Light.ppt.pdf

Light 1 of 47 © Boardworks Ltd 2008  What is light? 2 of 47 © Boardworks Ltd 2008 What is light? Light is produced by the Sun, and by objects such as light bulbs and matches. Light carries energy and travels as a wave. Light travels at a speed of 300,000,000 me...

Light 1 of 47 © Boardworks Ltd 2008  What is light? 2 of 47 © Boardworks Ltd 2008 What is light? Light is produced by the Sun, and by objects such as light bulbs and matches. Light carries energy and travels as a wave. Light travels at a speed of 300,000,000 metres per second, which is much faster than the speed of sound. Light waves travel in straight lines. 3 of 47 © Boardworks Ltd 2008 Light energy Light is a form of energy and can be changed from one form into another. Light energy can be used to make other useful forms of energy. It can be converted into electrical energy in a solar cell or chemical energy in the leaves of plants. solar cell plants 4 of 47 © Boardworks Ltd 2008 Which materials let light through? 5 of 47 © Boardworks Ltd 2008 How do we see things? An object that gives out light is described as luminous. How does light from a luminous object such as a light bulb reach the eye? Light travels in a straight line directly into the eye. An object that does not give out light is non-luminous. How does your eye see a non-luminous object such as a comb? Light hits the comb and some of it is reflected into the eye. 6 of 47 © Boardworks Ltd 2008 How the eye works 7 of 47 © Boardworks Ltd 2008  Reflection 8 of 47 © Boardworks Ltd 2008 Reflective materials Objects that reflect light well: Have smooth, shiny surfaces and are usually pale colours. Give clear images because they reflect light regularly. Objects that do not reflect light well: Have rough, matt surfaces and are usually dark colours. Give diffuse images (or do not give any images) because they reflect light irregularly. This is called scattering. 9 of 47 © Boardworks Ltd 2008 Reading in mirrors Working in pairs, decide who is the ‘timer’ and who is the ‘reader’. The ‘reader’ has to read a selection of words reflected in a mirror. They must read each word correctly before moving on to the next. The ‘timer’ measures the time taken and the results for the whole class are recorded in a table like this: 1. Why are the words so Name Time taken (s) difficult to read in the mirror Natasha – how do they appear? 2. What was the average time Shani taken in the class? Rajesh 3. Plot a bar chart of results. 10 of 47 © Boardworks Ltd 2008 What is lateral inversion? A plane mirror reflects light regularly so it produces a clear image, which is the same size as the object. The image appears the same distance behind the mirror as the object is in front of it. What is different about the image compared to the object? When an object is reflected in a plane mirror, left appears as right and right appears as left. This type of reversal is called lateral inversion. 11 of 47 © Boardworks Ltd 2008 Reflection at a mirror 12 of 47 © Boardworks Ltd 2008 Reflection ray diagram 13 of 47 © Boardworks Ltd 2008 Reflection investigation Fix a plane mirror to a piece of paper and draw around it. Draw a normal (a line at 90° to the mirror) through the centre of the mirror outline. Use a ray box to shine an angle of angle of incident ray at the mirror – plot incidence [i] reflection [r] the incident and reflected rays. Measure the angles of incidence [i] and reflection [r]. Repeat for another four angles of incidence. What do the results show? 14 of 47 © Boardworks Ltd 2008 Using reflection Reflection can be very useful. High-visibility strips are very reflective and make sure that this cyclist gets noticed when there is little light. How does a periscope use reflection? The two plane mirrors must be positioned at 45° from the vertical. Light is reflected at right angles from the top mirror onto the bottom mirror and into the eye of the viewer. 15 of 47 © Boardworks Ltd 2008 Reflection: summary 16 of 47 © Boardworks Ltd 2008 Reflection: true or false? 17 of 47 © Boardworks Ltd 2008  Refraction 18 of 47 © Boardworks Ltd 2008 Refraction at an air-glass boundary 19 of 47 © Boardworks Ltd 2008 Refraction investigation Place a rectangular glass block on a sheet of paper and draw around it. Draw a normal at 90° to the top surface of the block. Shine light rays, with angles of incidence [i] of 30°, 60° and 0°, into the block where the normal meets the glass surface. Record the angle of refraction [r] and the angle of the rays leaving the glass block [r2]. angle of angle of angle of incidence [i] refraction [r1] refraction [r2] 60 ° 30 ° 0° 20 of 47 © Boardworks Ltd 2008 Refraction in a glass block 21 of 47 © Boardworks Ltd 2008 Explaining refraction 22 of 47 © Boardworks Ltd 2008 Why is light refracted? The speed of light depends on the material through which the light is travelling. When light enters a different material (e.g. when moving from air into glass), the speed of light changes. This causes the light to bend or refract. air glass The speed of light is affected by the density of the material through which it is travelling. When light enters a more dense medium, its speed decreases and this is why refraction occurs. 23 of 47 © Boardworks Ltd 2008 Refraction ray diagram 24 of 47 © Boardworks Ltd 2008 What happens during refraction? 25 of 47 © Boardworks Ltd 2008 Effects of refraction Light from the part of the pencil in the water is refracted as it travels from the water into the air, making it appear bent. How does refraction make this stone look closer to the surface of the water than it really is? Light rays from the stone are refracted as they leave the water. The brain assumes the rays have travelled in straight lines, and is fooled into forming an image where it thinks the light rays came from. 26 of 47 © Boardworks Ltd 2008 Effects of refraction – the Archer fish The archer fish is a predator that shoots jets of water at insects near the surface of the water. The archer fish allows image of prey for the refraction of light at the surface of the water when aiming at its prey. The fish does not aim at the refracted prey image it sees but at location a location where it knows the prey to be. 27 of 47 © Boardworks Ltd 2008 Refraction – true or false? 28 of 47 © Boardworks Ltd 2008  Colour 29 of 47 © Boardworks Ltd 2008 Passing white light through a prism 30 of 47 © Boardworks Ltd 2008 Splitting white light A prism splits a ray of white light into the colours of the rainbow. This process is known as dispersion. The colours that make up white light are called the spectrum. Dispersion occurs because different colours of light refract differently. Red light refracts the least; violet light the most. The order of the colours in the spectrum is always the same. Use this phrase to remember the order of colours: Richard Of York Gave Battle In Vain 31 of 47 © Boardworks Ltd 2008 Natural dispersion If there are water droplets in the air and the sun is illuminating them from behind, then you may see a rainbow in the air. Light enters the water droplets and refracts. It then reflects off the back of the rain drop. The red light refracts the least and the violet the most. This causes dispersion of the sunlight. 32 of 47 © Boardworks Ltd 2008 The visible light spectrum 33 of 47 © Boardworks Ltd 2008 Recombining colours 34 of 47 © Boardworks Ltd 2008 Mixing coloured light 35 of 47 © Boardworks Ltd 2008 How do we see different colours? 36 of 47 © Boardworks Ltd 2008 Seeing different colours How do we see the different colours in this frog and lily? The frog’s red skin absorbs all of the colours except red and so it appears red. The black skin absorbs all colours. No colours are reflected and so it appears black. The lily’s centre absorbs all colours except red and green. It reflects red and green light, and so appears yellow. The leaves reflect all the colours and so appear white. 37 of 47 © Boardworks Ltd 2008 Using filters of primary colours A filter absorbs some colours of white light and lets other colours through to create coloured light. A red filter absorbs all colours… …apart from red light. A blue filter absorbs all colours… …apart from blue light. A green filter absorbs all colours... …apart from green light. 38 of 47 © Boardworks Ltd 2008 Using filters of secondary colours A magenta filter absorbs all colours… …apart from red and blue. A cyan filter absorbs all colours… …apart from green and blue. A yellow filter absorbs all colours... …apart from red and green. 39 of 47 © Boardworks Ltd 2008 Seeing colours in coloured light 40 of 47 © Boardworks Ltd 2008 How do we see colours in coloured light? 41 of 47 © Boardworks Ltd 2008 The electromagnetic spectrum 42 of 47 © Boardworks Ltd 2008 Colour summary 43 of 47 © Boardworks Ltd 2008  Summary activities 44 of 47 © Boardworks Ltd 2008 Glossary 45 of 47 © Boardworks Ltd 2008 Anagrams 46 of 47 © Boardworks Ltd 2008 Multiple-choice quiz 47 of 47 © Boardworks Ltd 2008