KS3 Physics - Light - Lessons 1-2 - PDF

- KS3 PHYSICS - Light Lessons 1-2 Travelling Light 3 MINUTES Luminous Or Not? Sort the objects into two lists. Click to start timer… Barcode Scanner Lens LUMINOUS NON-LUMINOUS Camera Film L.E.D. Barcode Scanner Camera Film C...

- KS3 PHYSICS - Light Lessons 1-2 Travelling Light 3 MINUTES Luminous Or Not? Sort the objects into two lists. Click to start timer… Barcode Scanner Lens LUMINOUS NON-LUMINOUS Camera Film L.E.D. Barcode Scanner Camera Film Carrot Light Bulb Fire Carrot Chair Mirror Glow Worm Chair Compact Disc Moon Laser Compact Disc Electric Fan Saturn’s Rings L.E.D. Electric Fan Fire Star Light Bulb Human Glow Worm Sun Star Lens Human Torch Sun Mirror Laser TV Screen Torch Moon TV Screen Saturn’s Rings A luminous object transfers one form of energy in order to emit light energy; a non-luminous object cannot give off its own light energy. Lesson 1-2 – Learning Objectives Using examples, explain the difference between luminous and non-luminous objects. Explain, with the aid of a diagram, why we are able to see luminous and non-luminous objects. Using examples, explain the difference between transparent, translucent and opaque objects. State that light rays always travel in straight lines. Explain the principles of operation of a pinhole camera, describing how the nature of the image may be varied. Compare the similarities and differences between the eye and the camera.  Luminous Objects How can we see non-luminous objects if they don’t give off their own light? We can see non-luminous objects with the aid of another light source. Light leaves the source and reflects off the object’s surface, then into our eyes. What do those symbols in the diagram mean? When drawing ray diagrams, we use standard symbols to represent the apparatus to improve the clarity of our work. Light Light Ray Source Triple Single Slit Eye Slit Comb (observer’s Mirror Glass Block, position) (stripes Lens, etc on back) (may be shaded)  Different Surfaces Does light always reflect off a surface? Opaque surfaces do not allow light to pass through – they either reflect or absorb any energy. This depends specifically on the surface – we will learn more about this in the next few lessons. Transparent surfaces allow all light to be transmitted. Translucent surfaces absorb some energy and allow some to pass through.  Travelling Light Light rays undergo rectilinear propagation – this means that light always travels in straight lines. Whenever you draw a ray diagram, you MUST use a ruler, draw continuous lines (not dotted lines), and add arrows to show the direction the light travels in. Shadows are created when light is unable to pass through an opaque object, but some light can pass around it. This is what happens during an eclipse – light from the Sun cannot pass through the Moon, therefore a shadow is created on the Earth’s surface. Task Complete p37 Q1-5. If you have time, continue to work through the remaining questions.  Eyes and Cameras Light rays reflect off different parts of the object and enters the pupil, projecting the image onto the retina at the back of our eye. Cameras work like our eyes – the light enters the aperture at the front, and is projected onto the film at the back.  The Pinhole Camera As light travels in straight lines, they only way they can enter the camera’s aperture is if they cross over at this point. The rays continue through the camera, forming an image on the screen. We call the images REAL, because: - They are inverted - They are smaller than the object - They appear closer to the pinhole than the object. If one of these conditions is broken, we say that the image is VIRTUAL. The Pinhole Camera You will now make a simple pinhole camera, then complete some experiments to investigate how the image changes when you make some adjustments. Complete each task in turn, then (with the aid of completing the ray diagram on the sheet) explain what you observed. The ‘arrow’ represents the object being observed (in your case, the lamp filament.  The Pinhole Camera What did you observe? Turning the camera upside-down makes no difference – the hole is still a hole! Moving closer makes the image larger; moving away makes the image smaller.  The Pinhole Camera What did you observe? Making more holes creates one image for each hole. Turning the camera upside-down turns the pattern upside-down, but the individual images do not change. Making the holes bigger allows more light in, so the image becomes blurred.  Lesson 2 - Summary Copy this paragraph, using the words below to fill gaps... Light must enter our eyes so that we can see. The light comes from _______________ such as the ____, light bulbs, ________ and camera flashes. It travels at very high ______. Light can shine through ____________ materials such as the front of out eyes or the lens on the front of a camera. The light that reaches the ____ in a camera makes an ________ image – the photograph. candles film inverted sources of light speed Sun transparent Lesson 2 – Extension Write a short report comparing the similarities and differences between a camera and your eye. You should now be able to… Using examples, explain the difference between luminous and non-luminous objects. Explain, with the aid of a diagram, why we are able to see luminous and non-luminous objects. Using examples, explain the difference between transparent, translucent and opaque objects. State that light rays always travel in straight lines. Explain the principles of operation of a pinhole camera, describing how the nature of the image may be varied. Compare the similarities and differences between the eye and the camera.  - KS3 PHYSICS - Light Lessons 3-4 Coloured Light and Coloured Filters 3 MINUTES Dispersing Colour Write down everything you know about rainbows. Click to start timer… Lesson 3-4 – Learning Objectives Describe how white light is made of a mixture of colours, and demonstrate how to separate the colours using a prism. Explain how we see coloured objects because they reflect light of the same colour and absorb light of all colours. Explain why some objects look different when coloured light is shone on them. Describe how filters can be used to change the colour of light.  Dispersing Light Rainbows are formed when sunlight shines through a raindrop – as the light refracts, it is split into into the colours of the spectrum. The process of splitting white light in this way is called dispersion.  Dispersing Light In the lab, we can use a triangular block of glass called a prism to split white light. In small groups, set up the apparatus provided to produce a spectrum. When successful, draw the ray diagram, and note the order of the colours.  Dispersing Light Remember... RED RICHARD ORANGE OF YELLOW YORK GREEN GAINED BLUE BATTLE INDIGO IN VIOLET VAIN Notice that the light refracts (bends) on entering and leaving the glass – red refracts the least, violet refracts the most. This demonstration shows us that white light is made up of different colours of light, which travel at different speeds through glass.  Seeing Colour Red surfaces will reflect only red light into our eyes (that’s why we see red); all other colours are absorbed. Green and blue objects behave similarly. Red, blue and green (NOT yellow!) are the three primary colours of light.  But why can we see Seeing Colour other colours? Mixing two primary colours in equal quantities gives us the secondary colours, magenta, yellow and cyan. Mixing the colours in different amounts gives us other colours. Mixing all three equally give us white light. We see black when no light is present. Seeing Colour What will happen in these situations? Objects in other colours reflect a combination of colours. White objects will reflect all colours of light. Black objects absorb all light, and reflect none. The absorbed energy may be converted to other forms, such as heat.  Seeing Colour What will happen in these situations? If only red light is present, a white object can only reflect red light, so it appears red. If the only light present matches the object’s colour, there is no change......however if the light does not match the object’s colour, no light is reflected, so the object appears black. Tasks Complete p48 Q2-4. Write your own summary of the rules for observing different coloured objects under different coloured lights. ◦ Use as few words as possible - you could even do this as a poem. 5 MINUTES Coloured Balloons Complete the ‘Coloured Balloons’ sheet, by stating how each balloon would appear under different coloured lights at a party. Click to start timer… Coloured Filters Imagine we have a white light source, but only need light of one particular colour... A light filter will only allow one colour of light to pass through (ie, to be transmitted). The filter will absorb all other colours of light. Filters are commonly used in theatres and photography to obtain different lighting effects.  Coloured Filters A red filter will only transmit red light; all others are absorbed. (Green and blue filters will work in a similar way). If a red object is viewed through a red filter, it will appear......red. If a white object is viewed through a red filter, it will appear......red. If a green object is viewed through a red filter, it will appear......black.  What will happen in these Coloured Filters situations? Yellow light is made up of red and green light – here, the red light is absorbed, and the green light is transmitted. If the filter is a secondary colour, both of its constituent primary colours will be transmitted; the third primary colour is absorbed. Here, only the green light is transmitted through the first filter, but it is absorbed by the second filter – no light is transmitted, so we see black.  Coloured Filters Copy and complete these sentences... 1. A light filter is used for... 2. Two places you may find light filters are... 3. A filter will ________ light of the same colour (allow it to pass), but ______ all other colours. For example, a green filter will transmit ____ light and will absorb... 4. Placing two filters together will... because... Task Look at how the coloured pictures change their appearance through the coloured filters – can you predict what they are going to look like? Colour what you think you would see on your blank sheet.  Task - Review Using the red filter......red areas remain red....white and yellow areas now appear red....all other areas now appear black.  Task - Review Using the green filter......green areas remain green....white and yellow areas now appear green....all other areas now appear black.  Task - Review Using the blue filter......blue areas remain blue....white areas now appear blue....all other areas now appear black.  Task - Review Using any two different filters......all of the images now appear entirely black. Why? Lesson 4 – Extension Complete the ‘What Colour?’ sheet. You should now be able to… Describe how white light is made of a mixture of colours, and demonstrate how to separate the colours using a prism. Explain how we see coloured objects because they reflect light of the same colour and absorb light of all colours. Explain why some objects look different when coloured light is shone on them. Describe how filters can be used to change the colour of light.  - KS3 PHYSICS - Light Lessons 5-6 The Law Of Reflection 3 MINUTES Seeing A Reflection When light is reflected in a mirror, we see our image, but when light is reflected off the surface of other objects, we don’t – explain why. Click to start timer… Lesson 5-6 – Learning Objectives Label a diagram showing light being reflected from a plane mirror, showing the normal line and angles of incidence and reflection. Recall and use the Law Of Reflection. Describe how plane mirrors may be used.  Seeing A Reflection Mirrors (also polished glass, still water, etc) have exceptionally smooth surfaces, so reflect light directly to us – this is known as specular or regular reflection. Other surfaces have a rough texture, so will scatter the light randomly. Some light is still reflected towards us (that’s how we see the object) – this is diffuse reflection. rough surface Experiment Time! CHALLENGE TASK ‘Law Of Reflection’ ◦ You will complete the experiment with a partner, but you must present your own work. ◦ You will be assessed on your ability to follow the instructions, take measurements, present data in a table and graph, draw conclusions and evaluate your work. ◦ READ THE SHEET CAREFULLY, AND ENSURE YOU MEET THE CRITERIA TO REACH YOUR TARGET GRADE! The Law Of Reflection normal line l e of an in ang ence ref gle o cid id lec f en inc tio n ra y d t ra y i r re f lecte mirror ANGLE OF INCIDENCE = ANGLE OF REFLECTION i=r Lesson 6 – Summary Complete p40 Q4, then Q2. For homework, ensure that all parts of the Challenge Task have been completed. You should now be able to… Label a diagram showing light being reflected from a plane mirror, showing the normal line and angles of incidence and reflection. Recall and use the Law Of Reflection. Describe how plane mirrors may be used.  - KS3 PHYSICS - Light Lessons 7-8 The Properties Of A Mirror Image 5 MINUTES Mirror Images Re-write the quotation on your card, as you would expect to see it in a mirror. Click to start timer… Mirror Images Now face your partner. One of you will be the ‘dummy’, the other will be their mirror image. The ‘dummy’ must perform some simple instructions – the ‘image’ must perform them correctly at the same time. Lesson 7-8 – Learning Objectives Recall and use the Law Of Reflection. Describe the properties of the image in a mirror. Show why a mirror image is laterally inverted. Describe how plane and curved mirrors may be used.  Mirror Images The image we see in a plane mirror is laterally inverted (flipped sideways); the image appears to be the same size as the object and appears to be as far behind the mirror as the image is in front. Therefore, we classify the image as a virtual image. Remember – real images must be fully inverted (upside-down as well), and appear closer to the mirror and smaller then the object. But how can we prove where the mirror image is located?  Locating The Reflected Image 1. Draw two light rays from the Object Image object, correctly reflecting off the mirror. You can add normal lines and use a protractor to ensure that the angles are correct. 2. Trace the reflected rays back behind the mirror (these are the virtual rays). 3. The image is located at the point where the virtual rays cross. If you have completed the diagram accurately, the image should be opposite the object, and the same distance from the mirror.  Real and Virtual Images Object Image Mirror images can also be described as being virtual because: - the image is formed using virtual rays – it is formed in a location that light rays do not actually reach (they can’t actually pass through from the back of the mirror); - light rays appear to come from the image but doesn’t actually pass through it. Tasks Complete p40 Q1,5. Complete the ‘Mirror Mysteries’ sheet. ◦ Ensure that diagrams are drawn as carefully as possible.  Curved Mirrors Look at your phrase again, using a curved mirror – what do you notice? Does the image appear the same when the mirror is held at different distances? Now watch what happens when three parallel light rays are directed at a curved mirror. CONVEX MIRROR CONCAVE MIRROR  Convex Mirrors Convex mirrors curve outwards. This type of mirror can be found in shops or at junctions for safety and security purposes. Convex mirrors spread the reflected rays further apart – this is called F divergence. The rays seem to come from a focal point behind the mirror. Convex mirrors always magnify the If the mirror is more curved, the image, therefore only produce focal point gets closer to the virtual images. mirror, and there will be more divergence.  Concave Mirrors Concave mirrors curve inwards. This type of mirror can be found in some types of telescope, solar furnaces and are used for shaving or applying make-up. Concave mirrors bring the rays closer together to focus on a point F – this is called convergence. If the mirror is more curved, the focal point gets closer to the Concave mirrors can form both mirror, and there will be more real and virtual images, convergence. depending on whether the observer is in relation to the focal point. Lesson 8 – Summary Complete the ‘Reflecting On Reflection’ sheet. ◦ Make as many sentences as possible using the keywords. EXTENSION – Find out more about the real-world uses of concave and convex mirrors. You should now be able to… Recall and use the Law Of Reflection. Describe the properties of the image in a mirror. Show why a mirror image is laterally inverted. Describe how plane and curved mirrors may be used. - KS3 PHYSICS - Light Lessons 9-10 Refraction Bending Light With Water Place a straw into a half-filled beaker of water – what do you notice? Now place a coin under the beaker of water – what do you notice? Bending Light With Water The straw appears to be ‘broken’ at the surface, and the coin is no longer visible (when viewed from the side). These effects occur because light travels at different speeds in different materials (air, glass, water), therefore the image we see is distorted. This concept is known as refraction. Lesson 9-10 – Learning Objectives Describe what refraction is and what causes it. Label a diagram of light being refracted, including the normal line and angles of incidence and refraction. Compare the sizes of the angles of incidence and refraction. Define ‘refractive index’, and explain what happens during refraction in terms of the optical density of the two media. Discuss some of the problems and applications of refracting light. Bending Light Follow the instructions on the worksheet to complete an investigation into refraction. Ensure that each partner has the construction diagram in their book, and your work is presented appropriately. What do you notice about the angles made by each ray as it enters and leaves the block?  This diagram shows the expected Bending Light results. Angle decreases on entering glass (i > r) inc ide nt ra i Angle increases on leaving y air glass (x < y) glass r Angles in air are equal (i = y) ref x r act Angles in glass are equal (r ed = x) ray em er ge Incident and emergent rays are y nt ra y parallel  Refraction occurs when light passes from Refraction one substance into another. AIR GLASS When light passes from air into glass, the light slows down (since glass is more ‘optically r dense’) – this causes the light to bend towards the normal line. i GLASS AIR r When the light leaves the glass back to the air, it gets faster, causing the light to bend away from i the normal line. Each substance has a refractive index – a higher refractive index means that the material is more optically dense, the light changes its speed and angle to a greater extent.  Refraction Refraction can be compared to a car driving between a tarmac road and a sandy beach… Both wheels of the red car hit the sand at the same time, so When returning there is no to tarmac, this redirection (if light wheel speeds enters the glass at up, causing the right angles to the car to turn surface, no back. refraction occurs. When driving onto the beach, this wheel slows down as it hits the sand, but the opposite wheel still travels faster, therefore the car turns until both wheels are in the sand.  Effects Of Refraction Refraction is the reason why swimming pools always appear shallower than they really are – an object at the bottom of the pool appears closer to the surface… ay d R cte fr a R e Apparent Depth Real Depth ay Image nt R of Lamp ide Inc Lamp at bottom of pool  Effects Of Refraction …or why the fisherman cannot locate the exact position of the fish… …or why a ninja hunting their victim from under-water must aim their blow-pipe lower than expected. Lesson 10 – Summary Complete p44 Q2,6. You should now be able to… Describe what refraction is and what causes it. Label a diagram of light being refracted, including the normal line and angles of incidence and refraction. Compare the sizes of the angles of incidence and refraction. Define ‘refractive index’, and explain what happens during refraction in terms of the optical density of the two media. Discuss some of the problems and applications of refracting light.  - KS3 PHYSICS - Light Lessons 11-12 Total Internal Reflection Lesson 11-12 – Learning Objectives Describe what refraction is and what causes it. Measure the critical angle of a glass or Perspex block. Describe what total internal reflection (TIR) is, and explain some of the uses of TIR. Describe how TIR occurs in an optical fibre, and list some uses of optical fibres. Reflecting Within A Block Use the equipment provided to make a light ray reflect of one of the semi-circular block’s surfaces like a mirror. This is what you should see. ◦ Try to make the angle as small as possible. ◦ Notice that refraction does not occur as the light enters / leaves through the curved surface, as the ray meets the surface at right angles. Draw this apparatus, and measure the size of the angle shown.  Total Internal Reflection r When leaving glass, as the angle of incidence increases, the angle of refraction, gets even larger, but this can i only happen up to a point. If the angle of incidence is too big, no refraction occurs – instead, we observe total internal reflection (TIR): the light reflects within the block, and the Law Of cc Reflection applies instead. The critical angle is the minimum angle at which TIR will occur (and refraction no longer occurs) – the larger the refractive index of the substance, the smaller the critical angle.  Using TIR Watch the demonstrations... As long as the critical angle is exceeded when the ray reaches the interface between materials, TIR will occur. TIR can occur more than once in the same piece of glass – a bike reflector is made up of many units like this one (cats’ eyes in the road work in a similar way). Tasks Complete the ray diagrams on the ‘Refraction and TIR’ sheet. Complete p44 Q4,7 (and Q3, if you have time).  Optical Fibres TIR also explains how optical fibres work. These are very thin, flexible, transparent fibres that light can pass through by TIR. Refraction occurs as light enters the fibre (towards normal) Refracts away from Law Of Reflection is obeyed each time normal on leaving light reaches a boundary fibre  Using Optical Fibres As well as decorative items, optical fibres are mostly used for communications – light (visible or infrared) flashes on and off in binary code to transfer data quickly. Usually, each ‘bundle’ contains many fibres, so more data can be sent at the same time. An endoscope is a flexible camera that can be inserted into the body to take images from within. Some of the fibres illuminate the area, while others carry the signal back to the computer; small tools can also be attached for simple procedures. Lesson 12 - Summary Summarise the benefits and drawbacks of using optical fibres, and discuss their importance in our daily lives. Lesson 12 – Homework Revise for the End-Of-Topic Test. You should now be able to… Describe what refraction is and what causes it. Measure the critical angle of a glass or Perspex block. Describe what total internal reflection (TIR) is, and explain some of the uses of TIR. Describe how TIR occurs in an optical fibre, and list some uses of optical fibres.