Science 8th grade
UNIT 12: Light 12.5 Coloured light
Science 8th grade
UNIT 12: Light 12.5 Coloured light
In the theatre, coloured lights are used to make interesting effects. To make coloured light, a filter is placed in front of a bright white light.
A filter is a piece of coloured plastic or glass. It only lets through some of the colours which make up white light. It absorbs the other colours.
For example, a red filter lets through red light (and a bit of orange).
It absorbs yellow, green, bluc, indigo and violet.
How coloured filters make light of different colours.
Questions
1) Which colours can pass through a blue filter? Which colours are absorbed?
$A+I$: 2) Imagine that you put a green filter next to a blue filter, so that white light had to pass through one filter after the other. Would any light come through?
$A+I$: 3) Which two filters, if placed together, would absorb all colours of light?
Adding coloured light
A prism divides white light into a spectrum. Here is how to put white light back together again.
Put red, green and blue filters on three torches. Shine them so that their different colours overlap. Where all three meet, you will see white. This is because each filter lets through one third of the spectrum. When you use them together, all colours of the spectrum are present and they add up to give white.
Red, green and blue are called the primary colours of light. When two or more colours of light are mixed together, this is called colour addition.
Reflecting coloured light
Grass is green. When you see grass, green light from the grass is entering your eye.
Grass is green because it reflects green light from the Sun It absorbs all of the other colours of sunlight. The grass has removed most of the colours of the sunlight. This is called colour subtraction. Other colours work the same way. For example, a red object reflects red light and absorbs all other colours. White objects reflect all colours of light and black objects absorb all colours.
Questions
4) Which colours of light are reflected by a black object?
5) Look at the diagram of the postbox. It is painted with red paint. Explain why it looks red when white light shines on it. Use the words 'reflected' and 'absorbed in your explanation
Activity 12.6 (Colour changes)
Grass does not always look green. At night it looks black because no light is falling on it, so no light is reflected to our eyes.
1) The picture shows a postbox with grass around it. The scene looks different when red light is shone on it, and when green light is shone on it.
With a partner, discuss why the appearance of the scene changes. When you have decided on your explanation, compare your ideas with another pair.
2) Now try to decide what the scene will look like if yellow and green light are shone on it.
3) Use coloured pens or pencils to draw a simple scene. Shine coloured light on your scene. Does it look different if you change the colour of the light?
