A reading lens of focal length is used as a simple magnifier

PHYSICS · Subjective · CBSE · Physics · Ray Optics and Optical Instruments · Ray Optics and Optical Instruments

Problem

A reading lens of focal length f=5 cmf = 5\,\text{cm} is used as a simple magnifier. (a) With the help of a suitable ray diagram, show that when the lens is adjusted so that the final image is at infinity, the angular magnification m∞m_\infty is given by m∞=Dfm_\infty = \dfrac{D}{f}, where DD is the least distance of distinct vision. (b) If the lens is instead adjusted to form the final image at the near point, derive the expression mD=1+Dfm_D = 1 + \dfrac{D}{f} for the angular magnification. (c) Taking D=25 cmD = 25\,\text{cm}, calculate the percentage increase in magnification obtained by shifting the final image from infinity to the near point.

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