The human eye

Light enters through the cornea — the transparent bulge at the front, where most of the eye's actual refraction happens. The crystalline lens behind it only fine-tunes the focus. The iris, a muscular diaphragm, controls the size of the pupil to regulate how much light gets in, and the lens throws a real, inverted image onto the retina at the back — a screen packed with light-sensitive cells that fire electrical signals down the optic nerve to the brain, which turns that signal into the picture you actually perceive.

Accommodation

The eye lens is soft and jelly-like, and ciliary muscles can squeeze or relax it to change its curvature — and therefore its focal length. Relaxed muscles thin the lens for distant objects; contracted muscles thicken it for close ones. This adjusting ability is called accommodation, and it has limits: the closest point you can focus on comfortably is the near point (about 25 cm for a young adult), and the farthest is the far point (infinity, for a normal eye).

Defects of vision and their correction

Three common refractive defects show up when the eye loses accommodation power or its shape changes:

The third, presbyopia, is simple ageing: the ciliary muscles weaken and the lens loses flexibility, so the near point drifts outward over time. Someone with both myopia and hypermetropia together typically needs bi-focal lenses — concave on top for distance, convex below for close work.

Refraction through a prism

A glass slab has parallel faces, so light bends in and bends back out by equal, opposite amounts — the emergent ray ends up parallel to the incident ray, just shifted sideways. A prism's two refracting faces are angled to each other instead, so the bending doesn't cancel out: the ray emerges at a genuine angle to where it came in, called the angle of deviation.

Dispersion of white light

Send white light through that same prism and it doesn't emerge as one deviated beam — it fans out into a full band of colour, because each colour bends by a different amount. Red bends least, violet bends most, and everything in between spreads out into a visible band called a spectrum. This splitting is dispersion, and the sequence is easy to remember as VIBGYOR:

Newton first produced this spectrum with a prism, then tried a second prism to split it further and got nothing new — but flipping that second prism upside-down recombined all seven colours right back into a beam of white light. That's what convinced him sunlight is made of seven colours to begin with.

A rainbow is this same dispersion happening naturally: tiny raindrops act as countless small prisms, refracting and dispersing sunlight, reflecting it internally, then refracting it again on the way out — which is also why a rainbow always appears in the sky opposite the Sun.

Atmospheric refraction

The atmosphere's density — and so its refractive index — changes gradually with height, which bends light continuously rather than at one sharp boundary. That's why hot air above a fire makes things behind it waver: the air's refractive index is fluctuating moment to moment.

Stars show the same effect on a cosmic scale: their light bends slightly and unevenly as it crosses the atmosphere, so a star's apparent position and brightness flicker — the twinkling effect. Since stars are effectively point sources, that flicker is visible. Planets are close enough to look like extended discs made of many such points, and the flickers from all of them average out to nothing — so planets shine steadily.

Scattering of light

The atmosphere is full of fine particles — dust, smoke, water droplets, air molecules — and light striking them scatters, which is why a sunbeam through a smoky room or a forest canopy becomes visible as a beam at all (the Tyndall effect).

How light scatters depends on particle size: very fine particles scatter short (blue) wavelengths far more than long (red) ones, and air molecules are exactly that small. That preferential scattering of blue is why the clear sky looks blue — and why it looks black from space or high altitude, where there's too little atmosphere left to scatter anything.