Lenses and Mirrors

Definition

Optical devices that form images by refracting light (lenses) or reflecting light (mirrors), used to magnify, focus, or redirect light.

Key points

• A convex (converging) lens is thicker in the middle and converges parallel light to a focus; a concave (diverging) lens is thinner in the middle and spreads light out.
• A concave mirror is reflecting on the inside of a curve and can converge light; a convex mirror bulges out and always gives a small, erect, virtual image with a wide field of view.
• Convex mirrors are used as vehicle rear-view and security mirrors because they show a wide area; concave mirrors are used in shaving mirrors, torches, and headlight reflectors.
• A convex lens corrects long-sightedness (hypermetropia) and a concave lens corrects short-sightedness (myopia); the power of a lens is measured in dioptres, the reciprocal of focal length in metres.
• Real images can be projected on a screen and are usually inverted; virtual images cannot be caught on a screen and are erect, such as the image in a plane mirror.

Why it matters for CAPF

Distinguishing convex from concave devices, their uses in vehicle mirrors and spectacles, correction of eye defects, and real versus virtual images are frequently tested physics facts.

Common confusion

A convex mirror diverges light but a convex lens converges it, so the same word means opposite optical behaviour for mirrors and lenses. Myopia (short sight) is corrected by a concave lens and hypermetropia (long sight) by a convex lens, which is easy to reverse.

Related concepts

concept total internal reflection, concept electromagnetic spectrum, concept optical fibre

Parent note

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