A child looking into a small polished hemispherical metal bowl sees an erect image of himself 21 cm from the bowl . He turns the bowl over and see a further image of himself 7 cm from the bowl .what is the radius of curvature of the bowl

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The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.