My Science School

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.

Picture Credit : Google

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.

Picture Credit : Google

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.

Picture Credit : Google

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.