Q.
A concave lens forms the image of an object such that the distance between the object and image is 10cm and the magnification produced is 1 / 4 The focal length of the lens will be
2553
227
Ray Optics and Optical Instruments
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Solution:
A concave lens forms virtual image I of a point object O.
As, m=41=uv ∴u=4v
If v=−x,u=−4x
Distance between object and image, ∣OI=4x−x=3x=10cm
or x=310cm ∴u=−340cm and v=3−10cm
Using lens formula, f1=v1−u1 f1=−103+403=−409 ; ∴f=−940cm=−4.4cm