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Question -

Abeam of light converges at a point P. Now a lens is placed in the path of theconvergent beam 12 cm from P. At what point does the beam converge if the lensis (a) a convex lens of focal length 20 cm, and (b) a concave lens of focallength 16 cm?



Answer -

In the given situation,the object is virtual and the image formed is real.

Object distance,┬аu┬а= +12 cm

(a)┬аFocal length of theconvex lens,┬аf┬а= 20 cm

Image distance =┬аv

According to the lensformula, we have the relation:

Hence, the image is formed7.5 cm away from the lens, toward its right.

(b)┬аFocal length of theconcave lens,┬аf┬а= тИТ16 cm

Image distance =┬аv

Accordingto the lens formula, we have the relation:

Hence,the image is formed 48 cm away from the lens, toward its right.

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