Spherical Lens Properties Explained

To identify the incorrect statement about spherical lenses, we need to understand their key components and definitions.

Understanding Lens Terminology

• Centres of Curvature (C₁, C₂): A spherical lens has two curved surfaces. Each surface forms part of a sphere. The centres of these spheres are called the centres of curvature for the lens surfaces. Therefore, every spherical lens has two centres of curvature. This applies to both concave and convex lenses.
• Principal Axis: The imaginary straight line that passes through both centres of curvature (C₁ and C₂) of the spherical lens is defined as the principal axis.
• Optical Centre (O): This is the central point of the lens, lying on the principal axis. For a thin lens, it's roughly at the geometric centre. Light rays passing through the optical centre travel undeviated. The optical centre is distinct from the centres of curvature.

Evaluating the Statements

• Statement 1 & 3: "A concave lens also possesses two centres of curvature." and "Every spherical lens has two centres of curvature." Both are correct descriptions of spherical lenses, as explained above.
• Statement 4: "The line joining the two centres of curvature is called the principal axis." This is the standard definition and is correct.
• Statement 2: "The optical centre coincides with the centre of curvature." This statement is incorrect. The optical centre (O) is a unique point on the principal axis, while the centres of curvature (C₁, C₂) are specific points related to the curvature of the lens's surfaces. They are not the same point for a standard lens.

Based on these definitions, the statement that is NOT correct is that the optical centre coincides with the centre of curvature.