Light: Reflection and Refraction – Spherical Lenses (Class X Physics)

Vocabulary flashcards covering essential terms, definitions, and formulas related to spherical lenses, their properties, sign conventions, magnification, and power, as discussed in Lecture No. 11 of Class X Physics (Light: Reflection and Refraction).

Optical Centre (O)

The central point inside a lens through which light passes undeviated; used as the origin for all distance measurements in lens sign convention.

Principal Axis

An imaginary straight line that passes through the optical centre and the two principal foci of a spherical lens.

Principal Focus (F)

The point on the principal axis where rays parallel to the axis either converge (convex) or appear to diverge from (concave) after refraction through the lens.

Convex Lens

A converging spherical lens that is thick at the centre and thin at the edges; it has a positive focal length.

Concave Lens

A diverging spherical lens that is thin at the centre and thick at the edges; it has a negative focal length.

Focal Length (f)

The distance between the optical centre of a lens and its principal focus; positive for convex, negative for concave lenses.

Sign Convention for Lenses

A set of rules: distances measured along the direction of incident light are positive, opposite directions are negative; heights above the principal axis are positive, below are negative.

Lens Formula

A quantitative relation given by 1/f = 1/v − 1/u, linking focal length (f), image distance (v) and object distance (u) for thin lenses.

Object Distance (u)

The distance from the optical centre to the object; taken as negative in the lens sign convention when the object is on the incident-light side.

Image Distance (v)

The distance from the optical centre to the image; positive for real images on the opposite side of incident light, negative for virtual images on the same side as the object.

Magnification (m)

The ratio of the height of the image to the height of the object; also equal to v/u for thin lenses.

Real Image

An image formed by actual convergence of refracted rays; it can be obtained on a screen and is inverted (m is negative).

Virtual Image

An image formed by apparent divergence of rays; cannot be captured on a screen and is erect (m is positive).

Inverted Image

An image that is upside-down relative to the object; produced by real image formation with m < 0.

Erect Image

An image oriented the same way up as the object; produced by virtual image formation with m > 0.

Power of a Lens (P)

A measure of a lens’s ability to converge or diverge light, defined as the reciprocal of focal length in metres: P = 1/f.

Dioptre (D)

The SI unit of lens power; 1 D equals the power of a lens with a focal length of 1 metre.

Converging Lens

Another name for a convex lens that brings parallel rays to a focus, producing positive power.

Diverging Lens

Another name for a concave lens that spreads parallel rays outward, producing negative power.

1 Dioptre

The power of a lens whose focal length is exactly 1 metre; numerically, 1 D = 1 m⁻¹.

Positive Power

Indicates a lens with a positive focal length (convex), capable of converging light rays.

Negative Power

Indicates a lens with a negative focal length (concave), capable of diverging light rays.

Combination of Lenses (Contact)

When thin lenses are placed coaxially in contact, their total power is the algebraic sum: P_total = P₁ + P₂ + … + Pn.

Combination of Lenses (Separated by d)

For two lenses separated by distance d (in metres), the effective power is P_total = P₁ + P₂ − d P₁P₂, valid for small separations.