Light_Concept_3

Subject Facts

The brightness of light

Light varies in intensity: light sources can emit smaller or greater amounts of light energy (or numbers of photons). The brightness of a light source can be reduced by reducing the energy input (for example, using a lower-voltage battery with a bulb) or by blocking some of the light using translucent materials such as tissue paper. Sometimes the brightness of one light source will mean that other, less bright light sources cannot be seem such as the stars in daylight see Earth and Space,

The colour of light

The cover of my diary is red, the ring binder is yellow, and my jeans are blue.. .but why? They are all illuminated by the same bulb, so why are they different colours? Light comes in many different colours, and scientists explain this fact two different ways.

Using the 'wave' theory of light, which is the more familiar and popular approach, you can say that each colour is a slightly different wavelength of light: violet has the shortest wavelength (highest frequency) and red the longest (lowest frequency) within the spectrum of light that we can see. Outside this range are ultraviolet and infrared fight. Using the 'photon' theory of light, you can say that colours are the result of photons having different energy levels, with red at the highest energy level and violet at the lowest. But it is enough to say that light comes in different colours!

White light

There is really no such thing as white light: there is no wavelength (or energy level) which is white, White light' is what we see when all the colours of light are mixed together in roughly equal quantities, Most of the light sources that we rely on, such as the Sun and domestic electrical lighting provide us with white or near-white light. Street lamps tend to use cheaper; longer-lasting light sources that produce yellow 'sodium' light, making use of a more limited range of wavelengths.

Mixing colours

Because of the way that we see colours (see below), it doesn't require all of the colours of the spectrum for us to see an object as white. There are three primary colours of light that we can use to produce white. By varying the relative intensities of these three colours, we can make all the other colours. The primary colours are red, green and blue. By mixing red and green (that is, by shining these coloured lights on a white surface), we can make yellow; mixing green and blue gives cyan; mixing red and blue produces magenta.

Coloured objects

Most objects scatter the light that hits them, so why does something yellow appear yellow? As was explained previously, when light hits an object it will be either absorbed or scattered. The relative proportions of fight absorbed and scattered varies for different colours of light. When white light falls onto a banana, blue light is absorbed and red and green light is scattered (remember red + green = yellow). Grass absorbs red and blue light from white light falling on it, and scatters green light.

The key point is that we see different colours according to which colours of light different pigments scatter or absorb. A white object scatters all the colours of light. So does something that is grey, but it also absorbs more of all the colours as well, Any colour appears in a darker shade if more of the light is absorbed.

Rose-tinted spectacles

Rose-tinted spectacles (or ones with any other colour of lens) affect how we see the world, Red glass or (transparent) plastic absorbs ail colours of light other than red Some red light is scattered from it (which is why it looks red), but it will only allow red light through it. Objects scattering white light will thus appear to be red and so will objects scattering red light. Objects that scatter some red (such as magenta or yellow objects) will appear red, since blue or green light will not get through the glasses,

Because green and blue objects don ^tt scatter any red light they appear to be black,

Why you need to know these facts

Children can normally see a range of colours, shades and degrees of brightness. they need to be able to explain how this is possible.

Vocabulary

Primary colours (red, blue and green) the colours of light that our eyes are able to detect.

White light — a mixture of all the colours of light (though for our eyesight, we only need to mix red, blue and green to make white).

Common Misconceptions

The most common colour of light is white.

Red, green or blue light is light at a particular wavelength White light cannot be described in the same way It is all the colours of visible fight combined in (more or less) equatl intensities. What we see as white light is thus a particular mixture of different colours.

The primary colours of light and paint (pigment) are the same.

This assumption can be tested if you have a room dark enough to mix fights. If possible it is best to use theatre lamps with red, blue and green colour filters. The control panel should allow you to dim each light independently, mixing the three colours to produce other colours of light. The primary colours of pigment are yellow, cyan and magenta (the secondary colours of light). These colours, along with black are the ones used in most colour printing processes (for example, the inks in a computer printer). When mixed, cyan and yellow will produce green; magenta and cyan, blue; and yellow and magenta, red. However, ail three together will only produce grey, If you try mixing the same colours in paint, the colours produced will not be the same. Light adds colours; pigment subtracts colours.

Ask the children to describe the gradual colour changes that they can see and try to identify the order. Why cant they see magenta? (Because red and blue are at opposite ends of the spectrum and so do not mix.)

Safety at night (testing and sorting)

Provide a collection of 'safety clothing' with highly fluorescent or bright surfaces. The children can test which of them are most visible in the light box (see above). They could make an orange light (similar to the colour of street lamps), then test different materials first with white light and then with orange Fight. Ask them: Which colours scatter fight best? Which colours absorb fight best?

Colour mixing (observing, using 'ICT)

With adult supervision, the children could investigate the effects of shining theatre lamps with blue and green filters on a white screen in a darkened room, or use art software on a computer to experiment with mixing primary colours of light at different intensities.

Secret messages (observing)

This fun activity children to remember the primary colours of light, and demonstrates the effect of viewing different colours through sheets of transparent coloured plastic (known as gels or colour filters). A red filter works best, but others can be used to show the different effects. The children can use the figure copied from a calculator display, as a template to write block capital letters in a message. They should use blue and green pens for lines to be seen and red, yellow, orange and pink pens for lines to disappear when the message is viewed through a red filter. This is a great activity for an 'indoor break', and can result in excellent displays.

Rose-coloured spectacles (observing)

The children can find out how the world looks when viewed through coloured gels. They could try using a different colour for each eye. Red and green work quite well together: Seen through the red gel, white and red objects appear to be the same colour and blue and green both appear black, Seen through a green gel, white appears to be green and red appears to be black The two eyes will see things very differently from each other!