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KS3 Physics

KS3 Physics

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KS3 Physics

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  1. KS3 Physics 8K Light

  2. Contents 8K Light What is light? Reflection Refraction Colour Summary activities

  3. 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.

  4. 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.

  5. 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. Shading chart 100% 2.5 % 75 % 5 % 50 % 10 % 30 % 15 % 25 % 20 % 0%

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. Contents 8K Light Whatis light? Reflection Refraction Colour Summary activities

  11. 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.

  12. Good and bad reflective materials best reflectors worst reflectors Arrange these items along the arrow: yellow banana red roses tarmac road blue car white paper polished black shoes tangerine green leaf aluminium foil tree bark

  13. 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.

  14. 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.

  15. 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:

  16. Reading in mirrors – words dog man ball bat bike ants park fins pink litter sandy shark No Cheating

  17. Reading in mirrors – results 1. Who read the words in the quickest time? 2. Plot a bar chart of your results: 3. What was the average time taken in your group?

  18. 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? Physics Physics When something is reflected in a plane mirror, left becomes right and right becomes left. This is calledlateral inversion.

  19. How far away is the image? I 1. Fix a plane mirror along the centre of a piece of A4 paper and draw around it. Place a pin as the object in front of the mirror. • 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. 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?

  20. The maths of reflection angle i angle r 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. Repeat for another five angles of incidence.

  21. 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.

  22. 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!

  23. 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

  24. 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)

  25. Contents 8K Light What is light? Reflection Refraction Colour Summary activities

  26. Bending light air glass 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 orrefract.

  27. Refraction at the air-glass boundary

  28. Refraction investigation angle i angle r 1. Place a rectangular glass block on a sheet of paper and draw around it. 2. Draw a normal line (at 90º) along the top surface of the block. • 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.

  29. Refraction investigation – results Record the results of the refraction investigation in a table: • 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.

  30. What happens in refraction: air to glass air glass When light is refracted as it travels from air to glass: angle of incidence > angle of refraction  i >  r 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.

  31. What happens in refraction: glass to air glass 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. i <r If the two surfaces of the block are parallel, then the ray at the start is parallel to the ray at the end.

  32. Refraction – angle of incidence = 0° air glass 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. undeviated light ray

  33. Refraction in a rectangular block

  34. 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

  35. 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.

  36. The speed of light in different media Light travels at 300,000 km/s in a vacuum. As light enters denser media, the speed of light decreases. From this bar chart, which material do you think is denser, Perspex or water? Perspex must be denser than water because light travels more slowly through Perspex than water.

  37. Why does light change direction? road tyre 2 tyre 1 mud 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.

  38. 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.

  39. Why does light change direction? • When light hits a medium at an angle to the n_____ the light ‘bends’ in a similar way to that described for the car in a muddy field. ormal • Part of the light ray s____ d____ before the rest and this causes the change of d_______. lows own 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___. ime ame

  40. 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?

  41. Apparent depth image actual location 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.

  42. The Archer fish image of prey prey location 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. The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

  43. 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?

  44. Refraction summary 1. When light bends this is calledrefraction. 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

  45. Contents 8K Light What is light? Reflection Refraction Colour Summary activities

  46. 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.

  47. Splitting white light with a prism • Shine a ray of bright white light at a prism and move the prism until colours appear. • Draw a diagram to show what you observed.

  48. Splitting white light animation

  49. Splitting white light into colours RichardOfYorkGaveBattleInVain Y A prism splits a ray of white light into a spectrum of colours. This is known asdispersion. When white light is split, the colours always follow the same order. Use this phrase to remember the order of colours:

  50. Dispersion Each of the colours of the spectrum [ROYGBIV] has a slightly different wave. What is the difference? Each colour has a different wavelength ().