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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 (A) a Convex Lens of Focal Length 20 Cm, and (B) a Concave Lens of Focal Length 16 Cm? - Physics

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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Solution

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

Object distance, = +12 cm

(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`

`:. v = 60/8 = 7.5 cm`

Hence, the image is formed 7.5 cm away from the lens, toward its right.

(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 = -1/16 + 1/12 = (-3+4)/48 = 1/48`

`:. v = 48  cm`

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

Concept: Refraction at Spherical Surfaces and by Lenses - Refraction by a Lens
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APPEARS IN

NCERT Class 12 Physics Textbook
Chapter 9 Ray Optics and Optical Instruments
Q 8 | Page 346
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