  Question

What is meant by power of a lens?  You have two lenses A and B of focal lengths +10 cm and -10 cm respectively. State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8 cm from the lens? Draw a ray diagram to justify your answer.

Solution

Power of a lens is defined as the ability of less to converge or to diverge a beam of light. It is also defined as reciprocal of focal length in metres. Let fA be focal length of lens A and PA be the power of lens A. For lens A: fA = + 10 cm = 0.1 m Since the focal length is positive, it is a convex lens. Now, PA=1fA=10.1=+10D. For lens B : fB = -10 cm = - 0.1 m Let fB be focal length of lens A and PB be the power of lens B. Since the focal length is negative, it is a concave lens. PB=1fB=1−0.1=−10D.  In case of a convex lens, image is virtual and magnified if object is placed between focus and optical centre. Biology

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