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in Ray Optics and Optical Instruments by (26.7k points)
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A beam of light converges at a point P. Now a lens is placed in the path of the convergent beam 12 cm from P.  At what point does the beam converge if the lens is

1. a convex lens of focal length 20 cm, and

2. a concave lens of focal length 16 cm?

1 Answer

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by (26.1k points)
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Best answer

Here the object is virtual and the image is real.

u = 12 cm object on right and virtual.

1. f = +20 cm

\(\frac{1}{v}\) = \(\frac{1}{f}+\frac{1}{u}\) = \(\frac{1}{20}+\frac{1}{12}\) = \(\frac{2}{15}\)

i.e., v = 7.5 cm. (image on right and real). It is located 7.5 cm from the lens.

2.  f = -16 cm

\(\frac{1}{v}=-\frac{1}{16}+\frac{1}{12}=\frac{1}{48}\)

i.e., v = 48 cm. (image on right and real). Image will be located 48 cm from the lens.

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