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LIGHT

LIGHT. Write down everything you know about light!. INTRODUCTION. We step outside on a sunny day and our surroundings are awash with every colour of the spectrum. How does the energy known as light produce such spectacular effects?

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LIGHT

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  1. LIGHT Write down everything you know about light!

  2. INTRODUCTION • We step outside on a sunny day and our surroundings are awash with every colour of the spectrum. How does the energy known as light produce such spectacular effects? • The answer is that white light, radiated from the sun, is perhaps not really white at all! White light is every colour radiating together, and it is only when this light is split that we see the dazzling array of colour that we know

  3. What are the colours that make up white light? • White light = • ROY G BIV

  4. HEADSTART BOOKLET • Successful completion of this booklet will allow you to: • Explain the refraction of light in various situations • Compare the effects of concave and convex lenses on light and explain how they form images • Describe how white light can be split into its constituent colours • Explain the interaction of white light with coloured objects and describe how coloured filters are used

  5. HEADSTART BOOKLET • Bring your booklet to every class • It will need to be submitted at the end of Headstart for assessment • Must choose ONE homework activity from the end to complete before the book is submitted for assessment • Your booklet is NOT for taking notes – it will be used for an assessment at the end of the unit. Your exercise book must be used for notes

  6. What do you notice about the pencil in the water? Can you explain what is happening?

  7. Put a coin in a polystyrene cup and move back until the coin is just out of sight. Stand in this position while a friend pours water into the cup. • What happens? • Draw a diagram to explain what has happened.

  8. REFRACTION OF LIGHT • As light moves from one medium to another, its speed changes • This causes the light wave to change direction, or bend (except when light hits a surface at right angles) Ray of light: the incident ray Air Water Where light would go if travelling in a straight line Where light really goes: the refracted ray

  9. REFRACTION VS. REFLECTION • Refraction and reflection animations

  10. REFRACTION OF LIGHT • To investigate the bending of light as it moves from one medium to another, we use a reference line called the normal • The normal is an imaginary line that is at right anglesto the boundary between the two materials Normal Air Boundary Water

  11. ANGLE OF REFRACTION • Angle between the normal and the path of the refracted light Normal Air Incident ray Glass Refracted ray Angle of refraction

  12. ANGLE OF INCIDENCE • Angle between the normal and the ray of light approaching a surface Normal Air Incident ray Glass Angle of incidence

  13. CRITICAL ANGLE • The angle at which there is total reflection of light as it hits a surface – called total internal reflection • i.e. There is NO refracted ray of light Critical angle Incident ray Reflected ray Air Glass Normal

  14. PRAC • Complete the ‘Demonstrating Refraction’ Prac

  15. REFRACTION OF LIGHT • The direction that light bends as it moves from one medium to another depends on the density of each material • How did the light bend (in relation to the normal) as it entered the glass block? • How did the light bend (in relation to the normal) as it left the glass block?

  16. REFRACTION OF LIGHT • When light moves from a less dense medium to a denser medium, the light slows down • This causes the light to bend towards the normal Normal Air Glass

  17. REFRACTION OF LIGHT • When light moves from a denser medium to a less dense medium, the light gets faster • This causes the light to bend away from the normal Normal Glass Air

  18. REFRACTION AND FISHING • Hunters who use spears to catch fish know about refraction. Light bends as it leaves the water so the fish always appears closer to the surface than it really is.

  19. REFRACTION AND FISHING • Where would the hunter aim to spear a fish? • Why?

  20. REFRACTION AND FISHING • Aim in front of the fish • Light from the fish comes out of the water and bends away from the normal (speed increases) • Our brain knows that light travels in straight lines. For this reason, we think that the fish is located in a straight line from our eyes to the water

  21. TOTAL INTERNAL REFLECTION • Why do you think diamonds sparkle?

  22. TOTAL INTERNAL REFLECTION • Light is refracted into the diamond, then totally internally reflected twice within the diamond, producing the sparkle

  23. TOTAL INTERNAL REFLECTION • What does total internal reflection mean? Use the following diagram to explain.

  24. LENSES • Look through a concave and then a convex lens at your classmates and at your book. • What do your friends look like through each of the lenses? • What does your book look like through each of the lenses? • Suggest some uses for each type of lens.

  25. CONVEX LENSES • Also called converging lenses because they focus parallel light rays to a point • Your eyes contain these type of lenses • Parallel rays come to a focus after passing through a convex lens

  26. PRAC • Complete ‘Investigating Convex Lenses’ Prac

  27. CONVEX LENSES • Were the images upright or inverted? • If the object is not past the focal length, the image is upright

  28. CONVEX LENSES • Were the images upright or inverted? • If the object is past the focal length, the image is inverted

  29. CONVEX LENSES • Was the image bigger or smaller? • Images may be bigger or smaller • However, images are mostly bigger than the real object

  30. CONVEX LENSES • Was the image bigger or smaller? Where would the object have to be for the image to be smaller than the object?

  31. CONVEX LENSES • Why did the plastic lens and glass lens have different focal lengths?

  32. CONVEX LENSES • What is the relationship between focal length and thickness?

  33. CONCAVE LENSES • A concave lens can be easily recognised by its thicker rim • Also called diverging lenses because they spread out parallel rays of light • Produce images that are always upright, smaller in size and virtual in nature

  34. REAL AND VIRTUAL IMAGES • Virtual images are seen on a mirror • Real images can be projected on a screen

  35. CONCAVE LENSES • Use a concave lens to try to obtain an image of your laboratory windows. • What do you find?

  36. PRAC • Complete ‘Investigating Concave Lenses’ Prac

  37. Re-Cap • What have you learnt about lenses and light so far?

  38. VISIBLE LIGHT & THE COLOUR SPECTRUM • Visible light is only one section of a wide variety of waves known as the electromagnetic spectrum • The electromagnetic spectrum also includes: UV rays, infrared rays, gamma rays

  39. VISIBLE LIGHT & THE COLOUR SPECTRUM • The electric field travels in waves – the energy alternates in positive and negative directions • Positive direction • Negative direction

  40. VISIBLE LIGHT & THE COLOUR SPECTRUM • All components of the electromagnetic spectrum consist of a changing electric field at right angles with a changing magnetic field. A field is an area in space where an introduced electrically charged/magnetic field will experience a force.

  41. VISIBLE LIGHT & THE COLOUR SPECTRUM • For example: a magnetic field is an area where if you introduce a magnet into it, the magnet will experience a force • A compass has a magnet in it that experiences a force which pushes the needle to point North

  42. PRAC • Complete the ‘Slinky’ Activity

  43. COLOUR SPECTRUM • The light that you can see is the visible spectrum • This includes red, orange, yellow, green, blue, indigo & violet • This can be abbreviated to ROY G BIV • Light wavelengths are extremely small, being less than 1,000 of a millimeter

  44. COLOUR SPECTRUM • Each colour of light has a different wavelength • Special cells called cones in the retinas of your eyes respond to different wavelengths and determine the colour you see • Colour your graph to match that on the screen

  45. COLOUR SPECTRUM • The white light that comes from the sun and light bulbs is a mixture of all the colours of the visible spectrum • It consists of mixed waves of different wavelengths and frequencies all travelling at the same speed • The frequency of a light wave refers the number of waves that pass a specific point in one second • Visible light is released from atoms when ‘excited’ electrons drop to a lower energy level

  46. COLOUR SPECTRUM • ROY G BIV – They Might Be Giants Video

  47. List the colours of the visible spectrum • Apart from their colour, what is different about the different colours in the visible spectrum? • Which colour of visible light has the greatest wavelength? • What colour of visible light has the highest frequency? • Which colours is the human eye most sensitive to?

  48. TRANSMISSION AND ABSORPTION OF LIGHT • When light hits a surface it can be reflected, refracted, transmitted or absorbed • If a material only absorbs light, no light will pass through it • If a material transmits light, the light is able to pass through it

  49. OPAQUE MATERIALS • If a material reflects or absorbs light, it is opaque • These materials do not transmit any light at all • This means that no light passes through the object and we cannot see what is on the other side • E.g. Wood, metal, mirror

  50. TRANSPARENT MATERIALS • Transparent materials transmit light • This means that they let light pass through, allowing us to see what is on the other side • E.g. Glass

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