Refraction

The bending of light

Refraction is a change of direction of a ray of light. Light that is traveling in a straight line alters course - bends - when it strikes light-transmitting substances at any angle other than perpendicular.

The density of the transmitting substance - which may be glass or water, for example - causes a change in the speed of the light, making it alter course to travel through the substance. It picks up its speed again as it leaves the substance, and in so doing, bends or refracts one more time.

WHAT CAUSES REFRACTION?

If you have ever driven a wide-tired bicycle into a patch of mud at an angle other than 90, you may have noticed that the bike tended to deviate slightly, as if the wheel turned of its own accord. One side of the tire was slowed down as it was gripped by the mud, while the other side (not yet affected by the mud) continued at the initial, faster speed, causing the wheel to veer slightly. A similar effect occurs when light enters a denser medium at an oblique angle.

Glass and many other light-transmitting substances are denser than air. Since light has more molecules to travel through when it passes through a dense material, it is slowed down. If the dense material has a flat surface and the light strikes it straight on, at a 90 angle, it does not bend or refract; it simply slows down and passes straight through. But, a beam of light that strikes the surface at an oblique angle becomes bent - that is, it alters course - or refracts because the edge of the light beam that enters the material first is slowed down before the opposite edge of the beam.

Think of the way a bulldozer turns. It is based on the same principle. When traveling in a straight line, the tracks of a bulldozer are both operating at the same speed. If you apply the brakes to one track only, the other track will continue at the higher rate of speed and will turn the bulldozer. Similarly, a beam of light changes direction when one side of it is slowed down. As the beam of light leaves the medium, the side that leaves first picks up speed before the other side, and the light is refracted (changes travel direction) once more.

REFRACTION ENABLES LENSES TO FOCUS LIGHT

Lenses are able to focus light because they control the direction in which refracted light is channeled. They gather the light striking the large front surface of the lens and control the direction of its refraction as it changes speed when passing from the medium of air to the medium of glass and out again. In a properly-designed lens, this controlled refraction results in the light becoming focused on one plane. In a camera, the film (or the sensor of a digital camera) is located on that plane, and therefore a focused image can be recorded on it.

You have seen the effect of lens refraction yourself if you have used a magnifying glass in sunlight to burn your initials into a piece of wood or to ignite paper. The lens of the magnifying glass converges all the parallel rays of sunlight that enter it, permitting you to focus the concentrated light on one point. This hot spot is actually a tiny image of the sun, refracted by the lens.

REFRACTION MAGNIFIES UNDERWATER OBJECTS

Because light bends when it passes through light-transmitting substances like water and glass at any angle except 90, underwater objects look closer than they actually are due to refraction (bending) of light by water. (See Lens choice and light refraction in our Underwater photography section.)

REFRACTION IS WHAT MAKES CLOUDS AND SUGAR WHITE

Pure water is clear and transparent. So, if clouds are made up of transparent water droplets, which they are, then why do they look white? The reason is refraction.

As we saw above, light waves traveling in a normally straight path become bent or refracted when going through transparent water particles. The surfaces and interior planes of water molecules (and particles of other clear substances such as sugar) reflect this broken-up, light, scattering it. The resultant jumble of red, blue and green light waves now traveling in so many different directions mixes to produce white.

REFRACTION MAKES DIFFUSION FILTERS WORK

Lines etched in a diffusion filter refract light, altering the distance it travels to the film. The refracted light is out of focus. The amount that is out-of-focus is minimal in relation to the overall amount of light that travels in straight lines from the subject to the film, but it’s enough to blur edges, resulting in a picture that looks properly-focused, but soft-edged.