8 k light (boardworks)

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Published on February 4, 2014

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ks3 science

KS3 Physics 8K Light 1 of 84 20 © Boardworks Ltd 2004 2005

Contents 8K Light What is light? Reflection Refraction Colour Summary activities 1 of 84 20 2 © Boardworks Ltd 2004 2005

What is light? Light carries energy and travels as a wave. Light travels much faster than sound at a speed of 300,000,000 m/s, which is the same as 300,000 km/s. Light waves travel in straight lines. 1 of 84 20 3 © Boardworks Ltd 2004 2005

Which materials let light through? Opaque materials do not allow light to pass through them – transparent materials do. A material that is translucent only lets part of the light through. Hold different materials between the lamp and the screen. Use the results table and shading chart on the next slide to estimate the opacity of different materials. 1 of 84 20 4 © Boardworks Ltd 2004 2005

Which materials let light through? Opaque materials do not allow light to pass through them – transparent materials do. A material that is translucent only lets part of the light through. Material Tracing paper 1 ply tissue 2 ply tissue Smoked glass Glass Perspex Wood 1 of 84 20 5 Opacity (%) Shading chart 100% 75 % 50 % 2.5 % 5% 10 % 30 % 15 % 25 % 20 % 0% © Boardworks Ltd 2004 2005

How does light travel? 1. Fill a clear glass trough or empty fish tank with smoke. 2. Use a slit to shine rays of light through the tank and describe what you see. Light waves travel in straight lines. 1 of 84 20 6 © Boardworks Ltd 2004 2005

Seeing light How do you see an object? Light from the object enters your eye. Do you see all objects in the same way? There are two ways you see objects:  You see some objects because they are light sources.  You see other objects by reflected light. 1 of 84 20 7 © Boardworks Ltd 2004 2005

Seeing a luminous object A luminous object gives out light and can also be called a light source. How does light from a light bulb and other light sources reach your eye? Light travels in a straight line directly into your eye. 1 of 84 20 8 © Boardworks Ltd 2004 2005

Seeing a non-luminous object Objects that do not give out light are non-luminous. How does your eye see non-luminous objects such as a book? Light from the light source strikes the book and some of the light is reflected into your eye. 1 of 84 20 9 © Boardworks Ltd 2004 2005

Contents 8K Light What is light? Reflection Refraction Colour Summary activities 1 of 20 10 of 84 © Boardworks Ltd 2004 2005

Good and bad reflective materials Objects that reflect light well:  have smooth, shiny surfaces and are usually pale colours;  give clear images because they reflect light regularly;  mirrors are excellent reflectors. Objects that do not reflect light well:  have rough, matt surfaces and are usually dark colours.  give no or diffuse images because they reflect the light irregularly. 1 of 20 11 of 84 © Boardworks Ltd 2004 2005

Good and bad reflective materials Arrange these items along the arrow: yellow banana r ed roses white pape r polished black shoes aluminium foil 1 of 20 12 of 84 tarm ac road best reflectors blue car angerin t tree bark e gree n leaf worst reflectors © Boardworks Ltd 2004 2005

Light that is not reflected What happens to light that is not reflected?  Some of this light may be absorbed, e.g. as heat.  Some of this may also be transmitted, e.g. glass reflects a small amount of light, absorbs some of the rest and allows most of it to pass through. 1 of 20 13 of 84 © Boardworks Ltd 2004 2005

Reflection investigations The following activities are designed to investigate the main laws of reflection. Summarize each investigation with a law based on the results of the exercise. 1. Reading in mirrors. 2. How far away is the image? 3. The maths of reflection. 4. Reflecting without mirrors. 1 of 20 14 of 84 © Boardworks Ltd 2004 2005

Reading in mirrors – instructions In small groups, take it in turns to read the list of words on the next slide with your back to the screen using a mirror. You can only move on to the next word when you have read the first word correctly. Put your results in a table like this: Name Natasha 46 Pashmina 56 David 1 of 20 15 of 84 Time taken to read (s) 85 © Boardworks Ltd 2004 2005

Reading in mirrors – words dog ball bat bike ants park fins pink litter 1 of 20 16 of 84 man sandy shark © Boardworks Ltd 2004 2005

Reading in mirrors – results 1. Who read the words in the quickest time? 2. Plot a bar chart of your results: Time taken [s] A graph showing the results of 'Reading in Mirrors' 100 50 0 Natasha Pashmina David Name 3. What was the average time taken in your group? 1 of 20 17 of 84 © Boardworks Ltd 2004 2005

Lateral inversion A plane mirror reflects light regularly so that it produces a clear image which is the same size as the object. What is different about the image? When something is reflected in a plane mirror, left becomes right and right becomes left. This is called lateral inversion. 1 of 20 18 of 84 © Boardworks Ltd 2004 2005

How far away is the image? 1. Fix a plane mirror along the centre of a piece of A4 paper and draw around it. I Place a pin as the object in front of the mirror. 2. Line up a ruler with the image of the pin and draw along the edge of the ruler on the paper. Repeat for three more positions of the ruler. 1 of 20 19 of 84 3. Remove the mirror and ruler. The point where the lines cross is the image position. What are the distances between the mirror and the object and its image? © Boardworks Ltd 2004 2005

The maths of reflection Fix a plane mirror to a piece of A5 paper and draw around it. Draw a normal line (at 90º) through the middle of the mirror outline. Use a ray box to shine an incident ray at the mirror – plot the incident and reflected rays. Measure the angles of incidence [i] and reflection [r] and record the results. angle i angle r Angle of Angle of incidence [i] reflection [r] Repeat for another five angles of incidence. 1 of 20 20 of 84 © Boardworks Ltd 2004 2005

Reflecting without mirrors Mirrors are good reflectors but not perfect - they give two reflections. Glass prisms are used instead of mirrors in good quality binoculars and other instruments. 1 of 20 21 of 84 © Boardworks Ltd 2004 2005

Reflecting without mirrors Shine rays of light into a prism as shown in these ray diagrams. Copy and complete the ray diagrams using a ruler and pencil. Don’t forget to include arrows on your rays! 1 of 20 22 of 84 © Boardworks Ltd 2004 2005

Using plane mirrors By positioning two plane mirrors at 45° to each other at either end of a tube we can make a periscope ___________. Periscopes are used in _____________. submarines 1 of 20 23 of 84 © Boardworks Ltd 2004 2005

Reflection summary 1. Pale and shiny surfaces are good reflectors, dark and rough surfaces are not. 2. The image in a plane mirror is laterally inverted. 3. The image is the same distance behind the mirror as the object is in front. 4. The image in a plane mirror is the same size as the object. 5. The law of reflection is: angle of incidence (i) = angle of reflection (r) 1 of 20 24 of 84 © Boardworks Ltd 2004 2005

Contents 8K Light What is light? Reflection Refraction Colour Summary activities 1 of 20 25 of 84 © Boardworks Ltd 2004 2005

Bending light The speed of light waves depends on the material they are travelling through. air = fastest glass = slower diamond = slowest If light waves enter a different material (e.g. travel from glass into air) the speed changes. This causes the light to bend or refract. air glass 1 of 20 26 of 84 © Boardworks Ltd 2004 2005

Refraction at the air-glass boundary 1 of 20 27 of 84 © Boardworks Ltd 2004 2005

Refraction investigation 1. Place a rectangular glass block on a sheet of paper and draw around it. angle i angle r 2. Draw a normal line (at 90º) along the top surface of the block. 3. Shine rays of light with incident [i] angles of 30º, 60º and 0º into the block, making sure they all hit where the normal line crosses the glass surface. Measure angle ‘r’ each time and record the results. 1 of 20 28 of 84 © Boardworks Ltd 2004 2005

Refraction investigation – results Record the results of the refraction investigation in a table: Angle of incidence [i] Angle of refraction [r] 30º 60º 0º Write two ‘rules’ to describe:  what happens to the ray as it enters the glass;  what happens to the ray as it re-enters the air. 1 of 20 29 of 84 © Boardworks Ltd 2004 2005

What happens in refraction: air to glass When light is refracted as it travels from air to glass: angle of incidence > angle of refraction ∠i > ∠r As the light ray travels from air into glass it moves towards the normal. In general, when light rays move from a less dense medium (air) to a more dense medium (glass) they ‘bend’ towards the normal. 1 of 20 30 of 84 ∠i > ∠r air glass © Boardworks Ltd 2004 2005

What happens in refraction: glass to air When light is refracted as it travels from air to glass: angle of incidence < angle of refraction ∠i < ∠r As the light ray travels from glass into air it moves away from the normal. In general, when light rays travel from a more dense medium (glass) to a less dense medium (air) they ‘bend’ away from the normal. If the two surfaces of the block are parallel, then the ray at the start is parallel to the ray at the end. 1 of 20 31 of 84 glass air ∠i < ∠r © Boardworks Ltd 2004 2005

Refraction – angle of incidence = 0° What happens to light travelling from air through a glass block when the angle of incidence is 0°? ∠ i = 0° When the angle of incidence is 0° the light ray is not deviated from its path. air glass undeviated light ray 1 of 20 32 of 84 © Boardworks Ltd 2004 2005

Refraction in a rectangular block 1 of 20 33 of 84 © Boardworks Ltd 2004 2005

Refraction – revision tip To remember what happens to light when it is refracted, think of the word: TAGAGA Towards (normal) Air Glass Away (from normal) Glass Air 1 of 20 34 of 84 © Boardworks Ltd 2004 2005

Travelling through different materials If you were running along a beach and then ran into the water when would you be moving slower – in the water or on the sand? In the water. In a similar way, as light moves from one medium to another of different density, the speed of light changes. Do you think light moves faster or slower in a more dense medium? Light moves slower through a more dense medium. 1 of 20 35 of 84 © Boardworks Ltd 2004 2005

The speed of light in different media Perspex From this bar chart, which material do you think is denser, Perspex or water? Speed of light (thousands km/s) Water As light enters denser media, the speed of light decreases. 300 270 240 210 180 150 120 90 60 30 0 Vacuum Light travels at 300,000 km/s in a vacuum. Perspex must be denser than water because light travels more slowly through Perspex than water. 1 of 20 36 of 84 © Boardworks Ltd 2004 2005

Why does light change direction? Imagine a car driving from the road into a muddy field.  In the muddy field it slows down as there is more friction.  If it enters the field at an angle then the front tyres hit the mud at different times.  Tyre 1 hits the mud first and will move more slowly than tyre 2. This causes the car to turn towards the normal.  When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal. 1 of 20 37 of 84 road tyre 1 tyre 2 mud © Boardworks Ltd 2004 2005

Why does light change direction? If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time. The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down. 1 of 20 38 of 84 © Boardworks Ltd 2004 2005

Why does light change direction?  When light hits a medium at an angle to the n_____ ormal the light ‘bends’ in a similar way to that described for the car in a muddy field.  Part of the light ray s____ d____ before the rest and lows own this causes the change of d_______. irection  If the light enters a new medium along the normal (i.e. angle of incidence = 0°) then it does not ‘bend’ because all of the light ray slows down at the s___ t___. ame ime 1 of 20 39 of 84 © Boardworks Ltd 2004 2005

Effects of refraction Many visual effects are caused by refraction. This ruler appears bent because the light from one end of the ruler has been refracted, but light from the other end has travelled in a straight line. Would the ruler appear more or less bent if the water was replaced with glass? 1 of 20 40 of 84 © Boardworks Ltd 2004 2005

Apparent depth The rays of light from a stone get bent (refracted) as they leave the water. Your brain assumes these rays of light have travelled in straight lines. Your brain forms an image at the place where it thinks the rays have come from – the stone appears to be higher than it really is. 1 of 20 41 of 84 image actual location © Boardworks Ltd 2004 2005

The Archer fish The Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. on a leaf. The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey. image of prey prey location The fish does not aim at the refracted image it sees but at a location where it knows the prey to be. 1 of 20 42 of 84 © Boardworks Ltd 2004 2005

Magic coins Place a coin in the bottom of a bowl and clamp an empty cardboard tube so that it points above the coin. Gradually add water to the bowl and watch the coin through the tube float up – can you explain this? 1 of 20 43 of 84 © Boardworks Ltd 2004 2005

Refraction summary 1. When light bends this is called refraction. 2. Refraction happens because the light changes speed. 3. When light enters a more dense medium (e.g. glass), it bends towards the normal. 4. When light enters a less dense medium [e.g. air], it bends away from the normal. 5. If the incident ray hits a surface at 0º, no refraction occurs. Remember that the angle of reflection [r] and the angle of refraction [r] use the same symbol. In reflection: i = r In refraction: i ≠ r 1 of 20 44 of 84 © Boardworks Ltd 2004 2005

Contents 8K Light What is light? Reflection Refraction Colour Summary activities 1 of 20 45 of 84 © Boardworks Ltd 2004 2005

Life without colour Imagine you could only see in black and white. How might this affect your life? Would it rule out any careers for you? What dangers could there be? Working in groups, each person has two minutes to give a presentation to the rest of the group about their ideas. 1 of 20 46 of 84 © Boardworks Ltd 2004 2005

Splitting white light with a prism 1. Shine a ray of bright white light at a prism and move the prism until colours appear. 2. Draw a diagram to show what you observed. 1 of 20 47 of 84 © Boardworks Ltd 2004 2005

Splitting white light animation 1 of 20 48 of 84 © Boardworks Ltd 2004 2005

Splitting white light into colours A prism splits a ray of white light into a spectrum of colours. This is known as dispersion. When white light is split, the colours always follow the same order. Use this phrase to remember the order of colours: Richard Of York Gave Battle In Vain 1 of 20 49 of 84 © Boardworks Ltd 2004 2005

Dispersion Each of the colours of the spectrum [ROYGBIV] has a slightly different wave. What is the difference? Each colour has a different wavelength (λ). 1 of 20 50 of 84 © Boardworks Ltd 2004 2005

Dispersion The different colours of light have different wavelengths, this means they are bent (refracted) by different amounts. Which colour is refracted the most? Red light is refracted least because it has the longest wavelength. Violet light is refracted the most because it has the shortest wavelength. 1 of 20 51 of 84 © Boardworks Ltd 2004 2005

Colours of the spectrum 1 of 20 52 of 84 © Boardworks Ltd 2004 2005

Recombining colours Remember how white light can be dispersed to give a spectrum of colours? To do the opposite – two prisms are needed! A similar effect can also be seen using a colour wheel (or Newton’s disc). 1 of 20 53 of 84 © Boardworks Ltd 2004 2005

Newton’s disc Colour in a paper or card circle with the colours of the spectrum. Using string or a pencil spin your disc around. What did you observe? What do you predict you will see? What does this tell you? 1 of 20 54 of 84 © Boardworks Ltd 2004 2005

Newton’s disc animation 1 of 20 55 of 84 © Boardworks Ltd 2004 2005

Seeing colours How do you see non-luminous objects such as a book? You see a non-luminous object when light hits the object and is then reflected into your eyes. So how do we see different colours? Why does a red dress look red? Why does a green apple look green? 1 of 20 56 of 84 © Boardworks Ltd 2004 2005

Primary colours animation 1 of 20 57 of 84 © Boardworks Ltd 2004 2005

Primary and secondary colours Colours are made by mixing other colours of light. There are three primary colours of light used to make all other colours. What are these colours? The three primary colours of light are red, green and blue. green red magenta blue The colours made by mixing two primary colours are called the secondary colours – magenta, yellow and cyan. 1 of 20 58 of 84 © Boardworks Ltd 2004 2005

Which primary colours? 1 of 20 59 of 84 © Boardworks Ltd 2004 2005

Seeing red Why does a red snooker ball look red in white light? White light is made up of a spectrum of colours. The snooker ball absorbs all the colours of the spectrum except red. Only red light is reflected into your eye, so the snooker ball appears red. 1 of 20 60 of 84 © Boardworks Ltd 2004 2005

Seeing green Why does a green snooker ball look green in white light? The snooker ball absorbs all the colours of the spectrum except green. Only green light is reflected into your eye, so the snooker ball appears green. 1 of 20 61 of 84 © Boardworks Ltd 2004 2005

Seeing black Why does a black snooker ball look black in white light? The snooker ball absorbs all the colours of the spectrum. No light is reflected into your eye, so the snooker ball appears black. 1 of 20 62 of 84 © Boardworks Ltd 2004 2005

Seeing white Why does a snooker ball look in white light? The snooker ball does not absorb any of the colours of the spectrum. The whole spectrum of light is reflected into your eye, so the snooker ball appears white. 1 of 20 63 of 84 © Boardworks Ltd 2004 2005

Seeing magenta Why does a magenta ball look magenta in white light? The snooker ball absorbs all the colours of the spectrum except red and blue. Red and blue light are reflected into your eye, so the snooker ball appears magenta. 1 of 20 64 of 84 © Boardworks Ltd 2004 2005

Which colour is reflected? Which colours of light are reflected by these clothes? 1 of 20 65 of 84 © Boardworks Ltd 2004 2005

Seeing different colours What colours are absorbed by this frog’s skin? What colours are reflected into your eyes? This part of the skin absorbs all the colours of the spectrum except red, and so reflects red light. 1 of 20 66 of 84 This part of the skin absorbs all the colours of the spectrum and none are reflected. © Boardworks Ltd 2004 2005

Seeing different colours What colours are absorbed by this flower? What colours are reflected into your eyes? This part of the flower absorbs all colours except red and green. It reflects red and green light, and so appears yellow. 1 of 20 67 of 84 This part of the flower absorbs no colours. It reflects them all and so appears white. © Boardworks Ltd 2004 2005

Using coloured filters Filters let certain colours of light pass through, but absorb all other colours. Using different coloured filters placed in front of your eye, look around the classroom and see what effect they have on your vision. object 1 of 20 68 of 84 filter © Boardworks Ltd 2004 2005

Red, blue and green filters 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. 1 of 20 69 of 84 © Boardworks Ltd 2004 2005

Magenta, cyan and yellow filters 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. 1 of 20 70 of 84 © Boardworks Ltd 2004 2005

Using colour filters 1 of 20 71 of 84 © Boardworks Ltd 2004 2005

Seeing colours in coloured light Why do colours look different in different coloured light? Consider a red ball in red light. The red light shines on the ball. 1 of 20 72 of 84 The red ball reflects the red light and so appears red. © Boardworks Ltd 2004 2005

Seeing colours in coloured light What colour does a red ball appear in green light? The green light shines on the ball. The red ball only reflects red light and so it absorbs the green light. So in green light, this ball does not reflect any light and so appears black. 1 of 20 73 of 84 © Boardworks Ltd 2004 2005

Seeing colours in coloured light What colour does a green ball appear in blue light? The blue light shines on the ball. The green ball only reflects green light and so it absorbs the blue light. So in blue light, this ball does not reflect any light and so appears black. 1 of 20 74 of 84 © Boardworks Ltd 2004 2005

Seeing colours in coloured light What happens when using a coloured filter which lets through more than one type of light? What will a red ball look like in magenta light? The magenta light, which is a mixture of red and blue light, shines on the ball. 1 of 20 75 of 84 The red ball only reflects red light and so absorbs the blue light. So in magenta light, this ball reflects the red light and appears red. © Boardworks Ltd 2004 2005

Coloured light activity – instructions The next two slides include a girl wearing a t-shirt and trousers. The girl is standing in a different coloured light each time. The colour of this light is written at the top of the slide. The aim of each activity is to decide what colours the girls’ clothes would appear in each type of coloured light. Drag the correct t-shirt and trousers onto the girl to find out if you have selected the correct colours. 1 of 20 76 of 84 © Boardworks Ltd 2004 2005

Coloured light activity 1 1 of 20 77 of 84 © Boardworks Ltd 2004 2005

Coloured light activity 2 1 of 20 78 of 84 © Boardworks Ltd 2004 2005

Flag colours in different coloured light How would the colours in this flag appear under these lighting conditions? a) red light b) green light c) blue light 1 of 20 79 of 84 © Boardworks Ltd 2004 2005

What colour does it appear? What colour does each object appear under the given lighting conditions? Object (Colour) Colour Filter red ball red ball blue blue book green blue book magenta green apple cyan green apple magenta red and blue tie 1 of 20 80 of 84 red red Appearance red black black blue green black red and black © Boardworks Ltd 2004 2005

Contents 8K Light What is light? Reflection Refraction Colour Summary activities 1 of 20 81 of 84 © Boardworks Ltd 2004 2005

Glossary absorption – When light is taken in by a material. dispersion – The separating of the colours in light, e.g. when white light passes through a prism. image – A copy of an object formed when light is reflected from a mirror. light – A form of energy that is detected by the eyes. law of reflection – When light is reflected, the angle of incidence equals the angle of reflection. prism – A block of glass, usually triangular, which separates the colours in light. reflection – The bouncing back of light from a surface. refraction – The bending of light when it passes into a different medium. spectrum – The range of colours that make up white light. 1 of 20 82 of 84 © Boardworks Ltd 2004 2005

Anagrams 1 of 20 83 of 84 © Boardworks Ltd 2004 2005

Multiple-choice quiz 1 of 20 84 of 84 © Boardworks Ltd 2004 2005

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