Light and Optics - Physics Textbook

Okay, here's the markdown conversion of the provided text: ## 6.3 Making Rainbows ### Newton's discovery about light The name Newton is often associated with forces, but Isaac Newton made many important discoveries. In the year 1666, Newton showed that white light could be split into different col...

Okay, here's the markdown conversion of the provided text: ## 6.3 Making Rainbows ### Newton's discovery about light The name Newton is often associated with forces, but Isaac Newton made many important discoveries. In the year 1666, Newton showed that white light could be split into different colours. The picture shows Newton using light from the Sun, coming through a hole. Newton is using a glass block in the shape of a triangle to split the light into its colours. He is shining the colours onto a screen. The next picture shows what the colours look like when seen on a screen. These are the colours that make up white light. ### The spectrum The range of colours that can be seen in white light is called a spectrum. In the spectrum, the colours are not separate but they merge from one to another. Some people think Newton originally saw five or six colours, but most people now agree that there are seven. In the order that they appear in the spectrum, these seven colours are: * red * orange * yellow * green * blue * indigo * violet You can remember the order of the colours using a made-up person's name: 'ROY G BIV'. ### Dispersion Dispersion means splitting light into different colours. Dispersion happens because light is refracted. Each of the different colours of light that make up white light is refracted through a slightly different angle. This can be shown by using a triangular prism. Triangular means in the shape of a triangle. A prism is a solid shape such as the one that Newton used. When a ray of white light passes through the prism, the ray is refracted. Violet light is refracted through the largest angle and red light is refracted through the smallest angle. You can see this in the picture. ## 6 Light ### Rainbows A rainbow is formed when drops of water in the air cause dispersion of light. The light is also reflected from inside the drops of water. That means that, for you to see a rainbow: * The Sun must be shining, to provide bright light * There must be rain or small drops of water in the air, to cause dispersion of light * The Sun must be behind you, because the water drops reflect the light inside them. The drops of water from the hosepipe in the photograph are making a rainbow. The Sun is shining and the Sun is behind the camera. ### Questions 1\. What name is given to white light being split into different colours? * refraction * dispersion * reflection * conduction 2\. The diagram shows a glass block used to separate white light. What name describes this piece of equipment? Write one letter. * A round glass cylinder * B square glass prism * C triangular glass prism * D hexagonal glass prism 3\. The diagram shows white light being separated into different colours. The colours are seen on a white screen. a. Which letter shows the position of red light on the screen? b. Which letter shows the colour of light that is refracted through the smallest angle? c. What word describes the range of colours seen on the screen? * refraction * reflected * separated * spectrum 4. Explain why drops of water are needed for a rainbow to be seen. 5. When looking at a rainbow, some people see indigo and violet as the same colour. How many colours will these people say they can see in the rainbow? ### Think like a scientist Making a rainbow In this activity, you will change variables and describe how observations change. Work in groups of two or three. You will need: * ray box and suitable power supply * triangular glass prism * piece of white paper or card to use as a screen **Safety** Do not put your eye closer to the prism than about one metre. The light will become very bright and could cause damage to your eye. Make the room as dark as possible for this activity. Set up the equipment as shown in the diagram. Method: Part 1 1. Adjust the positions of the ray box and the screen until you see the colours of the rainbow on the screen. 2. Move the screen closer to the prism. 3. Move the screen further away from the screen. ### Questions 1. a. How many colours can you see on the screen? b. List the colours in order, starting from red. 2. Name the colour that is refracted: a. through the smallest angle b. through the largest angle. 3. Describe two things that happened to the colours when: a. the screen was moved closer to the prism b. the screen was moved further away from the prism. ### Continued Method: Part 2 1. Remove the screen and stand about 1 metre from the prism, in the same direction as the screen had been. 2. Move so that you are looking into the refracted rays with one eye. You may find it easier to cover the other eye. 3. Move from side to side so that you can see the different colours. 4. Increase the distance between your eye and the prism to about 2 metres. 5. Again, move from side to side so that you can see the different colours. ### Questions 4. Describe two differences in the observations when you moved further away. 5. Describe one advantage and one disadvantage of *Method: Part 1* for observing the colours. 6. This activity is an analogy for how rainbows form. Describe one strength and one limitation of this analogy. ### Self-assessment Different people see different numbers of colours in this activity. The numbers of colours usually vary from 5 to 7. Did you see the same number as everyone else in the class? Suggest reasons why people see different numbers of colours. ### Summary checklist * I can recall that white light is made from different colours of light. * I can describe how to use a prism to produce dispersion of white light. * I can list the seven colours in order, starting from red. ## 6.4 Colours of light In this topic you will: * discover what happens when colours of light are added * discover what happens when colours of light are subtracted * discover why we see different colours. ### Getting started Work in pairs. Make a list of all the colours you can see in the picture of flowers. How many did you get? How does this number compare with other groups? ### Key words * absorbed * coloured filters * cyan * magenta * non-luminous * primary colours * subtraction * transmit ## 6 Light ### Primary colours In Topic 3.3 you learned that there are seven colours in white light. These are the colours that can be seen in the rainbow. However, there are three colours of light from which all other colours of light can be made. These are called the primary colours. The primary colours cannot be made by mixing any other colours of light. The primary colours of light are: * Red * Green * Blue The primary colours of light are different from the primary colours in paints. The colours in light mix differently from the colours in paint. ### Adding colours of light he diagram shows what happens when three beams of light, each of a different primary colour, overlap. You can see the effect of adding the primary colours: * red + green = yellow * red + blue = magenta * blue + green = cyan * red + green + blue = white The different colours that you see on a mobile phone, computer or television are all produced from combinations of the three primary colours of light. When you look very closely at some types of computer monitor, television or phone screen, you can see the individual sources of red, green and blue light. ## 6 Light The blue stage light in the picture has a white lamp and a blue filter. If you look carefully, you can see where the white lamp is inside the black case. The colours of the filters in stage lights can be changed to produce different colours. Photographers sometimes use coloured filters on a camera to get different effects. As with numbers, it is possible to subtract colours of light until the end result is zero. For example, if white light shines on a green filter, only green light will get through. The other colours of the white light are absorbed. If this green light then shines on a red filter, then no light will get through. That is because green is one of the colours that a red filter absorbs. ## 6.4 Colours of light ### Seeing colours When you look at a non-luminous object, you see the light that is reflected from the object. Non-luminous means the object does not emit its own light. Look at the flower in the picture. The flower is seen in white light. The flower looks yellow because it reflects only yellow light. The flower absorbs the other six colours in white light. This is shown in the diagram. A white object reflects all the colours in white light equally. A black object absorbs all the colours in white light and does not reflect any. These three balls appear black, red or white, according which colours of light they reflect and which they absorb. ## 6 Light Look at the two cubes in the diagrams. Red objects only reflect red light, and blue objects only reflect blue light. They absorb all other colours. So, in white light, the red cube appears red and the blue cube appears blue. In blue light, the red cube appears black: there is no red light for it to reflect, and the blue light is absorbed. In red light, the blue cube appears black for the same reason - it absorbs the red light and does not reflect any light. ### Questions 1. List the three primary colours of light. 2. Name the colour produced when a. red light and blue light are added together b. red light and green light are added together c. red, green and blue lights are added together. 3. A stage light uses a white lamp. What colour of light will be seen when: a. a yellow filter is used b. an orange filter is used. 4. a. A green ball appears green. What two colours of light could each be shining on the green ball? * blue * green * red * white * magenta b. A blue ball appears black. What two colours of light could each be shining on the blue ball? * blue * green * red * white 5. A T-shirt looks red. What could explain this? Write three letters. A. The T-shirt is red and is seen in white light. B The T-shirt is red and is seen in red light. C The T-shirt is blue and is seen in green light. D The T-shirt is white and is seen in red light. E The T-shirt is yellow and is seen in blue light. ## 6.5 Galaxies In this topic you will: * discover that galaxies contain dust, gas, stars and other solar systems. * Key words * elliptical * galaxy * irregular * spiral * stellar dust * Universe ### Getting started Work in pairs. Arrange these objects in order, from smallest to largest. * star * solar system * planet * galaxy * moon ## 6 Light ### The Milky Way If you look at the sky on a clear night, far away from any lights, you can see a milky band across the sky. This milky band is part of the galaxy where we live, called the Milky Way. This is what it looks like through a camera set for very low light conditions. The word 'galaxy' comes from a Greek word for 'milky'. ### Shapes of galaxies The reason why our own galaxy looks like a band across the sky is because of the shape of the galaxy. The Milky Way is a spiral galaxy. If you were to look at the Milky Way from far away, it would appear as a spiral. Because we live in a spiral galaxy, we can only see one 'arm' of the spiral, which is that band across the sky. In fact, most of the stars we see at night are in our own galaxy. There are an estimated 250 000 000 stars in the Milky Way, including our Sun. There are other galaxies in the Universe besides our own. The word Universe is used to describe all of space and everything in it. These other galaxies have different shapes, and they are classified according to shape. They are called elliptical galaxies or irregular galaxies. ## 6.5 Galaxies The closest known galaxy to the Milky Way is called the Canis Major Dwarf Galaxy. It is elliptical in shape and a distance of 240 000 million million km away from us. ### What are galaxies made of? Galaxies are made from stellar dust, gas, stars and solar systems held together by gravity. Stellar dust is the dust found in space. The Earth is travelling through a cloud of stellar dust that is estimated to contain one dust particle in every one million $m^3$ of space! The gravity holding a galaxy together is strong because galaxies have very large mass. Even if you could travel at the speed of light (300 000 km/s), it would take you more than 100 000 years to cross from one side of the Milky Way to the other! Scientists estimate the mass of the Milky Way to be 1 500 000 000 000 times the mass of the Sun. ### How many galaxies are there? Scientists have counted the galaxies in one part of space. The scientists then multiplied this number up to estimate the number of galaxies in the Universe. The answer they got was 100 000 000 000 galaxies! Estimates such as this may not be accurate. There could be more or fewer galaxies in the part of space that the scientists counted compared with the rest of the Universe. Also, the scientists may not know the total volume of the Universe accurately. ### Questions 1. List the three different shapes of galaxies. 2. Which of these are found in galaxies? Choose all that are correct from the list. * planet * star * Universe * stellar gas 3. Name the force that holds the parts of a galaxy together. 4. Explain why most of the stars we see in the night sky are from our own galaxy and not from other galaxies. 5. Suggest why scientists can only estimate the number of stars in the Milky Way and not know the number accurately. I have presented the text in a structured markdown format, using headings, subheadings, bullet points, questions, and key words to organize the information effectively. I have also converted any math formula into LaTeX format.