Light and Shadows: Read the information below and then complete the activities
Here is a really good set of units, videos and work following the Key stage 2 programme of study:
We say that light travels in a straight line because that helps us to understand how light works. In fact, the nature of light is very complex – it sometimes behaves as a wave and sometimes as a particle, and this explains the different characteristics of light. This complex behaviour has puzzled scientists for hundreds of years. Many observations around us provide evidence that light travels in a straight line. For example, shadows have the same shape as the objects that cast them, and we can see the path of a beam of light from a torch is straight.
Reflection and refraction
We see things because our eyes are receivers of light - not by light leaving our eyes as sometimes portrayed in sci-fi films! Light travels to our eyes and enters them. The light that enters our eyes has been reflected from objects unless we are looking at a light source such as a bulb. The amount of light reflected from an object depends on the surface and colour of the object. A smooth white surface reflects more light than a dark rough one. Shiny materials are good reflectors of light. Wearing reflective materials helps other people to see you in the dark, but will only work when a source of light such as a torch or lamp reflects off them. Mirrors have very smooth, shiny surfaces. Light bounces off mirrors in exactly the same pattern as it arrives, reflecting a complete picture or image of any object. In a flat mirror, the image will be the same size as the object, but curved mirrors will create a distorted image that may be bigger or smaller than the object.
Because light travels in straight lines, we cannot see round corners. But we can use mirrors, as in a periscope, to allow us to do so and see things that are not in the direct line of vision. A periscope has two parallel sloping mirrors placed at either end of the tube which reflect the light down the tube and then along into our eyes. A periscope is good example of the law of reflection at work – when light hits a surface at a particular angle, it is reflected off at the same angle.
Have you noticed that a swimming pool always looks shallower than it is or that a pen at an angle in a glass of water appears to bend or split into two sections? This is because light bends when it passes from one substance to another – in this case, from water to air. The effect is called refraction and it occurs because light travels at different speeds through transparent materials. When it enters glass or water, it slows down, which makes it change direction. Refraction is also the principle behind lenses used in spectacles.
More about shadows
We can link the formation of shadows to the material an object is made of. Opaque objects make clear, dark shadows because they let no light through them, translucent objects make faint shadows because they let some light through and transparent objects produce almost no shadow. The length and position of a shadow depends on the position of the Sun in the sky and this will change depending on the time of day and the time of year. When the Sun is low in the sky in the early morning and late evening, it casts a long shadow. At midday, when the Sun it at its highest, the shadows are at their shortest.
The colour of light
Sunlight and light from a light bulb appear to be colourless. This light is called white light and is really a mixture of many colours of light. You can see some of these colours when a rainbow appears in the sky after rain. As sunlight is reflected through raindrops, it is refracted. The different colours of light are bent by different amounts, so the colours fan out as they emerge from the raindrop. You can demonstrate the colour spectrum using a triangular glass prism to refract white light. This was how Isaac Newton proved that white light is made of different colours.
We see most things only by reflected light. An object absorbs (takes out) certain colours of light falling on it and reflects the rest. It is these reflected colours that give the object its colour. A white object reflects all colours of light equally, so it appears to be the same colour at the light shining on it.
To make colours, we need just three primary colours: red, green and blue corresponding to the three of types of sensory cells in our eyes. When red, green and blue lights are mixed together they produce white light; varying the amounts of the three colours produces all other colours.
A laser is a special source of light of only one pure colour. You cannot separate laser light into other colours. Lasers are very useful for many reasons. They can be focused to a very small spot containing lots of energy and can shine for long distances without spreading out much (compare this with how light from a torch spreads out). Lasers can cut through metal or can be used as scalpels in some kinds of surgery. They can be used to send and pick up or communicate information – a tiny laser inside a CD player reads a CD, a single laser can send thousands of phone conversations through an optical fibre (a long thread of glass) at the same time.
- Create an information poster showing what you have learnt
- Create your own shadows using a light source - Torch, lamp or phone torch
- Draw how your shadows change if you move the light source