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

The parallax method is commonly used to determine the focal length of a concave mirror by finding the point where an object and its image move together when viewed from different angles. Let's analyze the given options to determine the correct placement of the object: 

1. When using the parallax method, the object should be placed such that the image formed by the mirror can be observed moving with respect to the object. This allows us to identify when the object and its image coincide.
2. For a concave mirror, the image properties change as the object is moved across different regions relative to the mirror’s focal point \( F \), center of curvature \( C \), and pole \( P \).
3. Placing the object between the focus \( F \) and the center of curvature \( C \) ensures that a real, inverted image is formed and can be observed. In this region, the image will appear larger and will be located beyond the center of curvature towards the pole \( P \), making it possible to use the parallax method effectively.
4. Other given ranges, such as "at any point beyond the focus \( F \)," "beyond the center of curvature \( C \)," or "between the pole \( P \) and the focus \( F \)," do not allow for both the object and its real image to be viewed together for parallax observations.

Based on this reasoning, the correct placement of the object is between the focus \( F \) and the center of curvature \( C \) of the mirror ONLY. This position allows for a clear observation and adjustment of the parallax effect, thereby facilitating an accurate determination of the mirror's focal length.