Light and the electromagnetic spectrum Waves used in a mobile phone The visible spectrum The electromagnetic spectrum Wave properties of electromagnetic waves Check-point 2 ? ?

Light and the electromagnetic spectrum Waves used in a mobile phone camera Have you ever seen this mobile phone before?

Light and the electromagnetic spectrum Waves used in a mobile phone camera Do you know it makes use of different kinds of wave? Can you name some of them?

Light and the electromagnetic spectrum Waves used in a mobile phone camera Voice communication: Radio waves / microwaves Camera: Light waves Bluetooth: Radio wave

Light and the electromagnetic spectrum 1The visible spectrum When visible light passes through a prism, Wavelength range: 400 nm (violet) visible spectrum it spreads into a colour spectrum. 700 nm (red)

Light and the electromagnetic spectrum 1The visible spectrum wavelength (nm) ultraviolet infra-red

Light and the electromagnetic spectrum Experiment 11c The visible spectrum prism image of filament (no prism) converging lens ray box (no lens, no slit plate) white screen Project the image of the lamp filament on a screen. Note the different colours on the spectrum.

Light and the electromagnetic spectrum Experiment 11c The visible spectrum Video

Light and the electromagnetic spectrum 2 The electromagnetic spectrum The visible spectrum is only a small, visible part of the electromagnetic spectrum.

Light and the electromagnetic spectrum 2The electromagnetic spectrum 2in a vacuum, speed = 3  10 8 m s  1 in a material, speed is slower 3follow equation v = f f ↑, ↓ 4show reflection, refraction, diffraction, and interference 1transverse waves Waves in EM spectrum: 5do not require a medium to travel, can travel in a vacuum

Light and the electromagnetic spectrum 2The electromagnetic spectrum

Light and the electromagnetic spectrum 3Waves properties of electromagnetic waves Electromagnetic waves also show reflection, refraction, diffraction and interference. The following examples show the applications of these properties.

Light and the electromagnetic spectrum 3Waves properties of electromagnetic waves

Light and the electromagnetic spectrum Check-point 2 1The speed of light…The speed of light… 2The…The… 3Which of the following…Which of the following… 4Which of the following graphs…Which of the following graphs…

Light and the electromagnetic spectrum Check-point 2 - Q1 The speed of light = ___________ (in a vacuum) 3  10 8 m s  1

Light and the electromagnetic spectrum Check-point 2 - Q2 The ____________ of light determines its colour. frequency

Light and the electromagnetic spectrum Check-point 2 - Q3 Which statements about waves in the electromagnetic spectrum is INCORRECT ? AThey can all be detected by human eyes. BThey all travel at the same speed in a vacuum. CSome of them are used for communication. DVisible light can be thought of as a high- frequency radio wave, and radio waves as a low-frequency light.

Light and the electromagnetic spectrum Check-point 2 - Q4 Which graphs shows the relation between speed v and frequency f of electromagnetic waves in a vacuum? A B C v f v f v f

Light and the electromagnetic spectrum The End

Light and the electromagnetic spectrum Example 3 The electromagnetic spectrum (a)The frequencies of green and orange lights are 5.45 × Hz and 5.00 × Hz respectively. Find their wavelengths in a vacuum.

Light and the electromagnetic spectrum Example 3 (a)Speed of light in a vacuum = 3 × 10 8 m s –1 By v = f of green light = 5.45 × × 10 8 The electromagnetic spectrum = 5.50 × 10 –7 m of orange light = 5.00 × × 10 8 = 6.00 × 10 –7 m

Light and the electromagnetic spectrum Example 3 The electromagnetic spectrum radio waves Xinfra-red visible light YX-rays gamma rays (b)Name the parts X and Y in the electromagnetic spectrum. X : microwave Y : ultraviolet radiation

Light and the electromagnetic spectrum Example 3 The electromagnetic spectrum radio waves Xinfra-red visible light YX-rays gamma rays (c)Which part of the spectrum has the highest frequency? Gamma ray

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Light and the electromagnetic spectrum Example 4 Radar

Light and the electromagnetic spectrum Example 4 (a)What wave property is used by the radar system to detect aircrafts? (b)A radar pulse is bounced back from an aircraft 4 × 10 –5 s after the pulse is transmitted. What is the distance of the aircraft from the radar station? Radar

Light and the electromagnetic spectrum Example 4 Radar (a)What wave property is used by the radar system to detect aircrafts? Reflection. It detects microwaves reflected from the aircraft.

Light and the electromagnetic spectrum Example 4 (b)Time after bounce back = 4 × 10 –5 s Time for one-way journey = 2 × 10 –5 s Speed of radar pulse = 3 × 10 8 s Distance of aircraft = speed × time = 3 × 10 8 × 2 × 10 –5 = 6 × 10 3 m = 6 km Radar

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Light and the electromagnetic spectrum Example 5 Radio broadcasting (a)Calculate the wavelengths of waves for the broadcasting of RTHK 2 and RTHK 5.

Light and the electromagnetic spectrum Example 5 Radio broadcasting f v (a)For RTHK 2, wavelength == 94.8 × × 10 8 = 3.16 m f v For RTHK 5, wavelength == 783 × × 10 8 = 383 m

Light and the electromagnetic spectrum Example 5 (b)What happens when radio waves pass the top of an obstacle, e.g. a building or a hill? Diffraction occurs Radio broadcasting

Light and the electromagnetic spectrum Example 5 (c)Compare the reception of radio signals broadcasted by the two radio channels by the low building. Radio broadcasting Sketch wave patterns to illustrate your answer.

Light and the electromagnetic spectrum Example 5 (c)The shorter waves spread less and cannot reach the low building. The reception of RTHK 2 for the low building would be poorer. Radio broadcasting

Light and the electromagnetic spectrum Example 5 (c)The longer waves spread more and can reach the low building. The reception of RTHK 5 for the low building would be better. Radio broadcasting

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Light and the electromagnetic spectrum Example 6 Auto-focus cameras

Light and the electromagnetic spectrum Example 6 Auto-focus cameras

Light and the electromagnetic spectrum Example 6 (a)If the time difference between sending and receiving a pulse is 10 ns, what is the distance between the object and the camera? (b)Explain why the auto-focus function does not work well in the three cases mentioned in the article. Auto-focus cameras

Light and the electromagnetic spectrum Example 6 (a)If the time difference between sending and receiving a pulse is 10 ns, what is the distance between the object and the camera? Auto-focus cameras Distance 1 ns = 10 –9 s = speed × time = 3 × 10 8 × 2 1 × (10 × 10 –9 ) = 1.5 m

Light and the electromagnetic spectrum Example 6 Auto-focus cameras (b)(i)The object contains an open flame. The infra-red radiation from the open flame can confuse the infra-red sensor.

Light and the electromagnetic spectrum Example 6 Auto-focus cameras (b)(ii)The object is dull black in colour. The dull black object is a good absorber of radiation. It can absorb the infra-red pulses and reflect no radiation back to the camera.

Light and the electromagnetic spectrum Example 6 Auto-focus cameras (b)(iii)Shooting yourself in the mirror. The infra-red pulses are reflected by the mirror, which is in front of your image to be photographed.

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