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When an object is placed at a distance of 25   cm from a mirror, the magnification is m 1 . But the magnification becomes m 2 , when the object is moved 15   cm farther away with respect to the earlier position and m 1 m 2 = 4 , then find the focal length of the mirror and what type of mirror it is?

Options

  1. A20   cm , convex
  2. B20   cm , concave
  3. C10   cm , convex
  4. D10   cm , concave

Correct answer

B. 20   cm , concave

Step-by-step solution

m = - v u = f f - u so, m 1 = f f - - 25 = f f + 25 Similarly, m 2 = f f + 40 and m 1 m 2 = 4 ∴ f + 40 f + 25 = 4 f = - 20   cm Negative sign shows that the mirror is concave.

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