Q22 of 56 Page 1

What is meant by power of a lens? Define its S.I. unit.

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.

Power of the accommodation of an eye is defined as the ability of an eye to change its focal length so as to focus the distant objects as well as nearby objects on the retina. The S.I. unit of power of lens is Diopter.


Given: focal length f1 = +10 cm


In meters f1 = +0.1 m


P = 1/f = 1/0.1


P = +10 D


There is positive sign on power, so lens A having focal length +10 cm is convex lens


focal length f2 = -10 cm


In metres f1 = -0.1 m


P = 1/f = 1/-0.1


P = -10 D


There is negative sign on power, so lens A having focal length -10 cm is concave lens


So, when the object is placed at 8 cm between optical center and focus on a convex lens, then image formed will be virtual and magnified. The ray diagram will be as follows:



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Akshay, sitting in the last row in his class, could not see clearly the words written on the blackboard. When the teacher noticed it, he announced if any student sitting in the front row could volunteer to exchange his. Seat with Akshay. Salman immediately agreed to exchange his seat with Akshay. He could now see the words written on the blackboard clearly. The teacher thought it fit to send the message to Akshay's parents advising them to get his eyesight checked.


In the context of the above event, answer the following questions:


(a) Which defect of vision is Akshay suffering from? Which type of lens is used to correct this defect?


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(c) In your opinion, in what way can Akshay express his gratitude towards the teacher and Salman?