The Human Eye
& the Colourful World

The eye works like a camera. Light first bends at the cornea (which does most of the focusing), passes through the pupil — an aperture whose size the iris controls — and then the flexible eye lens fine-tunes the focus to form a real, inverted image on the retina. Light-sensitive cells there send signals to the brain through the optic nerve.

Accommodation: ciliary muscles change the curvature of the eye lens, changing its focal length. For distant objects the lens is thin (long focal length); for near objects the muscles squeeze it thicker (short focal length).

Myopia (near-sightedness): distant objects look blurred because their image forms in front of the retina — the eyeball is too long or the lens too converging. Corrected with a concave (diverging) lens.

Hypermetropia (far-sightedness): nearby objects look blurred because their image would form behind the retina. Corrected with a convex (converging) lens.

Presbyopia: with age the ciliary muscles weaken and the lens stiffens, so the near point recedes; often corrected with convex or bifocal lenses. A cataract (clouding of the lens) is treated by surgery, not lenses.

A glass prism has two triangular ends and three rectangular faces; its two refracting surfaces meet at the angle of the prism (A). A ray bends towards the base at each surface, so it emerges deviated from its original path. The total bending is the angle of deviation (δ).

White light is a mixture of seven colours. A prism splits it into VIBGYOR (violet, indigo, blue, green, yellow, orange, red) — this is dispersion. It happens because each colour travels at a slightly different speed in glass, so each has a slightly different refractive index and bends by a different amount. Violet bends the most, red the least.

Newton showed that a second, inverted prism recombines the spectrum back into white light — proving the colours were in the white light all along. A rainbow is nature's prism: dispersion, internal reflection and refraction of sunlight inside water droplets.

The atmosphere has layers of changing density, so light bends continuously as it passes through. Starlight reaching us keeps shifting slightly, so a star's apparent position and brightness flicker — the twinkling of stars. Planets are closer and look like extended sources, so their light averages out and they don't twinkle.

Atmospheric refraction also lifts the Sun's apparent position: we see the Sun about 2 minutes before the actual sunrise and after the actual sunset, and the Sun looks flattened/oval near the horizon.

Tyndall effect: when light passes through a medium with tiny particles (smoke, fog, a colloid), it scatters and the path of the beam becomes visible.

Why the sky is blue: air molecules scatter shorter wavelengths (blue) far more than longer ones (red), so scattered blue light reaches our eyes from all directions.

Why sunrise/sunset is red: near the horizon sunlight travels through much more air; most of the blue is scattered away, so mainly red reaches us. The same reason makes danger signals red — red scatters the least and is seen from farthest away.