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Index of refraction
Index of refraction
Light does not travel at the same speed in all materials. In space light travels at 300,000 kilometres per second. In glass it travels only two-thirds as fast. The refractive index is a term used to describe the power of a material to slow light compared to space (a vacuum). The refractive index of glass is about 1.5, and the refractive index of water is about 1.3.
Infrared light
Light that is just beyond our vision. The longest waves we
can see belong to red light.
Waves slightly longer than that
are called infrared waves. Some animals, especially insects, can see infrared waves. William Herschel, a German-born British astronomer, discovered infrared radiation by using a thermometer to look at
the heat produced by light passed through a prism (a glass wedge). He found that the thermometer increased in temperature when he placed it just outside the red part of the rainbow.
Interference
The pattern of light and dark
bands that are produced when
two or more light waves cross.
The principle can be imagined
by comparing light waves to two pebbles dropped into a pond. In some places where the waves cross, they add and make a higher wave, while in other places they subtract and make a lower wave. In light places of adding produce stronger, or brighter, light, while places of subtraction make less light. This pattern is at its simplest with light beams of a single colour. When interference occurs in white light, the pattern can be more complex. The coloured patterns on oil, soap bubbles, butterfly and bird wings,
Detector box: The light is switched on if something crosses any two sectors.
120° sweep 15m range
The shaded areas show neighbouring detecting sectors.
and some beetles are the result
of interference with white light.
In these cases interference occurs because light is reflected from both the upper and lower surfaces of thin sheets of transparent material.
Iris
Two sheets of muscle between the front of the eye (the cornea) and the lens. They have coloured cells that give the eye its colour. In the centre is a hole called the pupil. The size of the pupil is changed to let in more or less light as the iris muscles contract or expand.
L
Lamp
A device that burns fuel to produce light.
The earliest lamps used oil. A wick was placed in a dish of oil and ignited. The wick burned with
 Infrared light – This is a PIR (passive infrared) detector of the sort that is used to light up as you approach it. The heat from our bodies and from cars is emitted in the infrared range.
a yellow flame until the oil was used up.
Gas lamps burned a mixture of air and coal gas.
Arc lamps rely on a giant spark occurring between two electrically charged rods. That produces a very bright white light.
Gradually, all these forms of lamp were replaced in general use by the electric lamp. The most common form of electric light is a bulb that contains a filament of tungsten metal. When electricity passes through the filament, it glows brightly. It is called an incandescent lamp.
Laser
A device that produces an intense beam of light of a single colour. The word laser stands for ‘light amplification by stimulated emission of radiation’.
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