Optics 7 Question 27

28. Which of the following form(s) a virtual and erect image for all positions of the object?

(1996, 2M)

(a) Convex lens

(b) Concave lens

(c) Convex mirror

(d) Concave mirror

Show Answer

Solution:

  1. For a lens $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$, i.e. $\frac{1}{v}=\frac{1}{f}+\frac{1}{u}$

For a concave lens, $f$ and $u$ are negative, i.e. $v$ will always be negative and image will always be virtual.

For a mirror :

$$ \frac{1}{v}+\frac{1}{u}=\frac{1}{f} \text { i.e. } \frac{1}{v}=\frac{1}{f}-\frac{1}{u} $$

Here, $f$ is positive and $u$ is negative for a convex mirror. Therefore, $v$ is always positive and image is always virtual.



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