The Human Eye and the Colourful World

Light entering the eye is bent first by the transparent cornea, passes through a hole called the pupil (whose size the iris controls), and is focused by the eye lens onto the light-sensitive retina at the back. The retina sends signals along the optic nerve to the brain. The clever part is the lens: it is made of flexible living tissue, and the ciliary muscles around it can squeeze it fatter or let it go thinner. A fatter lens has a shorter focal length for near objects; a thinner, flatter lens has a longer focal length for distant objects. This ability to change focal length is called the power of accommodation.

A normal eye can see distant objects clearly right out to infinity (the far point) and can see near objects clearly down to about 25 cm (the near point, also called the least distance of distinct vision). You cannot focus on an object held closer than the near point because the lens cannot be made fat enough; it becomes blurred and the eyes feel strained. With age the lens stiffens, the near point recedes, and reading becomes hard, a condition called presbyopia.

In myopia (short-sightedness) the eyeball is too long or the lens too powerful, so distant rays meet in front of the retina; the person sees near objects clearly but distant ones blurred. A concave (diverging) lens spreads the rays slightly before they enter, pushing the image back onto the retina. In hypermetropia (long-sightedness) the eyeball is too short or the lens too weak, so near rays would meet behind the retina; the person sees far objects clearly but near ones blurred. A convex (converging) lens adds the missing convergence and brings the image forward onto the retina.

White light is a mixture of colours. When it passes through a glass prism each colour is refracted (bent) by a different amount because each colour has a slightly different speed in glass: violet bends the most, red the least. The light fans out into a band of seven colours, VIBGYOR (violet, indigo, blue, green, yellow, orange, red), called the spectrum. A rainbow is nature's prism: tiny raindrops act as prisms, dispersing and internally reflecting sunlight, so a rainbow always appears in the part of the sky opposite the Sun.

Air molecules are tiny, and they scatter short-wavelength light (blue) far more strongly than long-wavelength light (red). So when you look at the daytime sky away from the Sun, you see scattered blue light coming from all directions, which is why the sky looks blue. At sunrise and sunset the Sun's light travels a much longer path through the atmosphere; almost all the blue is scattered out of the line of sight, leaving mainly red and orange to reach your eye, so the Sun and the surrounding sky look red. The scattering of light by fine particles is called the Tyndall effect.

The air is denser near the ground and thinner higher up, and light bends as it passes between layers of different density. This atmospheric refraction makes stars appear to twinkle: their tiny point of light is refracted by ever-changing air, so its apparent position and brightness flicker. Planets do not twinkle because they are nearer and look like small discs, averaging out the flicker. Refraction also lets us see the Sun about two minutes before it actually rises and two minutes after it actually sets, because the bent rays reach us while the Sun is still just below the horizon.