Q20 of 58 Page 1

List the sign conventions that are followed in case of refraction of light through spherical lenses. Draw a diagram and apply these conventions in determining the nature and focal length of a spherical lens which forms a four times magnified, real image of an object placed 20 cm from the lens.

The sign conventions that are followed in case of refraction of light through spherical lenses are as follows:

1. All the distances are measured from the optical centre of the lens.


2. All the distances which are measured in the same direction of the incident light ray will be taken as positive.


3. All the distance which are measured against the direction of incident light are taken as negative.


4. The perpendicular distances to principal axis in upward direction will be positive and those in downward direction will be taken as negative.


Given: h2 =-4 h1 (image formed in downward direction)


Object distance u= -20 cm


Magnification = +4 (Real image)


Magnification is given by



Therefore, v=-4×-20



Since image distance is positive, so it will form on the right side of the convex lens.


The ray diagram will be as follows:


Image is formed 80 cm from the lens on the opposite side of the object, i.e. on the right side of the lens.


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