(a) For which position of an object, a real, diminished image is formed by a convex lens?

(b) Two convex lenses A and B have focal lengths of 2.5 cm and 12.0 cm, respectively. Which lens has a greater power?
(c) Describe with the help of a labelled ray-diagram, the formation of image when an object is placed in front of a convex lens between focus and optical centre. State any two characteristics of the image formed.

(a) A diminished, real and inverted image is formed by a convex lens when the position of an object is beyond the $2F'$.

Image is posted for reference only

(b) Among the given two convex lenses, lens A will has greater power because power is inversely proportional to focal length $(P=\frac {1}{f})$. Therefore, a lens of a small focal length which is 2.5 cm would be of greater power in this case.

Explanation

Power of the lens is given as-

$P=\frac {1}{f}$

Putting the focal length $(f_A=2.5cm\ and\ f_B=12cm)$, in the formula we get-

$P_A=\frac {1}{f_A}=\frac {1}{2.5}=\frac {10}{25}=+0.4D$

$P_B=\frac {1}{f_B}=\frac {1}{12}=+0.08D$

Thus, the power of the lens A $(P_A)$ is greater than the power of the lens B $(P_B)$.

(c) The labelled ray-diagram given below shows, when an object is placed in front of a convex lens between focus and optical centre, then the image formed is virtual, erect, magnified and on the same side as that of the object (on the left).

Two characteristics of the image formed are:

1. Nature of the image - Virtual and Erect.

2. Size of the image - Enlarged or Magnified.

3. Position of the image - On the same side as that of the object (on the left side).

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Updated on: 10-Oct-2022

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