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JAMB Physics 1982 Question 7

Question 7

Light and Reflection

In order to produce a parallel beam of light, a lamp is placed at a distance from the concave mirror equal to

  1. A.the radius of curvature
  2. B.half the radius of curvature
  3. C.half the focal length
  4. D.twice the focal length
  5. E.twice the radius of curvature.
Show the answer and explanation

Answer: B (half the radius of curvature)

Explanation

Light from a lamp at the principal focus is reflected as a parallel beam, and the focal length is half the radius of curvature.