Geometrical Optics – Lens Basics & Applications

25 vocabulary flashcards summarising key terms and definitions related to lenses, image formation, and optical instruments from the lecture notes.

Lens

A transparent medium bounded by two refracting surfaces, each of which is part of a sphere.

Optic Centre (O)

The midpoint of a lens through which light passes without deviation.

Centres of Curvature

The centres of the imaginary spheres of which the refracting surfaces of a lens are parts.

Optic Axis

An imaginary straight line that passes through the centres of curvature and the optic centre of a lens.

Aperture (of a lens)

The effective area of a lens through which light actually passes.

Principal Focus (F) – Convex Lens

The point on the optic axis where rays near and parallel to the axis converge after refraction by a convex lens.

Principal Focus (F) – Concave Lens

The point on the optic axis from which rays near and parallel to the axis appear to diverge after passing through a concave lens.

Focal Length (f)

The distance between the optic centre of a lens and its principal focus.

Real Image

An image formed by actual convergence of light rays that can be projected onto a screen (e.g., cinema screen, camera sensor).

Virtual Image

An image formed by apparent divergence of rays; it cannot be captured on a screen and is seen only by looking into the lens.

Cartesian Sign Convention

A system where distances are measured from the optic centre; along the incident ray and above the optic axis are positive, opposite directions are negative.

Magnification (m)

The ratio of the height of the image (hᵢ) to the height of the object (h₀); a dimensionless quantity.

Convex Lens (Converging Lens)

A lens thicker at the centre than at the edges, which converges parallel rays to a focus.

Concave Lens (Diverging Lens)

A lens thinner at the centre than at the edges, which diverges parallel rays as if originating from a focus.

Power of a Lens (P)

The reciprocal of focal length in metres (P = 1/f); it indicates the converging or diverging ability of a lens.

Dioptre (D)

The SI unit of lens power, equal to one reciprocal metre (1 m⁻¹).

Objective Lens (Telescope)

The large-aperture lens with a long focal length that forms a small, real image of a distant object.

Eyepiece (Telescope or Microscope)

The lens of shorter focal length that magnifies the real image produced by the objective.

Simple Microscope

An optical instrument that uses a single convex lens to magnify small, nearby objects.

Compound Microscope

A microscope employing an objective of short focal length and an eyepiece to obtain high magnification of tiny objects.

Telescope

An optical device with an objective and eyepiece designed to observe distant objects by producing a magnified virtual image.

Diminished Image

An image whose size is smaller than that of the object.

Erect Image

An image with the same orientation as the object (upright).

Inverted Image

An image that is upside-down relative to the object.

Spectacles

Corrective lenses mounted in a frame, used to adjust the focal length of the eye for clear vision.