Category Light and Lasers

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.

When light travels from one transparent material to another, it changes direction. Light bends as it travels from air into glass or water. It bends again as it leaves glass or water and re-enters the air. We call this bending of light ‘refraction’. The reason that light refracts is that it travels more slowly in glass or water than it does in air.

Refraction of light has some strange effects. It can make a stick look bent when it is lowered into water; it makes the bottom of a swimming pool seem closer than it really is; it can even make a traveller ‘see’ lakes in the desert as in a mirage.

When you look at a spoon in a glass, you see the light that the spoon reflects. Light from the spoon handle travels to your eyes in a straight line. But light from the rest of the spoon changes its speed and its direction as it passes from water to air. However, your brain assumes that the light reaching your eyes has all travelled in straight lines. You see a bent spoon which seems closer to you than it is.

The light of torch is travelling from cool air to the warmer air above the candle. Warm air is less dense than cold air, so light travels faster in it and bends, or is refracted. The same shimmering effect is produced on a hot day. Light travels faster through the hot air rising from the ground than through the cooler air above.

Mirages are caused by light bending as it passes through warm air.

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Light can travel through some materials. Materials that allow light to pass through them are ‘transparent’. When light shines onto an opaque object, like wood or bricks, a shadow is formed.

You can make shadows yourself by shining a torch onto the wall of a dark room. An opaque object, such as a pen, placed between the torch and the wall, will cast a shadow on the wall. On a sunny day, you can see clear shadows outside. Shadows fall wherever the light of the Sun is blocked by any opaque object.

Shadows can be used to tell the time. When the Sun shines, the pencil casts a shadow onto the base of the ‘sundial’. As the Sun appears to move across the sky, the shadow falls in a different direction and points to the correct time.

Sometimes the Moon passes between the Sun and the Earth in such a way that all three are in a straight line. The Moon is opaque and so it casts a shadow onto the surface of the Earth. The part of the Earth in the shadow is suddenly thrown into darkness in the daytime!

If you are standing in a shadowed area looking at the Sun, you may only see part of it (a ‘partial’ eclipse) or it may be obscured altogether by the Moon — a ‘total’ eclipse.

 The light of the Sun can be blinding. You must never look directly at the Sun.

An eclipse of the Sun: the Moon passes over the Sun.

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Light travels faster than anything else we know of. The Sun is about 150 million kilometres away from Earth and yet its light takes only about eight minutes to reach us! Some of the stars are so far away that their light takes many years to reach us. We do not see them as they are, but as they were hundreds, thousands or even millions of years ago!

Sometimes you can see rays of light from the Sun as they light up dust particles in the air. The rays do not bend — they seem to travel in straight lines.

Light travels in straight lines

Light from the torch travels through the holes in the first screen. But only the rays travelling through the centre hole have a straight path through all three screens. Light rays travel in straight line.

Our most important source of light is the Sun. The Sun has an enormous amount of energy which is given out in the form of heat — at its centre, the temperature of the Sun is about 13 million degrees centigrade! It is some of this energy which reaches us as light.

Since the Earth spins around once every 24 hours, we only face the Sun part of the time — the time we call ‘day’. At night, light from the Sun can no longer reach us. But even at night there is some light. The stars, like the Sun, produce light. The Moon also provides light. But the Moon has no light of its own — it simply reflects light which has reached it from the Sun.

            Electric lights enable us to see well at night. At night, we see how the Moon reflects the Sun’s light.

Our sense of sight is one of our most important links with the world. We can see thousands of colours and shapes which help us to recognize the people, places and things around us. But our eyes are limited. Not until the discovery of lenses were we able to see the things which were either too small or too far away for our eyes to focus on.

Telescopes enable us to see faraway objects such as galaxies.

Lenses in microscopes allow us to see tiny forms of life, helping us to understand how living things function. And lenses in telescopes have enabled us to understand something of the solar system, and the universe, of which we are a tiny part. You will learn how a special type of light, laser light is changing our lives.

Picture Credit : Google