Question

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

• A. Convex lens
• B. Concave lens
• C. Convex mirror
• D. Concave mirror

Answer 1

Answer: (B), (C)

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,/and u are negative, i.e. v will always be
negative and image will always be virtual.
For a mirror:
$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}i.e.\frac{1}{v}=\frac{1}{f}-\frac{1}{u}$
Here, / is positive and u is negative for a convex mirror.
Therefore, v is always positive and image is always virtual.