Question

Object distance (u), image distance (v), focal length (f) are all measured from

• A. focus (F)
• B. optic centre (P)
• C. centre of curvature (C)
• D. principal axis

Answer 1

The Correct Option is B

To solve this problem, we need to understand the reference point from which object distance (\( u \)), image distance (\( v \)), and focal length (\( f \)) are measured in lens or mirror systems.

1. Standard Reference Point:
In the ray diagram for both mirrors and lenses, all distances (object distance, image distance, and focal length) are measured from a common point called the optic centre in case of lenses and the pole in case of mirrors.

2. Definitions:

• Object distance (\( u \)) – Distance from the object to the optic centre.
• Image distance (\( v \)) – Distance from the image to the optic centre.
• Focal length (\( f \)) – Distance from the optic centre to the principal focus.

3. Evaluating the Options:

• (A) Focus (F) – Incorrect; it is the end point of focal length, not the reference.
• (B) Optic centre (P) – Correct; all distances are measured from here in a lens system.
• (C) Centre of curvature – Incorrect; mainly used in spherical mirrors.
• (D) Principal axis – Incorrect; it's a line, not a point of measurement.

Final Answer:
The correct answer is (B) Optic centre (P).