Human Eye and Colourful World Cheat Sheet

Definitions

Cornea

The transparent front part of the eye through which light enters.

Iris

The coloured muscular ring that controls the size of the pupil.

Pupil

The opening in the iris through which light enters the eye.

Crystalline lens

A flexible convex lens in the eye that focuses light on the retina.

Ciliary muscles

Muscles that change the curvature of the eye lens and hence its focal length.

Retina

The light-sensitive screen at the back of the eye on which the image is formed.

Rods and cones

Cells of the retina; rods respond to brightness and cones to colour.

Optic nerve

The nerve that carries signals from the retina to the brain.

Accommodation

The ability of the eye lens to change its focal length to focus on objects at different distances.

Least distance of distinct vision

The nearest distance (about 25 cm) at which the eye can see an object clearly.

Far point

The farthest distance from the eye at which an object can be seen clearly.

Myopia

Short-sightedness, in which distant objects appear blurred; corrected with a concave lens.

Hypermetropia

Long-sightedness, in which nearby objects appear blurred; corrected with a convex lens.

Presbyopia

The age-related loss of accommodation, corrected with bifocal lenses.

Cataract

Clouding of the eye lens that makes vision hazy; cured by surgery.

Dispersion

The splitting of white light into its component colours.

Spectrum

The band of colours obtained when white light is dispersed.

Atmospheric refraction

The bending of light by the atmosphere, whose density changes with height.

Scattering

The spreading of light in different directions by particles present in the medium.

Tyndall effect

The scattering of light by minute particles in a colloid, which makes the path of the beam visible.

Key Points

• Light enters the eye through the cornea, passes through the pupil and is focused by the lens onto the retina, forming a real, inverted image.
• The ciliary muscles change the focal length of the eye lens (accommodation) so that objects at different distances are focused.
• The least distance of distinct vision for a normal human eye is about 25 cm; the far point is at infinity.
• Myopia is corrected with a concave lens and hypermetropia with a convex lens; presbyopia is corrected with bifocal lenses.
• A prism disperses white light into seven colours — violet, indigo, blue, green, yellow, orange and red (VIBGYOR).
• A rainbow is formed by the dispersion, refraction and internal reflection of sunlight in water droplets.
• Atmospheric refraction makes stars appear to twinkle and advances sunrise and delays sunset.
• The sky appears blue because shorter (blue) wavelengths are scattered more than longer wavelengths.
• The sun appears red at sunrise and sunset because light travels a longer path and most blue light is scattered away.
• Red is used for danger signals because it is scattered the least and can be seen from far.
• Clouds appear white because large water droplets scatter all colours of light equally; the Tyndall effect explains light scattering in colloids and fog.