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:
- 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.
