Question

In parallax method for the determination of focal length of a concave mirror, the object should always be placed:

• A. at any point beyond the focus \(F\) of the mirror
• B. between the focus \(F\) and the centre of curvature \(C\) of the mirror ONLY
• C. beyond the centre of curvature \(C\) of the mirror ONLY
• D. between the pole \(P\) and the focus \(F\) of the concave mirror ONLY

Hint:

In the parallax method, correct positioning of the object ensures that the image coincides with the object, allowing accurate focal length measurement.

Answer 1

The Correct Option is B

Step 1: Principle of parallax method.
In the parallax method, the position of the object is adjusted such that the image formed by the concave mirror coincides with the object itself. The absence of parallax confirms this condition.

Step 2: Required position of the object.
For a concave mirror, an object placed between the focus \(F\) and the centre of curvature \(C\) forms a real, inverted image beyond \(C\), which can be made to coincide with the object using a pin arrangement.
If the object is placed elsewhere, the condition of no parallax cannot be satisfied correctly for focal length determination.

Step 3: Conclusion.
Therefore, the object must be placed strictly between the focus and the centre of curvature of the concave mirror.

Final Answer: \[ \boxed{\text{Between the focus } F \text{ and the centre of curvature } C \text{ ONLY}} \]