1. P.1 Book 3A Section 4.2 The electromagnetic spectrum How do bees see? The electromagnetic spectrum Applications of electromagnetic waves Check-point 2 4.2 The electromagnetic spectrum

2. P.2 Book 3A Section 4.2 The electromagnetic spectrum How do bees see? Can you explain the difference? Vision of human beings Vision of bees bees can see ultra-violet light

3. P.3 Book 3A Section 4.2 The electromagnetic spectrum 4.2The electromagnetic spectrum Expt 4b Beyond the visible spectrum Is there any invisible ‘light’ beyond the ends of the visible spectrum?

4. P.4 Book 3A Section 4.2 The electromagnetic spectrum Experiment 4b Beyond the visible spectrum 1.Produce a visible spectrum. 2.Use a temp sensor to measure the temp inside the visible spectrum and the region just outside the red end of the spectrum

5. P.5 Book 3A Section 4.2 The electromagnetic spectrum Experiment 4b 3.Place a fluorescent paper strip on the screen, leaving the two ends un-illuminated. Observe the strip. Beyond the visible spectrum 4.3 Expt 4b - Beyond the visible spectrum Video

6. P.6 Book 3A Section 4.2 The electromagnetic spectrum 1 The electromagnetic spectrum Expt 4b shows that… 1.Beyond red light, there is some invisible ‘light’ that heats up the temp sensor.  infra-red radiation 2.Beyond violet light, there is some invisible ‘light’ that causes the fluorescent paper strip to glow.  ultra-violet radiation They all belong to electromagnetic waves.

7. P.7 Book 3A Section 4.2 The electromagnetic spectrum 1 The electromagnetic spectrum Other members: radio waves, microwaves, X-rays and gamma rays. All these form the electromagnetic spectrum. They all travel in a vacuum at 3  10 8 m s –1.

8. P.8 Book 3A Section 4.2 The electromagnetic spectrum 2 Applications of electromagnetic waves a Infra-red radiation (IR radiation) emitted by all objects the amount depends on temperature of the object hotter object emits more can be detected by thermometers and infra-red detectors. Our skin is also sensitive to it.

9. P.9 Book 3A Section 4.2 The electromagnetic spectrum a Infra-red radiation (IR radiation) i Infra-red imaging: thermographs Thermograph is a kind of IR image  can show the temp variation of an object  can be applied in many areas 4.4 Thermographs Video

10. P.10 Book 3A Section 4.2 The electromagnetic spectrum a Infra-red radiation (IR radiation) ii Night vision An IR imaging camera forms images by detecting the IR radiation emitted.  can see things with it even in complete darkness Since IR can penetrate smoke, firemen can locate victims or fire sources through smoke by using an IR imaging camera.

11. P.11 Book 3A Section 4.2 The electromagnetic spectrum a Infra-red radiation (IR radiation) iii Data transmission Expt 4c Data transmission by infra-red radiation

12. P.12 Book 3A Section 4.2 The electromagnetic spectrum Experiment 4c Data transmission by infra-red radiation 1.Connect an infra-red transmitter to the output of a radio, and an infra-red receiver to an amplifier and a loudspeaker. 2. Align the infra-red transmitter and receiver. Observe what happens.

13. P.13 Book 3A Section 4.2 The electromagnetic spectrum Experiment 4c Data transmission by infra-red radiation 3.Turn the transmitter away from the receiver and observe what happens. 4.Insert a piece of paper between the receiver and the transmitter. Observe what happens. 4.5 Expt 4c - Data transmission by infra-red radiation Video

14. P.14 Book 3A Section 4.2 The electromagnetic spectrum a Infra-red radiation (IR radiation) remote controls infra-red thermometers heat lamps burglar alarm systems cooking auto-focusing, etc Example 1 Roof thermography Other applications of IR radiation:

15. P.15 Book 3A Section 4.2 The electromagnetic spectrum Example 1 Roof thermography When a roof leaks, we may to find where the leak is by roof thermography. Water cools down slower than the building material of the roof. When the building material cools down after sunset, water remains at a higher temp. Taking a thermograph of the roof will therefore tell you where the water is.

16. P.16 Book 3A Section 4.2 The electromagnetic spectrum Example 1 Roof thermography (a) Which member of the electromagnetic spectrum is used in roof thermography? What is the advantage of using it? IR radiation is used. The amount of IR radiation emitted by an object is related to its temp. ∴ Objects of different temp can be distinguished easily.

17. P.17 Book 3A Section 4.2 The electromagnetic spectrum Example 1 Roof thermography (b) A thermograph of a roof taken after sunset is shown below. The brighter parts represent areas of higher temp. Which part of the roof is wet, A or B ? Part A.

18. P.18 Book 3A Section 4.2 The electromagnetic spectrum a Infra-red radiation (IR radiation) Example 2 Auto-focus cameras

19. P.19 Book 3A Section 4.2 The electromagnetic spectrum Example 2 Auto-focus cameras The camera emits IR pulses to the object to be photographed and receives the pulses reflected from it. Many auto-focus cameras use IR radiation to judge the distance of an object from the camera. The distance is calculated from the time difference between emitting and receiving the pulses. The camera lens is then driven by a built-in motor to the correct position to focus the object.

20. P.20 Book 3A Section 4.2 The electromagnetic spectrum Example 2 Auto-focus cameras The auto-focus function makes focusing easy. But the function may not work well in some cases: The object contains an open flame. The object is dull black in colour. You take a photograph of yourself in the mirror.

21. P.21 Book 3A Section 4.2 The electromagnetic spectrum Example 2 Auto-focus cameras (a) If the time difference between sending and receiving a pulse is 10 ns, what is the distance between the object and the camera? (1 ns = 10 –9 s) Distance = speed  time = 3  10 8   (10  10 –9 ) = 1.5 m 1212

22. P.22 Book 3A Section 4.2 The electromagnetic spectrum Example 2 Auto-focus cameras (b) (i) Explain why the auto-focus function does not work well in this case. The object contains an open flame. The infra-red radiation from the open flame can confuse the infra- red sensor.

23. P.23 Book 3A Section 4.2 The electromagnetic spectrum The object is dull black in colour. Example 2 Auto-focus cameras (b) (ii) Explain why the auto-focus function does not work well in this case. The dull black object is a good absorber of radiation. It absorbs the IR pulses and reflects no radiation back to the camera.

24. P.24 Book 3A Section 4.2 The electromagnetic spectrum You take a photograph of yourself in the mirror. Example 2 Auto-focus cameras (b) (iii) Explain why the auto-focus function does not work well in this case. The IR pulses are reflected by the mirror, which is in front of your image to be photographed.

25. P.25 Book 3A Section 4.2 The electromagnetic spectrum 2 Applications of electromagnetic waves emitted by very hot objects, such as our sun and mercury lamps. induces the production of vitamin D in the skin gives us sun-tans a small amount of it is necessary to our health. excessive exposure can cause cataracts, sunburn and skin cancer. b Ultra-violet radiation (UV radiation)

26. P.26 Book 3A Section 4.2 The electromagnetic spectrum b Ultra-violet radiation (UV radiation) i Detection of fake banknotes  The marks are difficult to duplicate but can be verified easily using a UV lamp.  The fluorescent ink glows when it absorbs UV radiation Each banknote has some numbers, characters and symbols printed on it in fluorescent ink.

27. P.27 Book 3A Section 4.2 The electromagnetic spectrum b Ultra-violet radiation (UV radiation) ii Sterilization Since UV radiation can kill bacteria,  it is commonly used in sterilizing drinking water and tools used in laboratories and medical facilities.

28. P.28 Book 3A Section 4.2 The electromagnetic spectrum 2 Applications of electromagnetic waves c Radio waves can be produced by radio transmitters and detected by radio antennae Mainly used in data transmission, e.g: radio and TV broadcasting telecommunications mobile phone communications, etc

29. P.29 Book 3A Section 4.2 The electromagnetic spectrum 2 Applications of electromagnetic waves d Microwaves i Satellite telecommunications Microwaves can pass through the Earth’s atmosphere.  used in satellite telecommunications. We can watch live TV broadcasts around the world via satellites.

30. P.30 Book 3A Section 4.2 The electromagnetic spectrum i Satellite telecommunications Communication satellites revolve around the Earth at the same rate as the rotation of the Earth  appear stationary when seen on the ground. Antennae on the ground receive signals from one place and re-transmit to another.

31. P.31 Book 3A Section 4.2 The electromagnetic spectrum d Microwaves ii Radar Radar = Radio Detection And Ranging Example 3 Radar Uses of radar: for locating aircraft in the sky gathering weather information for forecasting

32. P.32 Book 3A Section 4.2 The electromagnetic spectrum Example 3 Radar The transmitter sends out short pulses of microwaves. If the pulses hit an object such as an aircraft, some of the pulses are bounced back to the receiver. Radar can locate the positions of aircraft. A radar system consists of a transmitter and a receiver. It whirls around continuously to scan the surrounding area.

33. P.33 Book 3A Section 4.2 The electromagnetic spectrum Example 3 Radar The distance of the object can be calculated from the time lag between the transmitted pulse and the return pulse. The results are shown instantly on a screen. The direction in which the signal is received gives the direction of the object.

34. P.34 Book 3A Section 4.2 The electromagnetic spectrum Example 3 Radar (b) A radar pulse is bounced back from an aircraft 4  10 –5 s after the pulse is transmitted. Distance of the aircraft from the radar station = ? Distance of aircraft = speed  time = 3  10 8   4  10 –5 = 6  10 3 m = 6 km 1212 (a) What is the speed of the microwave pulses? 3  10 8 m s –1

35. P.35 Book 3A Section 4.2 The electromagnetic spectrum d Microwaves iii Microwave ovens In microwave ovens, water molecules in the food absorb energy from the microwave and vibrate vigorously. This heats up the food as the vibrating molecules hit other molecules into motion.

36. P.36 Book 3A Section 4.2 The electromagnetic spectrum 2 Applications of electromagnetic waves e X-rays can be produced by hitting a metal target with fast-moving electrons. have high energy and high penetrating power. can damage living cells deep in the body. long-term or strong exposure is dangerous. With proper safety precautions, X-rays can be used in medical imaging and security checks.

37. P.37 Book 3A Section 4.2 The electromagnetic spectrum e X-rays i Medical imaging Low-energy X-rays can penetrate flesh but not the bones  commonly used for medical imaging. ii Security checks Used in airports to check whether luggage contains dangerous objects such as knives and guns.

38. P.38 Book 3A Section 4.2 The electromagnetic spectrum have the highest energy and penetrating power in the EM spectrum. emitted by radioactive substances. f Gamma rays i Radiotherapy gamma rays can damage living cells deep in the body.  can be used to kill cancer cells  radiotherapy

39. P.39 Book 3A Section 4.2 The electromagnetic spectrum f Gamma rays ii Medical imaging A small amount of radioactive substance is injected into the patient’s body  a gamma camera detects the gamma rays emitted and forms images.

40. P.40 Book 3A Section 4.2 The electromagnetic spectrum f Gamma rays iii Sterilization Gamma rays kill microbes and are used to sterilize food and medical equipment. The irradiated food can keep fresh for a longer time. Normal strawberries Irradiated strawberries

41. P.41 Book 3A Section 4.2 The electromagnetic spectrum Check-point 2 – Q1 The speed of light is _______________ in a vacuum. 3  10 8 m s –1

42. P.42 Book 3A Section 4.2 The electromagnetic spectrum A They can all be detected by human eyes. B They all travel at the same speed in a vacuum. C Some of them are used for communications. Check-point 2 – Q2 Which of the following statements about the members in the electromagnetic spectrum is incorrect?

43. P.43 Book 3A Section 4.2 The electromagnetic spectrum Check-point 2 – Q3 Which members of the electromagnetic spectrum can be detected at positions X and Y ? X : Y : Infra-red radiation Ultra-violet radiation

44. P.44 Book 3A Section 4.2 The electromagnetic spectrum The End