What is meant by power of a lens? Define its S.I. unit. |
You have two lenses A and B of focal lengths \[+\mathbf{10}\text{ }\mathbf{cm}\]and \[\mathbf{10}\text{ }\mathbf{cm}\]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. |
Answer:
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. For lens A: \[{{f}_{A}}=+10\,cm=0.1\,m\] Since the focal length is positive, it is a convex lens. Now, \[{{P}_{A}}=\frac{1}{{{f}_{A}}}+\frac{1}{0.1}=+10D\] For lens B: \[{{f}_{B}}=-10\,cm=-\,0.1\,m~~\] Since the focal length is negative, it is a concave lens. \[{{P}_{B}}=\frac{1}{{{f}_{B}}}=\frac{1}{-\,0.1}=-10D\] In case of a convex lens, image is virtual and magnified if object is placed between focus and optical centre.
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