Colored Light - Chapter 9 PDF

# Chapter 9. Topic 3 Coloured Light ## You Will Learn: - That white light is made of many colours. - To use a prism to show the dispersal of white light. - To describe what can be done with colours of light. - To reach conclusions studying results and explain their limitations. - To carry out pr...

# Chapter 9. Topic 3 Coloured Light ## You Will Learn: - That white light is made of many colours. - To use a prism to show the dispersal of white light. - To describe what can be done with colours of light. - To reach conclusions studying results and explain their limitations. - To carry out practical work safely. - To describe the application of science in society, industry and research. - To describe how scientific progress is made through individuals and collaboration. ## Starting Point - You should know that... light refracts when it travels from air into glass, clear plastic or water. - You should be able to... Draw accurate diagrams to show how light travels. ## Making A Spectrum - If you shine white light into a prism, coloured light comes out at the other side. It looks like a rainbow. Why does this happen? Where do the colours come from? - White light is a mixture of different coloured lights. - The prism splits the white light into these different colours, this is called *dispersion*. - In figure 9.14, the light entering the prism is refracted when it goes into the prism and again when it leaves. - White light is a mixture of all colours. As white light passes through the prism, different colours of light are refracted by different amounts. - Figure 9.14 shows that violet light is refracted more than red light. - The colours in the light spectrum are: Red Orange Yellow Green Blue Indigo Violet - You can remember the order of the colours from 'Rinse out your greasy bottles in vinegar'! You can make up your own saying to help you remember the colours too. - There are other ways of showing that white light is a mixture of the colours of the spectrum. - If you look at white light through a *diffraction grating*, you can also see a spectrum. ## Key Terms: | **Term** | **Description** | | - | - | | Dispersion | The splitting of white light into a spectrum of colours | | Prism | Transparent object that refracts light | | Diffraction grating | Transparent piece of glass or plastic which has many lines drawn onto it. Light can pass through the spaces between the lines. | ## A Diffraction Grating: - A diffraction grating is a transparent piece of glass or plastic. - It has many lines drawn onto it. - Light can pass through the spaces between the lines. - There are usually about 3000 lines per centimetre. - The closer together the lines are, the clearer the spectrum is. - If the lines are too far apart, you won’t see a spectrum at all. ## Activity 9.6: Making A Newton's Disc - Cut a circle of card. - Divide it into seven equal sections. - Colour each section as shown in figure 9.16. - Make a hole through the centre of your disc and put a pencil through the hole. - Spin the disc as fast as you can. What do you see? - What does this experiment show? ## Combining Colours - Look at figure 9.17. Why does the shirt on the right look blue and the shirt on the left look red? - It is because the blue shirt only reflects blue light and the red shirt only reflects red light. ## Primary and Secondary Colours of Light - There are three primary colours of light: - Red. - Blue. - Green. - These three colours can be added together to produce the secondary colours, this is called *colour addition*. | Primary Colours of Light | Secondary Colours of Light | |---|--- | | Red + Green = Yellow | Yellow | | Red + Blue = Magenta | Magenta | | Blue + Green = Cyan | Cyan | - *Yellow, magenta and cyan are called secondary colours*. - Red and green light are added to form yellow light. - Red and blue light are added to form magenta light. - Green and blue light are added to form cyan light. - All three of the primary colours of light can be added together to produce white light. - Objects appear black when they do not reflect any light at all. - We say that a black object *absorbs* all the light that shines on it. - You can make many different colours by mixing different amounts of each primary colour. - This is how you get coloured images on TVs and computer screens. ## Key Terms | Term | Description | | - | - | | Absorption | The way in which an object takes in the energy reaching its surface. | | Primary Colours | Red, blue and green. Combining these colours of light will make all other colours of light. | | Secondary Colours | Yellow, magenta and cyan. | ## Colour Filters and Coloured Lighting - A colour filter will only let light of its own colour pass through it. The filter absorbs all other colours of light. - For example, a red filter will only allow red light to pass through it. - Figure 9.19 shows what happens if you use different colours of light with different filters. ## Key Term | Term | Description | |---|---| | Filter | A colour filter will only allow light of its own colour to pass through it. | ## What Would You See If You Looked At: - A yellow shirt using a red filter? - A magenta shirt using a blue filter? - A cyan shirt using a green filter? - A blue shirt using a red filter? ## You Can Use Colour Filters To Make Different Coloured Lights: - You can use different coloured lights to make objects look a different colour too. - In the process of *colour subtraction* one or more colours of light from white light may be absorbed by a material while the other colour or colours are reflected. - The colours reflected are the colours seen. - White light is made up of red, blue and green light. - If white light is shone onto a shirt that absorbs red light, the red light is subtracted from the white light. - The remaining blue and green light would be reflected, and the shirt would therefore appear cyan to the observer due to the combination of the blue and green light. - This process of colour subtraction is further illustrated in figure 9.20. ## Imagine You Are Looking At A Sheet Of White Paper: - White can reflect all the primary colours. - Figure 9.20 shows you what happens when different colour lights are shone on different coloured papers. ## 2 What Would You See If You Looked At: - A magenta shirt under red light? - A red shirt under yellow light? - A cyan shirt under blue light? - A blue shirt using green light? ## Scattering - Almost everything scatters light. - As we saw earlier in this chapter, when light hits a rough surface or particles in the atmosphere it reflects. - Depending on the shape of the surface, the light will reflect in different directions. - Different colours of light are scattered by different amounts. - Violet light is scattered the most and red light is scattered the least. - In the daytime, the sky looks blue because the Sun is high in the sky. - Light from the Sun is scattered by the particles in the atmosphere. - The angle of scattering means that more scattered blue light reaches your eyes. - At sunrise and sunset, the Sun is lower in the sky. - The angle of scattering means that more scattered red light reaches your eyes. ## Activity 9.7: Looking Through A Colour Filter - Look at objects around you through different coloured light filters. What do you notice? - Use a red pen, a green pen and a blue pen to write on white paper. - Look at the writing through a red filter, a green filter and a blue filter. - What do you see? - You could also try writing a secret message that can only be seen using a colour filter. - For example, when viewed through a red filter WHESELXLLWCOOVMDE says WELCOME. ## Activity 9.8: The Effects Of Coloured Lights - Use a ray box (or torch) and colour filters to make different coloured beams of light. - Shine these onto different coloured objects and note any colour changes you see. - Try to do as many colour combinations as you can. - You could also try writing a secret message that can only be seen using a coloured light. - For example, when viewed under green light FHOSELXLLWDOB says HELLO. ## Science In Context: Colour Blindness - The human eye contains cells called cones which detect colour. - There are three different types of cone, each type detecting specific colours of light. - If someone is colour blind it is usually because one or more type of cone does not work correctly. - This means that the eye has problems detecting the difference between specific pairs of colours. - The most common type of colour blindness is *red-green* where the colour blind person cannot tell the difference between these two colours though they have no problems with other colours. - Red-green colour blindness is usually diagnosed by using the *Ishihara Plate test*. - This was developed by Dr Shinobu Ishihara in 1917. It is not as good at diagnosing other forms of colour blindness. Other tests have been developed more recently, for example the Farnsworth-Munsell 100 hue test which was developed in the 1940s. - It was originally done using coloured tiles but is now usually done using blocks of colour on a computer screen. - Figure 9.21 shows three examples of the plates used in the Ishihara plate test. - Inside each plate there is a number or a pattern created by one or more lines. - A person who is unable to see these patterns or numbers in the plates may have red-green colour blindness, and they should get tested further. ## Key Facts - A prism can split white light into a spectrum. This is *dispersion*. - The primary colours of light are red, blue and green. - Adding primary colours can make secondary colours and white. - Filters only allow certain colours of light to pass through them. - Filters absorb other colours of light, causing them to be subtracted from the reflected light seen. ## Check Your Skills Progress - I can do experiments to show how a prism can form a spectrum. - I can use filters to make different coloured lights and make objects look a different colour. - I can predict what something will look like under different colour lights and through different coloured filters.