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Define the power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm. What is the nature of the lens and its power used by each of them?

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  • Degree of convergence and divergence provided by a lens is called power of the lens. 
  • Unit of power of lens is Diopter D. 
  • Focal length of lens used by first student is in positive hence it is a convex lens.
  • The lens of second student is a concave lens.

p = 1/f 

= 1/0.5

= 2 

Power of lens (first student) =+2 

Power of lens (second student) = -2

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