Optics Glossary

The distance from the optical center of a lens to the focal point, where parallel rays of light converge (or appear to diverge from). Measured in meters or centimeters.

A dimensionless number that describes how fast light travels through a material compared to vacuum. Higher values indicate slower light propagation. For air, n ≈ 1.00; for glass, n typically ranges from 1.4 to 1.9.

The radius of the sphere from which a lens surface is a portion. A larger radius means a flatter surface. The sign indicates the direction of curvature relative to the light source.

A lens that causes parallel light rays to converge to a focal point. Has a positive focal length. Thicker in the middle than at the edges (e.g., biconvex lens).

A lens that causes parallel light rays to spread apart, appearing to originate from a virtual focal point. Has a negative focal length. Thinner in the middle than at the edges (e.g., biconcave lens).

A lens whose thickness is negligible compared to its radii of curvature and focal length. The lens maker formula applies directly to thin lenses without thickness correction.

A lens where thickness significantly affects optical properties. Requires additional terms in the lens maker formula to account for the optical path through the lens material.

A lens with both surfaces curving outward (convex). The most common type of converging lens, used in magnifying glasses and many optical instruments.

A lens with both surfaces curving inward (concave). A common diverging lens used in eyeglasses for nearsightedness and beam expanders.

A lens with one flat (plano) surface and one convex surface. A simple converging lens often used in focusing applications.

A lens where both surfaces curve in the same direction, but with different radii. Can be either converging or diverging depending on the relative radii.

The point on the principal axis where parallel rays of light converge (real focus) or appear to diverge from (virtual focus) after passing through the lens.

An imaginary line passing through the centers of curvature of both lens surfaces. Light rays along this axis pass through without bending.

The point on the principal axis through which light rays pass without deviation. For a thin lens, it is at the geometric center of the lens.

The fundamental law of refraction relating the angles of incidence and refraction to the refractive indices of two media. Forms the basis for deriving the lens maker formula.

The reciprocal of the focal length measured in meters. Expressed in diopters (D). A 0.5 m focal length lens has a power of 2 D.

The ratio of image size to object size. Can be lateral (height ratio) or angular (angle ratio). Positive for upright images, negative for inverted.

A set of rules for assigning positive or negative values to optical quantities. The Cartesian convention measures all distances from the optical center, with positive to the right.