Question

The focal length of a concave mirror is 20 cm. The distance at which an object should be placed to get a real image of same size is:

• A. 10 cm
• B. 20 cm
• C. 40 cm
• D. 30 cm Correct Answer: (C) 40 cm Solution:

Hint:

For a concave mirror: \[ \text{Same-size real image} \Rightarrow \text{Object at } C = 2f \] This is one of the most important standard ray-diagram results.

Answer 1

The Correct Option is C

Step 1: Know the special position.
A concave mirror gives a real image that is the same size as the object only when the object sits at the centre of curvature.

Step 2: Link it to the focal length.
The centre of curvature is twice the focal length away from the mirror.
\[ R = 2f \]

Step 3: Plug in the value.
Here the focal length is 20 cm.
\[ R = 2 \times 20 = 40 \text{ cm} \]
So the object must be placed 40 cm from the mirror.
\[ \boxed{40 \text{ cm}} \]