expert Reply
Fri November 18, 2011
The plus/minus sign in the magnification just shows that whether image formed is real image or virtual image.
For real images, we have m = v/u
For virtual images , we have m = -v/u
That's why magnification is m = v/u = f/(u-f) for the real images formed by the spherical mirror
The minus sign in the magnification is showing whether image formed is inverted (positive for it) or erect (minus sign is considered) . Otherwise , magnification in terms of u and f is always f/(u-f)
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