planoconcave

Planoconcave, or plano-concave, describes a lens or optical element with one flat (planar) face and one concave face. Such an element is a diverging lens, meaning it tends to spread incoming light and create a virtual, upright, smaller image of a real object when used alone. In standard sign conventions, planoconcave lenses have a negative focal length.

In a thin-lens approximation, the optical power of a plano-concave lens is primarily set by the concave surface. If the concave surface has a radius of curvature R and the lens material has refractive index n, the focal length is roughly f ≈ -R/(n-1). The plane surface contributes no focusing power. As the concave curvature increases (smaller R), the magnitude of the negative focal length decreases, producing stronger divergence; as R grows (flatter surface), divergence weakens and |f| increases. Temperature and wavelength changes modify the refractive index and thus the focal length.

Materials commonly used for planoconcave lenses include crown glass (such as BK7) and fused silica, often with anti-reflection coatings for specific wavelengths. Lenses may be manufactured in standard sizes or custom-diameter formats. Surface quality, polishing, and coating affect transmission and aberrations; thermal stability and dn/dT of the glass influence focal length with temperature.

Applications for planoconcave lenses include beam deflection, beam shaping, and integration into optical systems where a negative lens is required. They are also used in eyeglasses to correct myopia, typically paired with other lenses to form specific focal and aberration corrections. Compared with plano-convex lenses, planoconcave elements are inherently diverging and are chosen to achieve the desired optical power and aberration control in combination with other components.