Short Summary

Chapter 11, Human Eye and the Colourful World, explains the structure and functioning of the human eye, common defects of vision, and atmospheric optical phenomena involving dispersion, refraction, and scattering of light. Light enters the eye through the cornea, while the iris controls the size of the pupil and regulates the amount of light entering the eye. The ciliary muscles change the curvature and focal length of the eye lens, providing the power of accommodation needed to focus real and inverted images onto the retina, which converts light into electrical signals that are transmitted to the brain through the optic nerve. Common defects of vision include myopia, or near-sightedness, which may result from an elongated eyeball or excessive curvature of the eye lens and is corrected using a concave lens; hypermetropia, or far-sightedness, which may result from a shorter eyeball or a longer focal length and is corrected using a convex lens; presbyopia, an age-related reduction in the power of accommodation that is commonly corrected using bifocal lenses; and cataract, in which the eye lens becomes cloudy and vision can be restored through surgery. A glass prism separates white light into its constituent colours through dispersion, with red light deviating the least and violet light deviating the most. Rainbows are formed when water droplets suspended in the atmosphere act like tiny prisms and produce a sequence of refraction, dispersion, internal reflection, and refraction of sunlight. Atmospheric refraction occurs because light passes through layers of air having different optical densities, causing effects such as the twinkling of stars, changes in the apparent positions of celestial objects, and the advancement of sunrise and delay of sunset by approximately two minutes. Scattering of light occurs when light interacts with small particles, and the scattering of light by colloidal particles is known as the Tyndall effect. Fine particles and molecules in the atmosphere scatter shorter wavelengths of light, such as blue light, more strongly, making the sky appear blue, while during sunrise and sunset, sunlight travels through a longer path in the atmosphere, causing much of the blue light to be scattered away and making the Sun appear reddish.