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Question
Name the type of lens whose power is positive.
The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40 cm from the lens, what is the nature and power of the lens ? Draw ray diagram to justify your answer.
Solution
The power of a convex lens is positive.
It is given that the image of the object formed by the lens is real, inverted and of the same size as the object. Also, we know that only convex lens can form a real and inverted image. Thus, the given lens is a convex lens. In the given case, the object is placed at 2F1 and the image is formed at 2F2.
It is clear from the figure that the focal length of the lens (f) is 40 cm.
Now, power (P) is given by
\[P = \frac{1}{f}\]
\[\Rightarrow P = \frac{1}{20 \times {10}^{- 2} m}\]
\[ \Rightarrow P = + 5 D\]
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