Centre of biconvex lens is called –
**Core Concept**
A biconvex lens is a type of convex lens that is thicker in the middle than at the edges. The centre of a biconvex lens is a critical point where the curvature of the lens is maximum, affecting its refractive power and optical properties.
**Why the Correct Answer is Right**
The correct term for the centre of a biconvex lens is the **vertex**. This is because the vertex is the point on the lens where the two curved surfaces meet, creating a sharp focus of light rays. The vertex is crucial in optometry and ophthalmology for calculating the power of lenses and correcting vision disorders. The curvature of the lens at the vertex determines its refractive power, which is essential for understanding how lenses work.
**Why Each Wrong Option is Incorrect**
* **Option A:** This is not a recognized term in optics or ophthalmology related to the centre of a biconvex lens.
* **Option B:** This term is not specific to the centre of a biconvex lens and could refer to various points on a lens.
* **Option D:** This term is not relevant to the centre of a biconvex lens and is not a recognized concept in optics or ophthalmology.
**Clinical Pearl / High-Yield Fact**
When calculating the power of a lens, it's essential to consider the vertex distance, which is the distance between the vertex of the lens and the eye. A small change in the vertex distance can significantly affect the refractive power of the lens.
**Correct Answer: C. Vertex**