Unit 15 Light - F2 Physics PDF

F2 Physics Unit 15 Light 1 of 84 20 © Boardworks Ltd 2005 2004  Contents Light What is light? Reflection Refraction...

F2 Physics Unit 15 Light 1 of 84 20 © Boardworks Ltd 2005 2004  Contents Light What is light? Reflection Refraction Light Colour 1 of 84 2 20 © Boardworks Ltd 2005 2004  Learning objective To define luminous and non-luminous objects To state the speed of light To construct ray diagrams and illustrate the process of reflection and refraction To state the law of reflection To explain the process dispersion and list out the spectrum of light To apply knowledge on primary colours to predict results of colour addition / subtraction 1 of 84 3 20 © Boardworks Ltd 2005 2004  8K Light We are surrounded by light. For most people, it’s hard to imagine life without it. But where does the light around us come from? Answer: Light comes from light sources. A light source is anything that produces light. (name three in this classroom) 1 of 84 4 20 © Boardworks Ltd 2005 2004  Do you know that light is a type of electromagnetic radiation? Fine…calm down …it is absolutely safe!!! 1 of 84 5 20 © Boardworks Ltd 2005 2004  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 6 20 © Boardworks Ltd 2005 2004  Luminous objects Examples of luminous objects Campfire Burning candle 1 of 84 7 20 © Boardworks Ltd 2005 2004  Luminous objects Examples of luminous objects Stars Light bulbs 1 of 84 8 20 © Boardworks Ltd 2005 2004  Luminous objects Examples of luminous objects Neon lights Firefly 1 of 84 9 20 © Boardworks Ltd 2005 2004  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 ofof20 10 84 © Boardworks Ltd 2005 2004  Which one is luminous? Which one is non-luminous? 1 ofof20 11 84 © Boardworks Ltd 2005 2004  Non-luminous objects We see them only when they reflect light into our eyes. Non-luminous objects do not give out light. 1 ofof20 12 84 © Boardworks Ltd 2005 2004  Seeing light - conclusion Q 1: How do you see an object? Light from the object enters your eye. Q 2: What are the two ways you see objects? There are two ways you see objects: 1. You see some objects because they are light sources / luminous objects – light travels in a straight line directly into your eye. 2. You see other objects by reflected light – light from luminous objects strike on a non-luminous object and some of the light is reflected into your eyes. 1 ofof20 13 84 © Boardworks Ltd 2005 2004  Which one is luminous? Which one is non-luminous? 1 ofof20 14 84 © Boardworks Ltd 2005 2004  Light travels at a very high speed Speed (m/s) Light travelling in a vacuum 300 000 000 Sound travelling in air 340 Flying jumbo jet 260 The fastest moving train 160 1 ofof20 15 84 © Boardworks Ltd 2005 2004  Light travels in straight lines 1 ofof20 16 84 © Boardworks Ltd 2005 2004  Light ray Since light travels in straight lines, we use straight lines with arrows to represent light. Ray diagram straight line (representing path) arrow (representing direction) light ray 1 ofof20 17 84 © Boardworks Ltd 2005 2004  Light ray Light rays from a nearby object are spreading out. 1 ofof20 18 84 © Boardworks Ltd 2005 2004  Light ray Light rays from a distant object are almost parallel. meters away the rays seem to be parallel 1 ofof20 19 84 © Boardworks Ltd 2005 2004  Shadows light is blocked forms opaque objects shadows 1 ofof20 20 84 © Boardworks Ltd 2005 2004  screen light source near object far object 1 ofof20 21 84 © Boardworks Ltd 2005 2004  A ray diagram Now draw a ray diagram to show the path that the light follows! Don’t forget to draw the shadow. Example 1: 1 ofof20 22 84 © Boardworks Ltd 2005 2004 1 ofof20 23 84 © Boardworks Ltd 2005 2004  Pinhole camera A pinhole camera has a hole on one side and a screen on another side What do you notice about the image that is formed? How do you think the image will change if the distance is shorter? 1 ofof20 24 84 © Boardworks Ltd 2005 2004  How shadow is formed? Classwork: (2) draw light ray diagram to show how sunlight creates the shadow of a tree 1 ofof20 25 84 © Boardworks Ltd 2005 2004  8K.2 Light hitting objects When light hits something, it can: 1. Be transmitted – it passes straight through 2. Be absorbed and heat up the object 3. Be reflected – it bounces back 1 ofof20 26 84 © Boardworks Ltd 2005 2004  Letting light through transparent materials allow light to be transmitted through. translucent materials only let part of the light through. 1 ofof20 27 84 © Boardworks Ltd 2005 2004  Stopping the light Opaque materials do not allow light to pass through Opaque surface can absorb / reflect most of light depending on the colours: 1 ofof20 28 84 © Boardworks Ltd 2005 2004  Contents Light What is light? Reflection Refraction Colour 1 ofof20 29 84 © Boardworks Ltd 2005 2004  Reflection at plane surfaces The laws of reflection describe how light is reflected. 1 ofof20 30 84 © Boardworks Ltd 2005 2004  Laws of reflection plane mirror (reflecting surface) 1 ofof20 31 84 © Boardworks Ltd 2005 2004  Laws of reflection Incident ray: travelling towards the reflecting surface plane mirror (reflecting surface) 1 ofof20 32 84 © Boardworks Ltd 2005 2004  Laws of reflection Reflected ray: travelling away from the reflecting surface plane mirror (reflecting surface) 1 ofof20 33 84 © Boardworks Ltd 2005 2004  Laws of reflection Normal: imaginary straight line perpendicular to the reflecting surface plane mirror (reflecting surface) 1 ofof20 34 84 © Boardworks Ltd 2005 2004  Laws of reflection Angle of incidence:(i) angle between the incident ray and the normal plane mirror (reflecting surface) 1 ofof20 35 84 © Boardworks Ltd 2005 2004  Laws of reflection Angle of reflection:(r) angle between the reflected ray and the normal plane mirror (reflecting surface) 1 ofof20 36 84 © Boardworks Ltd 2005 2004  Laws of reflection 1: 1 ofof20 37 84 © Boardworks Ltd 2005 2004 Laws of reflection 2: Incident ray and reflected ray are on the same plane incident ray normal (perpendicular to mirror) reflected ray i r mirror 1 ofof20 38 84 © Boardworks Ltd 2005 2004  Can you successfully shoot your target? Imagine you all are the scientists work in NASA…there is a destructive meteorite hitting our planet…. We have only three trials… A power laser weapon is fixed , but you can adjust the mirror on your table 1 ofof20 39 84 © Boardworks Ltd 2005 2004  light reflection Youtube video: https://www.youtube.com/watch?v=dwxaq 4c9K6k 1 ofof20 40 84 © Boardworks Ltd 2005 2004  Laws of reflection Your turn (class work.1) 1. Draw a horizontal line 2. Use a protractor to measure 45 degrees from the horizontal line to draw the incident ray 3. Draw the 1) normal 2) reflected ray 3) angle of incidence 4) angle of reflection 1 ofof20 41 84 © Boardworks Ltd 2005 2004  Class work 2: Please draw a diagram of light in periscope? 1 ofof20 42 84 © Boardworks Ltd 2005 2004 1 ofof20 43 84 © Boardworks Ltd 2005 2004  Regular reflection and diffuse reflection Reflection of the Taj Mahal from: a calm water surface a wavy water surface Finding the difference 1 ofof20 44 84 © Boardworks Ltd 2005 2004  Regular reflection and diffuse reflection The image is The image is clear. NOT clear. Why? a calm water surface a wavy water surface 1 ofof20 45 84 © Boardworks Ltd 2005 2004 1 ofof20 46 84 © Boardworks Ltd 2005 2004  Regular reflection incident parallel rays smooth surface 1 ofof20 47 84 © Boardworks Ltd 2005 2004  Regular reflection clear images are formed as parallel rays are reflected smooth surface 1 ofof20 48 84 © Boardworks Ltd 2005 2004  Diffuse reflection incident parallel rays rough surface 1 ofof20 49 84 © Boardworks Ltd 2005 2004  Diffuse reflection no clear images are formed as rays reflected with different angles of reflection rough surface 1 ofof20 50 84 © Boardworks Ltd 2005 2004  Good and bad reflective materials Objects that reflect light well (regular reflection):  have smooth, shiny surfaces and are usually pale colours;  give clear images;  mirrors are excellent reflectors. Objects that do not reflect light well (diffuse reflection):  have rough, matt surfaces and are usually dark colours (unless it has been polished);  give no or diffuse images. 1 ofof20 51 84 © Boardworks Ltd 2005 2004  Anyone wants an invisible cloak? Invisible technology https://www.youtube.com/watch? v=TJvGOI263po 1 ofof20 53 84 © Boardworks Ltd 2005 2004  Contents Light What is light? Reflection Refraction Colour 1 ofof20 54 84 © Boardworks Ltd 2005 2004  Objectives At the end of today’s lesson, you will learn: 1. the meaning of refraction of light. 2. how to draw and label ray diagram to show how a light ray is refracted. 3. the change of direction of the light ray when it enters or leaves the glass block / prism. 1 ofof20 55 84 © Boardworks Ltd 2005 2004  Refraction of light The straw seems bent. 1 ofof20 56 84 © Boardworks Ltd 2005 2004  Refraction of light refraction of light 1 ofof20 57 84 © Boardworks Ltd 2005 2004  Refraction – slowing of light 1 ofof20 58 84 © Boardworks Ltd 2005 2004  Refraction of light Incident ray: travelling towards the boundary air glass 1 ofof20 59 84 © Boardworks Ltd 2005 2004  Refraction of light Refracted ray: travelling away from the boundary air glass 1 ofof20 60 84 © Boardworks Ltd 2005 2004  Refraction of light Normal: imaginary straight line perpendicular to the boundary air glass 1 ofof20 61 84 © Boardworks Ltd 2005 2004  Refraction of light Angle of incidence: angle between the incident ray and the normal air glass 1 ofof20 62 84 © Boardworks Ltd 2005 2004  Refraction of light Angle of refraction: angle between the refracted ray and the normal air glass 1 ofof20 63 84 © Boardworks Ltd 2005 2004  Refraction of light Refraction of light occurs medium 1 medium 2 1 ofof20 64 84 © Boardworks Ltd 2005 2004  Activity 3 Bending of light in glass Bending of light when passing through a rectangular glass block 1 ofof20 65 84 © Boardworks Ltd 2005 2004  Activity 3 pencil glass block A4 paper 1 ofof20 66 84 © Boardworks Ltd 2005 2004  Activity 3 What happens to the light ray? light ray ray box 1 ofof20 67 84 © Boardworks Ltd 2005 2004  Activity 3 1 The ray splits into two parts when it hits the glass block. reflected by the glass surface. One part is __________ The other part enters the glass block with a change in direction _________. Then it leaves the glass block on the other side with a direction again. change in __________ 1 ofof20 68 84 © Boardworks Ltd 2005 2004  Activity 3 Mark all the paths of the light ray. ray box 1 ofof20 69 84 © Boardworks Ltd 2005 2004  Activity 3 Complete the path of the light inside the glass 1 ofof20 70 84 © Boardworks Ltd 2005 2004  Activity 3 normal incident ray normal 1 ofof20 71 84 © Boardworks Ltd 2005 2004  Activity 3 b ∠a = ____ a ∠b = ____ ∠c = ____ P c 1 ofof20 72 84 © Boardworks Ltd 2005 2004  Activity 3 ∠d = ____ ∠e = ____ d Q e 1 ofof20 73 84 © Boardworks Ltd 2005 2004  Activity 3 incident ray 0° ∠a = ____ P ∠c = ____ 0° When the angle of incidence is 0 the light ray is not deviated ______________ from its path. undeviated light ray 1 ofof20 74 84 © Boardworks Ltd 2005 2004  Activity 3 Discussion 1 Do the laws of reflection apply when the incident ray is reflected by the glass surface? Yes. 2 Does the ray bend towards or away from the normal when it enters the glass block? Towards the normal. 3 Does the ray bend towards or away from the normal when it leaves the glass block? Away from the normal. 4 Does the ray bend away from the normal when it enters the glass block along the normal? No. 1 ofof20 75 84 © Boardworks Ltd 2005 2004  Activity 3 Conclusion When light enters a glass block from air, it bends towards ____________ the normal; when light leaves the glass away from the normal. block into air, it bends ____________ Light does not bend if it enters the glass block along the ____________. normal 1 ofof20 76 84 © Boardworks Ltd 2005 2004  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 ofof20 77 84 © Boardworks Ltd 2005 2004  Refraction of light … from air to glass normal air bends towards glass the normal 1 ofof20 78 84 © Boardworks Ltd 2005 2004  Refraction of light … from glass to air normal bends away glass from the normal air 1 ofof20 79 84 © Boardworks Ltd 2005 2004  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 ofof20 80 84 © Boardworks Ltd 2005 2004  Refraction phenomena Let’s learn about some refraction phenomena. 1 ofof20 81 84 © Boardworks Ltd 2005 2004 1 ofof20 82 84 © Boardworks Ltd 2005 2004  5 Complete the ray diagram. 1 ofof20 83 84 © Boardworks Ltd 2005 2004  7 Complete the ray diagram. 1 ofof20 84 84 © Boardworks Ltd 2005 2004  Refraction phenomena When an object is placed under water or glass … seems to be nearer to the surface no water water added 1 ofof20 85 84 © Boardworks Ltd 2005 2004  Refraction phenomena seems to be nearer to the surface 1 ofof20 86 84 © Boardworks Ltd 2005 2004  Refraction phenomena Similarly, at a depth deeper than we expect 1 ofof20 87 84 © Boardworks Ltd 2005 2004  Refraction phenomena Many daily phenomena, such as why a chopstick looks bent in water and objects look closer to the surface of water, result from the refraction of light. Your brain thinks that light travels in straight lines; it doesn’t realise that the rays of light may have been bent before they reach your eyes. refracted 1 ofof20 88 84 © Boardworks Ltd 2005 2004  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 ofof20 89 84 © Boardworks Ltd 2005 2004  Contents Light What is light? Reflection Refraction Colour 1 ofof20 90 84 © Boardworks Ltd 2005 2004  Objectives At the end of today’s lesson, you will be able to: - To explain the process dispersion and the production of a light spectrum - To explain how a rainbow is formed - To state the three primary colours of light 1 ofof20 91 84 © Boardworks Ltd 2005 2004  White light and colours Why can we see so many colours? 1 ofof20 92 84 © Boardworks Ltd 2005 2004  94 Dispersion of white light What is the composition of white light? 1 ofof20 94 84 © Boardworks Ltd 2005 2004  Splitting white light into colours A prism splits a ray of white light into a spectrum of colours. This is known as dispersion. a narrow beam of white light prism constituent colours of white light When white light is split, the colours always follow the same order. Richard Of York Gave Battle In Vain 1 ofof20 96 84 © Boardworks Ltd 2005 2004  Dispersion of white light Different coloured lights bend by different amounts Bends and slows down the least ight red ite l orange wh yellow green blue indigo violet prism Bends and slows down the most 1 ofof20 97 84 © Boardworks Ltd 2005 2004  Dispersion of white light colour spectrum ight red ite l orange wh yellow green blue indigo violet prism 1 ofof20 98 84 © Boardworks Ltd 2005 2004  Recombining colours Remember how white light can be dispersed to give a spectrum of colours? To do the opposite – two prisms are needed! 1 ofof20 99 84 © Boardworks Ltd 2005 2004  Dispersion of white light White light is a mixture of coloured lights. splits white light into the colour spectrum. A prism ________ due to dispersion of sunlight Rainbows 1 of of 100 2084 © Boardworks Ltd 2005 2004  Rainbows After raining … 1 of of 101 2084 © Boardworks Ltd 2005 2004  Rainbows water droplets in the air sunlight 1 of of 102 2084 © Boardworks Ltd 2005 2004  Rainbows Only one colour from each droplet reaches our eye re d violet 1 of of 103 2084 © Boardworks Ltd 2005 2004  Rainbows also appear near fountains Rainbows are formed by the dispersion of white light by water droplets in the air. 1 of of 104 2084 © Boardworks Ltd 2005 2004  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 of 105 2084 © Boardworks Ltd 2005 2004  Primary and secondary colours Colours are made by mixing other colours of light. Primary colours can combined to form secondary colours. This is called colour addition. The three red green primary colours of light are red, green and blue. magenta blue The colours made by colour addition of two primary colours are secondary the called colours – magenta, yellow and cyan. 1 of of 106 2084 © Boardworks Ltd 2005 2004  Which primary colours? 1 of of 107 2084 © Boardworks Ltd 2005 2004  Three primary colours of light red blue green mixed in suitable proportions different colours can be seen 1 of of 108 2084 © Boardworks Ltd 2005 2004 Colours of objects The world we live in is so colourful! 1 of of 109 2084 © Boardworks Ltd 2005 2004  Colours of objects in white light The car appears red. reflects white light red light absorbs all other coloured lights 1 of of 111 2084 © Boardworks Ltd 2005 2004  Colours of objects in white light Green shoes green light white light absorbs all other coloured lights 1 of of 112 2084 © Boardworks Ltd 2005 2004  Colours of objects in white light Blue mug white light blue light absorbs all other coloured lights 1 of of 113 2084 © Boardworks Ltd 2005 2004  Colours of objects in white light Black cat white light absorbs almost all coloured lights 1 of of 114 2084 © Boardworks Ltd 2005 2004  Colours of objects in white light White mouse white light white light 1 of of 115 2084 © Boardworks Ltd 2005 2004  Which colour is reflected? Which colours of light are reflected by these clothes? 1 of of 116 2084 © Boardworks Ltd 2005 2004  Seeing different colours What colours are absorbed by this frog’s skin? What colours are reflected into your eyes? This part of the skin This part of the skin absorbs all the colours of absorbs all the colours the spectrum except red, of the spectrum and and so reflects red light. none are reflected. 1 of of 117 2084 © Boardworks Ltd 2005 2004  Seeing different colours What colours are absorbed by this flower? What colours are reflected into your eyes? This part of the flower absorbs This part of the flower all colours except red and green. absorbs no colours. It reflects red and green light, It reflects them all and so appears yellow. and so appears white. 1 of of 118 2084 © Boardworks Ltd 2005 2004  Colours of objects in white light The colour of an object is determined by the colours of light reflected by the object. What happens if the light shining on an object is coloured instead of white? How to make coloured light? 1 of of 119 2084 © Boardworks Ltd 2005 2004  Using coloured filters Coloured Filters let certain colours of light pass through, but absorb all other colours. This is called subtraction. Using different coloured filters placed in front of your eye, look around the classroom and see what effect they have on your vision. object filter 1 of of 120 2084 © Boardworks Ltd 2005 2004  Colour filters white light Red filter absorbs other coloured lights red light 1 of of 121 2084 © Boardworks Ltd 2005 2004  Colour filters white light Blue filter absorbs other coloured lights blue light 1 of of 122 2084 © Boardworks Ltd 2005 2004  Colour filters white light Green filter absorbs other coloured lights green light 1 of of 123 2084 © Boardworks Ltd 2005 2004  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 of 124 2084 © Boardworks Ltd 2005 2004  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 of 125 2084 © Boardworks Ltd 2005 2004  Colours of objects in coloured lights What colour can you see? green green light light white object 1 of of 126 2084 © Boardworks Ltd 2005 2004  Colours of objects in coloured lights What colour can you see? green green light light green object 1 of of 127 2084 © Boardworks Ltd 2005 2004  Colours of objects in coloured lights What colour can you see? green reflects light no light red object 1 of of 128 2084 © Boardworks Ltd 2005 2004  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. The red ball reflects the red light and so appears red. 1 of of 129 2084 © Boardworks Ltd 2005 2004  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 of 130 2084 © Boardworks Ltd 2005 2004  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 of 131 2084 © Boardworks Ltd 2005 2004  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 The red ball only reflects red and blue red light and so absorbs light, shines the blue light. on the ball. So in magenta light, this ball reflects the red light and appears red. 1 of of 132 2084 © Boardworks Ltd 2005 2004  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 of 133 2084 © Boardworks Ltd 2005 2004  What colour does it appear? What colour does each object appear under the given lighting conditions? Object (Colour) Colour Filter Appearance red ball red red red ball blue black blue book green black blue book magenta blue green apple cyan green green apple magenta black red and blue tie red red and black 1 of of 134 2084 © Boardworks Ltd 2005 2004