Question

A point object $\left(O\right)$ is moving along the principal axis of a concave mirror with uniform velocity towards the pole. Initially, the object is at infinite distance from pole on right side of mirror as shown in the figure. Before the object collides with mirror, find the number of times for which the distance between object and its image is $40\text{cm}$.

• A. Two times
• B. Three times
• C. Four times
• D. Five times

Answer 1

The correct option is B Three times


In the following cases, it can be possible that distance between object and image is $40\text{cm}$ -
$\left(1\right)$ When the object is beyond the centre of curvature, then its image will form between focus and centre of curvature.
$\left(2\right)$ When the object is between centre of curvature and focus, then its image will form beyond the centre of curvature.
$\left(3\right)$ When the object is between focus and pole, then its image will form behind the mirror.
$\begin{array}{c}\text{Why this question ?}\\ \text{It challenges you to apply the}\\ \text{fundamentals of ray diagram or image formation}\\ \text{for a spherical mirror.}\\ \text{Any finite distance between the object and the image will appear same number of times i.e three times for the conditions mentioned above.}\end{array}$