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Light
theory of how lenses worked, allowing people to make lenses more accurately.
Lenses work because, when light passes from one material to another (from air to glass, then glass to air again), it is bent. By curving the lens, the rays of light can be made to come together at a point and produce a clear image (an image that is in focus, as you might say if using a camera).
The shape of the lens is vital to the image that is formed. Lenses that bulge in the centre are called convex lenses, or converging lenses, and they are used to magnify (see: Magnification). Lenses that are thinnest in the centre are called concave lenses, or diverging lenses, and make things look smaller. They have fewer uses.
Convex lenses are used in magnifying glasses, binoculars and cameras. The lens in the eye is also a convex lens, as are eyeglasses.
The type of image you get from a convex lens depends on how far from the lens you are. If you are very close to the lens, as when you wear eyeglasses, you see an upright, magnified image. If you are further away, like the film in a camera, the image is smaller and upside down. Among other things, this allows a large area to be recorded on a small piece of film.
 Lens – Many naturally bulbous transparent materials act as lenses. Raindrops are a common example.
Light
The waves of energy that can be detected by our eyes. The world around us gives out all kinds of energy, like heat and sound. But our eyes are designed to detect just a small range of waves (see: Light waves and Sound waves) – those whose waves are between 380 to 780 billionths of a metre long. Red
colours are made by the waves with longer lengths, then orange, yellow, green, cyan, blue and violet. Violet colours are produced by the shortest waves.
If waves throughout this range reach our eyes, we see white light. But if some of the waves are blocked, perhaps by a coloured glass or by the paint on a surface, we only see part of the range of waves, and that is when we see a colour. The colour we see is made of the waves that reach our eyes. So, for example, if we see ‘pure’ blue, we know that nearly all of the other waves have been blocked in some way. If we see a greenish blue, on the other hand, we know that a group of waves is reaching our eyes. The waves that reach our eyes and make colours
are different from the waves that come directly from a light source. Waves from a light source are called transmitted light, and waves that bounce off a surface before they reach our eyes are called reflected light (see: Reflection).
Light travels in straight lines. You can see that when you look
at shadows: The shadow has well- defined, not fuzzy, edges, as would be the case if light could follow a curved path. As a result, scientists can draw paths of light (called rays) when they are working out how light behaves when it is bounced (reflected) off a mirror, or when it passes through a lens.
(See also: Absorption of light; Brightness; Diffraction of light; Dispersion of light; Doppler effect of light; Fibre optics; Interference; Light energy; Light-year; Mirage; Optics; Photon; Refraction of light; Scattering of light; Spectrum; Speed of light.)
(For types of light see: Bioluminescence; Electric light; Fluorescence; Incandescence, incandescent light; Infrared light; Luminescence; Monochromatic light; Phosphorescence; Polarised light; Skylight; Sunlight; Ultraviolet light.)
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