Question

In which of the following, the image of an object placed at infinity will be highly diminished and point-sized?

• A. Concave mirror only
• B. Convex mirror only
• C. Convex lens only
• D. Concave mirror, convex mirror, concave lens and convex lens

Answer 1

The correct option is D

Concave mirror, convex mirror, concave lens and convex lens

The explanation of the correct option

In case of option D

When light rays from infinity strike a concave mirror, convex mirror, concave lens, or convex lens, a drastically decreased picture are generated at the focus, as illustrated in the figure.

The mirror equation and lens equation can be used to confirm this.
Mirror equation is given by: $\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$
Lens equation is given by: $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
Substituting $u=-\infty$, in the mirror formula or the lens formula, we get $v=f$
Magnification of the image formed by either lens or mirror is $m=\pm \frac{v}{u}=\pm \frac{f}{\infty }=0$

Thus, the image is point size.

Hence, option D is the correct answer.