I. What It Is
A Plano-Concave Lens is a single lens with one flat surface and the other being a concave spherical surface. The radius of the concave surface determines the focal length, which is always negative, hence it is also known as a negative lens or a diverging lens.

II. Working Principle
When a parallel beam of light enters along the optical axis, the light diverges in a fan shape after refraction by the concave surface; the backward extensions converge at the same point, known as the virtual focus F. Using the Gaussian lens formula
1/f = (n?1)(1/R??1/R?)
the focal length can be quickly estimated, where R?=∞ (flat surface) and R? is negative (concave surface).

III. Key Parameters
? Material: UV fused silica (190 nm-2.1 μm), BK7 (visible-near infrared), CaF? (infrared)
? Diameter: ?6-?100 mm standard stock
? Focal Length: ?10 mm to ?1000 mm available
? Surface Accuracy: λ/4 @632.8 nm
? Coating: MgF? single-layer anti-reflection (R<1.5%) or broadband BBAR (400-700 nm, R<0.5%)

IV. Typical Applications

1. Laser Beam Expansion: Combined with a plano-convex lens, it achieves 2×-50× continuously adjustable beam expansion, reducing power density and protecting subsequent optical components.

2. Aberration Compensation: Adding a plano-concave lens to a collimating lens group offsets spherical overcorrection, improving spot roundness.

3. Teaching Demonstration: Using a 532 nm laser pointer + plano-concave lens, a clear diverging light ring is formed on the wall, visually demonstrating the concept of a virtual focus.

4. Optical Metrology: Paired with an interferometer, it detects the radius of curvature of concave mirrors with an accuracy of up to ±0.1%.

V. Usage Tips
? Face the concave surface toward the incoming light direction to reduce spherical aberration and achieve a better divergence angle.
? For high-power laser applications, choose quartz material with AR coating to prevent thermal lensing effects.
? When cleaning, first use an air blower to remove dust, then gently wipe with a drop of anhydrous ethanol to avoid scratching the soft coating.