Class 12 Physics
CBSE and UP Board
Ray Optics and Optical Instruments
Chapter-9 Exercise 9.8
NCERT Solutions for Class 12 Physics Chapter 9 Question-8
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 (a) a convex lens of focal length 20 cm, and (b) a concave lens of focal length 16 cm?
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In the given situation, the object is virtual and the image formed is real.
Object distance, u = +12 cm
Ans (a).
Focal length of the convex lens, f = 20 cm
Image distance = v
According to the lens formula, we have the relation:
1/v – 1/u = 1/f
1/v -1/12 = 1/20
1/v = 1/20 + 1/12 = (3+5)/60 = 8/60
Therefore , v = 60/8 = 7.5 cm
Hence, the image is formed 7.5 cm away from the lens, toward its right.
Ans (b).
Focal length of the concave lens, f = -16 cm Image distance = v
According to the lens formula, we have the relation:
1/v – 1/u = 1/f
1/v – 1/12 = 1/16
=> 1/v = 1/(-16) + 1/12 = -1/16 + 1/12
=>1/v = (-3+ 4)/48 =1/48
v = 48 cm
Hence, the image is formed 48 cm away from the lens, toward its right.