Light: Mirrors, Lenses, Refraction and the Human Eye

Drafted with AI assistance and fact-checked on 28 October 2026; not yet read line by line by a human editor. Spotted a mistake? Tell us. Editorial standards & corrections

Reflection and plane mirrors

Light is an electromagnetic wave that travels in straight lines at about 3 x 10^8 m/s in vacuum; sunlight takes about 8 minutes 20 seconds to reach the Earth. The angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal lie in one plane. A plane mirror forms a virtual, erect, same-sized image as far behind the mirror as the object is in front, and laterally inverted, which is why AMBULANCE is written in mirror letters on ambulances. Two mirrors inclined at an angle θ form (360/θ) - 1 images, so mirrors at 90 degrees form 3 images.

Spherical mirrors

A concave mirror converges light and a convex mirror diverges it; the focal length is half the radius of curvature (f = R/2) and the mirror formula is 1/v + 1/u = 1/f. A concave mirror gives real, inverted images, except an enlarged, erect, virtual image when the object is between the focus and the mirror, so it serves as a shaving or dentist's mirror and, with a bulb at its focus sending out a parallel beam, as the reflector of torches, headlights and searchlights. A convex mirror always gives a virtual, erect, diminished image over a wide field of view, so it is used as the rear-view mirror of vehicles.

Refraction and refractive index

Light bends when it passes from one medium into another because its speed changes; from a rarer to a denser medium it bends towards the normal. Snell's law says sin i / sin r is constant for a given pair of media. The refractive index, n = speed of light in vacuum / speed in the medium, is about 1.33 for water, about 1.5 for glass and 2.42 for diamond. On refraction the frequency stays the same while the speed and wavelength change. Refraction makes a pool look shallower than it is and a stick look bent in water. Atmospheric refraction makes stars twinkle (planets, being nearer, do not) and shows us the Sun about 2 minutes before actual sunrise and after actual sunset.

Lenses and optical instruments

A convex lens, thicker in the middle, converges light; a concave lens, thinner in the middle, diverges it. The lens formula is 1/v - 1/u = 1/f, and power P = 1/f (f in metres) is measured in dioptres, positive for convex and negative for concave lenses, so a convex lens of focal length 0.5 m has a power of +2 D. Convex lenses are used in magnifying glasses, cameras and projectors. A compound microscope has an objective and an eyepiece of short focal length; an astronomical telescope has an objective of large focal length and large aperture. A lens's effect depends on its surroundings: an air bubble in water acts as a diverging lens.

Total internal reflection

When light goes from a denser to a rarer medium at an angle of incidence greater than the critical angle, all of it is reflected back. The critical angle is about 42 degrees for glass and about 24 degrees for diamond; this small angle traps light inside a cut diamond and makes it sparkle. Optical fibres, used in telecommunication and in endoscopes to see inside the body, carry light by repeated total internal reflection. A mirage on a hot road or in a desert, and the prisms in binoculars, are further examples.

Dispersion, rainbow and scattering

A prism splits white light into seven colours, VIBGYOR, as Newton showed; red deviates least and has the longest wavelength (about 700 nm), violet deviates most and has the shortest (about 400 nm). A rainbow forms by dispersion, refraction and internal reflection in raindrops, so it is seen only with the Sun behind the observer, with red on its outer edge. The tiny particles of air scatter short wavelengths far more than long ones, so the sky looks blue. At sunrise and sunset light crosses a longer path through the air, the blue is scattered away and the Sun looks red; danger signals are red because red scatters least and is seen far in fog. C. V. Raman discovered the Raman effect, a scattering of light by molecules, on 28 February 1928, now National Science Day, and won the Nobel Prize in Physics in 1930.

The human eye and its defects

Most refraction in the eye happens at the cornea; the iris controls the size of the pupil; the convex eye lens forms a real, inverted image on the retina, whose rods sense dim light and cones sense colour. Ciliary muscles change the lens's focal length (power of accommodation), so a normal eye sees clearly from 25 cm (the near point) to infinity. An image persists on the retina for about 1/16 of a second, which makes cinema possible. In myopia (short sight) the image falls in front of the retina and a concave lens corrects it; in hypermetropia (long sight) it falls behind the retina and a convex lens corrects it; presbyopia, the weakening of accommodation with age, needs bifocals; astigmatism needs a cylindrical lens; cataract, a clouding of the eye lens, is treated by surgery.

Key facts

The speed of light is about 3 x 10^8 m/s in vacuum; sunlight reaches the Earth in about 8 minutes 20 seconds.
Plane mirror image: virtual, erect, same size, laterally inverted; two mirrors at 90 degrees give 3 images, from (360/θ) - 1.
Concave mirror: shaving mirror, dentist's mirror, torch and headlight reflectors, solar furnace. Convex mirror: rear-view mirror (wide field, small erect image).
Refractive index n = c/v: water 1.33, glass about 1.5, diamond 2.42; refraction makes pools look shallow and stars twinkle.
Power P = 1/f (f in metres), unit dioptre; convex lens positive, concave lens negative; an air bubble in water acts as a diverging lens.
Total internal reflection needs light going from denser to rarer at more than the critical angle (glass about 42 degrees, diamond about 24 degrees): diamond sparkle, optical fibre, mirage.
VIBGYOR: red has the longest wavelength and deviates least, violet the shortest and deviates most; a rainbow is seen with the Sun behind you.
Blue sky and red sunset are due to scattering; danger signals are red because red scatters least; Raman effect discovered on 28 February 1928 (National Science Day), Nobel Prize 1930.
Myopia: concave lens; hypermetropia: convex; presbyopia: bifocal; astigmatism: cylindrical; cataract: surgery; near point of a normal eye: 25 cm.
Telescope objective: large focal length; microscope objective: short focal length; persistence of vision about 1/16 second makes cinema work.

Quick revision

c = 3 x 10^8 m/s; angle of incidence = angle of reflection; plane mirror image laterally inverted; concave mirror in headlights and shaving, convex as rear-view; n = c/v, water 1.33, diamond 2.42; P = 1/f in dioptre; total internal reflection in diamond, optical fibre, mirage; VIBGYOR, red least deviated; blue sky by scattering; Raman effect 1928; myopia concave, hypermetropia convex, presbyopia bifocal, astigmatism cylindrical; near point 25 cm.

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