Category Lights & Laser, Rainbow

We have seen that white light is a mixture of many colours which can be separated. It helps to think of these colours as waves. Each different colour of light has a different length of wave. Red light has long waves. Blue light has short waves. However, the light produced by a laser is the entire same wavelength.

This means that a beam of light produced by a laser can be easily concentrated onto a tiny point. It can produce enough heat to turn a metal into a vapour! Lasers can be used as accurate cutting tools which can even cut through diamond, the hardest substance known.

Laser light and wavelength

White light from a torch can be thought of as a mixture of waves. Each wavelength represents a certain colour. The waves making up a laser beam are quite different.

Not only are all the waves the same length (colour), but they are lined up so that the tops (peaks) of the waves coincide.

The various wavelengths making up white light can be separated by a prism. We know that laser light is all of one wavelength because it cannot be separated by a prism.

Waves of laser light are all bent to the same extent by the prism since they all travel at the same speed through glass.

Beams of laser light are powerful enough to cut through metal.

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Lenses are pieces of transparent material, such as glass or plastic, which have been made into special shapes. They refract (bend) light in certain ways depending on their shape. Lenses may be convex or concave. Convex lenses are thicker in the middle than they are at the edges. Concave lenses are thinnest in the middle.

A convex lens

Light rays from a small, close object travel in straight lines to the lens. But as they pass through the lens and towards your eye, they bend inward. Since your brain expects light to travel in straight lines, you see a magnified (larger) image.

A concave lens

Rays of light from a tennis ball travel in straight lines to the lens. As they pass through the lens, they bend outward towards your eyes. Again, the brain expects these rays to have arrived in straight lines and you see a smaller image.

Convex and concave lenses are very useful. They are found in many of the instruments which help us to see things which we could not see with our eyes alone. Lenses are used in telescopes which help us see stars and planets, in binoculars which enable us to watch birds and animals in the wild, and in microscopes which magnify tiny living things.

People use lenses to carry out detailed work.

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You are seeing the light it is reflecting. It is reflecting sunlight or electric light, both of which are ‘white’. Yet you can see many different colours. You must remember that white light is really a mixture of colours. The white part of the page is reflecting all the colours of the spectrum. But the printed words are reflecting almost no light. Black is the absence of colour, or light. The colours we see depend on the type of light being reflected. Red, green and blue are known as the primary light colours. It is possible to make any colour by mixing different amounts of these colours.

You can see some of the effects of mixing the primary colours of light. Red and green together make yellow light; green and blue combine to make cyan; and blue and red give magenta. Any other colour can be produced by varying the amounts of each of the primary colours. Red, blue and green together make white.

MIXING COLOURED PAINT

Red, blue and yellow are said to be the primary colours of paint. Blue paint reflects green light as well as blue. Yellow paint reflects green and red light. A mixture of blue and yellow paint appears green since this is the only colour reflected by both. An artist can mix paints to produce any colour.

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It is light which enables our eyes to see. Light reflected from this page enters each eye and passes through a hole called the ‘pupil’. In dim surroundings, your pupils get larger to let in more light. In bright light, they become smaller.

Your eyes each contain a lens. This lens is jelly-like and can change shape. The lens bends the light entering your eyes so that you always see a clear picture. At the back of the eye is the ‘retina’. When light rays fall onto the retina, they cause messages to be sent to the brain. Your brain interprets the messages it receives and you are conscious of ‘seeing’.

Opticians use different lenses to check a patient’s eyesight.

The pinhole camera

This simple camera is a box with a pinhole at the front. Rays of light from the candle travel in straight lines through the pinhole to the screen at the back. The rays cross over as they pass through the hole and so the image is formed upside down.

Hold the camera between you and the candle. Look at the tracing paper — you will see an upside down candle!

The eye works a little like the pinhole camera. An apple held in your hand reflects rays of light which pass through your eye. The lens becomes short and fat to focus the light rays onto your retina.

To focus on the apple tree, your lens gets longer and thinner. The image formed on your retina is upside down in both cases. When the information is relayed from your retina to your brain, you ‘see’ things the right way up.

A ray of light from the Sun, or from an electric light bulb, looks white. But this white light is really a mixture of lights of different colours! To see these colours, we must split up the white light by shining it through a glass ‘prism’.

White light is refracted as it enters and leaves the prism. Different colours of light travel at slightly different speeds through the glass. As they leave the prism, they bend different amounts. The colours red, orange, yellow, green, blue and violet can be seen. They’re called the ‘spectrum’. We can see the colours of the spectrum naturally in soap bubbles, thin films of oil or rainbows.

You can make white light by mixing light of different colours together. A spinning wheel is divided into equal sections. Each section is painted with a different colour from the spectrum. As the wheel spins, the colours ‘mix’ together and the wheel looks white!

White sunlight may be split into the colours of the spectrum by raindrops. White light bends as it enters the edge of the water droplet. It is then reflected back into the drop and is bent once more as it leaves the drop. The colours of the spectrum are now spread out. Thousands of raindrops together may separate sunlight in this way and form a rainbow, one of the most beautiful natural sights of all.

The colours of the rainbow range from red on the outside to violet on the inside.

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We know that sunlight shines onto every object we see. Some of the light bounces off the object again. We say it is ‘reflected’. We can only see objects when they reflect light. Most objects have no light source of their own. We see them because they reflect the Sun’s light.

Every substance reflects some light. Shiny, smooth surfaces, such as metals, are the best reflectors of light. A mirror, made from a sheet of glass with a thin layer of silver or aluminium on the back, reflects light almost perfectly. However, a mirror image can be misleading. You appear the wrong way round in an ordinary mirror — left appears right and vice versa, and your reflection may be very distorted in a curved mirror.

Mirror images

Letters held in front of a mirror appear the wrong way round in the reflection. We say they are ‘laterally inverted’. But if a second mirror is added, at right angles to the first, the image is turned round again.