1. Light and electromagnetic waves 4 th form IGCSE Textbook: Chapters 13 & 12

2. Students will be assessed on their ability to: 3.14 recall that light waves are transverse waves which can be reflected, refracted and diffracted 3.15 recall that the angle of incidence equals the angle of reflection 3.16 construct ray diagrams to illustrate the formation of a virtual image in a plane mirror 3.17 describe experiments to investigate the refraction of light, using rectangular blocks, semicircular blocks and triangular prisms 3.18 recall and use the relationship between refractive index, angle of incidence and angle of refraction: 3.19 describe an experiment to determine the refractive index of glass, using a glass block 3.20 describe the role of total internal reflection in transmitting information along optical fibres and in prisms 3.21 recall the meaning of critical angle c 3.22 recall and use the relationship between critical angle and refractive index:

3. Reminder: light is... Energy – Not matter... A wave – Doesn’t need a medium to travel – Travels at c in a vacuum

4. Reminder: reflection Light that is not absorbed or transmitted is reflected Angle of incidence=angle of reflection Find images by drawing ray diagrams Smooth, shiny objects give specular reflections (like mirrors) Rough ones give diffuse reflections

5. Reflection Virtual ripple tank

6. Refraction Refraction is the change in wave speed when a wave passes from one medium to another. This may be accompanied by a change in direction. Refraction + curved surfaces → lenses

7. Photo: George Silk (1962)

8. Wave slows down and bends towards the normal due to entering a more dense medium Wave speeds up and bends away from the normal due to entering a less dense medium Wave slows down but is not bent, due to entering along the normal Refraction

9. Passing into a denser medium: – Wave slows down – Bends towards normal Passing into a less dense medium: – Wave speeds up – Bends away from normal Air to glass Glass Air Light moves slowly in glass Glass to air Glass Air Light moves faster in air

10. Investigating refraction Shine light ray through a glass block and measure where it goes – Record incident angle and refracted angle Why might it be useful to use a semi-circular block? – If a ray travels along a radius, it always strikes the curved surface along the normal – Only one change of direction Air Glass Air Glass i r

11. Refraction Why the change in direction? Section of wavefront in second medium travels more slowly Also change in wavelength

12. Refraction

13. Apparent depth Refraction causes things underwater to appear to be at a shallower depth when viewed from the air – Difficult to spear fish!

14. Prisms There is refraction at each surface, as the ray enters and leaves the block The speed of light in glass depends slightly on the wavelength – Dispersion – “blue bends best” – So which travels faster in glass, red or blue light?

15. Investigation results Not a straight line... This one is... Air Glass Refractive index i r

16. Refractive Index Refractive index of a material is defined as A ratio, so no units n is never smaller than 1 – be careful! The denser a material is, the higher its refractive index – for air, n is very close to 1 Note: going from air into the material Example: A light ray is incident on a diamond of refractive index 2.4 at an angle of 60° to the normal. What angle does it travel at in the diamond, relative to the normal?

17. Snell’s Law v1tv1t v2tv2t Medium 1 Medium 2 11 22 A D C B Refraction is described by Snell’s Law (Actually first discovered by Ibn Sahl in 984 AD, 630 years earlier!)

18. Going into a less dense material Which way does the light bend? – Note there is always a weak reflection What happens as we increase i? – When r=90°, i is called the critical angle, c – If i>c, we get total internal reflection No light is transmitted, All the energy is reflected Angle of incidence = angle of reflection c Note: this only happens going from a denser to a less dense material, not the other way around ir

19. Critical angle The critical angle is the angle of incidence for which the refracted ray runs along the boundary – Angle of refraction = 90° Example: calculate the critical angle for water, n=1.33. Describe what you would see looking at a light at the bottom of a pool.

20. Uses of TIR To avoid multiple reflections sometimes associated with silvered mirrors – Reflective coating is on the back of the glass, but some light reflects from the front surface. – This can result in multiple images – Problem is avoided if we use prisms to reflect

21. Uses of TIR A prism can be used as a retro- reflector, eg for a bicycle. – Why is this better than a mirror? There are even some on the moon!on the moon (Used to measure distance from Earth)

22. Uses of TIR In binoculars – Fold the path of light to make instrument more compact – Produce a non- inverted image Would usually be upside down or L-R inverted otherwise!

23. Uses of TIR Optical fibres – A thin strand made of two types of glass: a denser core surrounded by a less dense cladding – Rays undergo multiple TIR and are guided through the fibre, even around corners A “light pipe”; very useful for communications

24. Optical fibres Different types of fibre for different jobs – For general illumination can use thick fibres – Telecoms fibre is very thin, to allow higher speed data transmission Cladding diameter is same as a human hair, and the core diameter is only 6% of that! – A bundle of fibres can give an image from inaccessible places

25. Fibrescopes

26. Rainbows The result of TIR in water droplets – Need the Sun behind you – Different wavelengths refract at different angles, leading to dispersion Rainbows are bows because each wavelength refracts at a different angle 42 o 40' 40 o 35' Sun raindrops rainbow Sun

27. Students will be assessed on their ability to: 3.14 recall that light waves are transverse waves which can be reflected, refracted and diffracted 3.15 recall that the angle of incidence equals the angle of reflection 3.16 construct ray diagrams to illustrate the formation of a virtual image in a plane mirror 3.17 describe experiments to investigate the refraction of light, using rectangular blocks, semicircular blocks and triangular prisms 3.18 recall and use the relationship between refractive index, angle of incidence and angle of refraction: 3.19 describe an experiment to determine the refractive index of glass, using a glass block 3.20 describe the role of total internal reflection in transmitting information along optical fibres and in prisms 3.21 recall the meaning of critical angle c 3.22 recall and use the relationship between critical angle and refractive index:

28. Students will be assessed on their ability to: 3.10 understand that light is part of a continuous electromagnetic spectrum which includes radio, microwave, infrared, visible, ultraviolet, x-ray and gamma ray radiations and that all these waves travel at the same speed in free space 3.11 recall the order of the electromagnetic spectrum in decreasing wavelength and increasing frequency, including the colours of the visible spectrum 3.12 recall some of the uses of electromagnetic radiations, including: radio waves: broadcasting and communications microwaves: cooking and satellite transmissions infrared: heaters and night vision equipment visible light: optical fibres and photography ultraviolet: fluorescent lamps x-rays: observing the internal structure of objects and materials and medical applications gamma rays: sterilising food and medical equipment 3.13 recall the detrimental effects of excessive exposure of the human body to electromagnetic waves, including: microwaves: internal heating of body tissue infra-red: skin burns ultraviolet: damage to surface cells and blindness gamma rays: cancer, mutation.

29. What is light? An electromagnetic wave But what’s one of those? – A periodic variation in the electric and magnetic field – No matter is involved! Magnetic field electric field Travel direction

30. There is more than we can see... Visible light is just a small portion of a whole range of different types of electromagnetic waves Can you name some of the other types? Can you say what they are useful for? What do they have in common? What are the differences between them?

31. Electromagnetic spectrum Increasing wavelength All EM waves: – are vibrations of an electric and magnetic field – Transfer energy and require no medium – Can be reflected, absorbed or transmitted (and also refracted or diffracted) – travel at the same speed in free space about 30,000,000 ms -1

32. You need to learn the spectrum radio VLF radio UHF (TV) radio (AM radio) radio VHF (radio) UV X-ray gamma rays Visible light infrared microwaves Increasing frequency/Decreasing wavelength red orange yellow green blue indigo violet Indigo was included by Newton just because he liked the number 7! Remember this beard! Richard III can be your friend There is a song...

33. Uses of EM waves Increasing wavelength First let’s look at the waves with less energy than visible light… i.e. longer wavelength

34. Radiowaves Used for communications – TV – Radio Radio waves produced by moving electrons at transmitter Induce current in receiver aerial

35. Microwaves Microwaves are high frequency radio waves Uses include: – Cooking – Communications

36. The atmosphere is a selective filter Only some wavelength can pass through it Need to choose the right frequency if you want to communicate with a satellite Microwaves are preferable to radio waves because: They spread out less They can carry data faster

37. Microwave cookers Microwaves’ varying electric field causes water and fat molecules to rotate rapidly This causes heating Microwaves cannot pass through metal – Metal case and grid in door prevent them from escaping superheating

38. Infrared radiation All objects emit infra-red radiation – The hotter they are, the more they emit – Can be used for imaging and diagnostics Also used for – heating and cooking – Communications* * Although the exam board say visible light is used with optical fibres – they are wrong!

39. Infrared imaging The colours are “artificial” but show temperature differences Which picture shows before the ball was bounced? What happened to the ball when it was bounced? Annoying woman video Fart video(L6 folder)

40. Infrared imaging Also finds military and security applications. Emergency services can use IR cameras to see through smoke.

41. Visible light It’s what we see! – Lots of uses... ?

42. Electromagnetic spectrum Increasing wavelength Now let’s look at those waves with more energy than visible light…

43. Ultraviolet Invisible! Produced in mercury vapour tubes Can cause substances to fluoresce – Convert UV to visible light

44. Fluorescent tubes Electric current excites mercury vapour in the tube Excited Mercury emits UV light UV light is absorbed by the white fluorescent coating, some of the energy is dispersed, the rest is emitted as visible light - fluorescence

45. Uses of UV Fluorescence – Eg forensics or security Curing glue Vitamin D production photolithography Sun bed (not recommended)

46. Uses of x rays X-rays are transparent to soft materials but absorbed by dense materials Used to produce shadow pictures of bones in our bodies or of objects in aircraft passengers’ luggage Looking for cracks and faults in buildings or machinery

47. Gamma Rays Shortest wavelength Highest frequency Most energy carried in each chunk of waves (quantum or photon) Go through almost anything! Stopped by several centimetres of lead or metres of concrete

48. Uses of gamma rays Medical Imaging (gamma camera) Treating cancer – radiotherapy Sterilisation (seal then irradiate,  ) – Food – Surgical instruments

49. The Gamma Camera Injecting a weak short- lived gamma source into the blood lets a gamma camera image the blood pattern in a body. Hopefully most of the gamma escapes without doing too much damage!

50. Absorbing radiation When a material absorbs electromagnetic radiation it gains energy. Absorbed radiation can: – Induce electric currents in metals e.g. radio waves absorbed by an aerial – Heat up an object, by increasing the vibration of its particles – Cause chemical changes e.g. photosynthesis or photocells in the eye – Cause ionisation, breaking up molecules or atoms. Increasing energy

51. Ionising and non-ionising radiation radio VLF radio UHF (TV) radio (AM radio) radio VHF (radio) UV X-ray gamma rays Visible light infrared microwaves Ionising Non-ionising Increasing photon energy

52. EM radiation and living things The health risk posed by radiation depends on the type and intensity of the radiation. In general, exposure to very high levels of non- ionising radiation can be harmful, but at lower doses they are relatively “safe”. Ionising radiation can damage or kill cells, cause mutations and cancer. There is no “safe” dose, but the risk increases with length and strength of exposure.

53. Effects of radiation on tissue radio VLF radio UHF (TV) radio (AM radio) radio VHF (radio) UV X-ray gamma rays Visible light infrared microwaves No detectable effect? Heating. Can be damaging – burns, chemical changes Ionising. Can be damaging – cancer, genetic mutation

54. The atmosphere is a selective filter It absorbs and so protects us from ionising radiation from space (UV, x-ray, gamma) It allows through IR, visible and a little UV, and radio waves (satellite communications)

55. Protecting tissue from radiation

56. Risks and Benefits Exposure to ionising radiation carries risks but can also have benefits

57. Risks and Benefits What are the risks and benefits of sunbathing? – Burns and skin aging – Skin cancer – Cataracts – Skin colour (?) – Vitamin D production – Makes people happy What precautions might you take? – Sun block – Limit exposure – Protective clothes – sunglasses On balance, is sunbathing a good idea?

58. Skin Cancer Ultraviolet (UV) photons harm the DNA molecules of living organisms in different ways. In one common damage event, adjacent bases bond with each other, instead of across the “ladder.” This makes a bulge, and the distorted DNA molecule does not function properly.

59. Microwaves and Mobiles Microwave ovens and mobile phones both use microwaves, but: – Wavelength of microwave ovens is ~12 cm – Wavelength of mobiles is ~16-33 cm There is no evidence that phone use is harmful in the short term Phone use does cause measureable brain heating, but no more than moderate exercise Precautionary principle is recommended, meanwhile we are all guinea pigs.

60. People at risk from radiation Hospital radiologists Nuclear workers Pilots and flight crew Some miners Industrial radiation workers Exposure is regularly monitored Exposure is kept as low as reasonably achievable Exposure time is adjusted to ensure annual dose is within accepted levels

61. EM weapons demo

62. Students will be assessed on their ability to: 3.10 understand that light is part of a continuous electromagnetic spectrum which includes radio, microwave, infrared, visible, ultraviolet, x-ray and gamma ray radiations and that all these waves travel at the same speed in free space 3.11 recall the order of the electromagnetic spectrum in decreasing wavelength and increasing frequency, including the colours of the visible spectrum 3.12 recall some of the uses of electromagnetic radiations, including: radio waves: broadcasting and communications microwaves: cooking and satellite transmissions infrared: heaters and night vision equipment visible light: optical fibres and photography ultraviolet: fluorescent lamps x-rays: observing the internal structure of objects and materials and medical applications gamma rays: sterilising food and medical equipment 3.13 recall the detrimental effects of excessive exposure of the human body to electromagnetic waves, including: microwaves: internal heating of body tissue infra-red: skin burns ultraviolet: damage to surface cells and blindness gamma rays: cancer, mutation.