1. ultraviolet radiation
The electromagnetic spectrum
The electromagnetic spectrum is made up of:
gamma rays
radio waves
visible light
ultraviolet radiation
X-rays
microwaves
infrared radiation
They occupy different wavelength ranges and have different applications.

2. Visible light Human eye can only detect the visible spectrum.
The visible spectrum consists of light of different wavelengths.
Each wavelength has its own colour.
The colours of a rainbow correspond to light of different wavelengths.

3. When white light passes through a prism, it undergoes dispersion.
The colours red, orange, yellow, green, blue, indigo and violet are produced.

4. Range of wavelengths: from 4  10–7 m to 7  10–7 m.
Violet light has the shortest wavelength of 4  10–7 m.
Red light has the longest wavelength of 7  10–7 m.

5. Infrared radiation
Infrared radiation is emitted by any object with a temperature above absolute zero.
The hotter the object, the more energy and higher frequency the emitted infrared radiation has.

6. Typical wavelength: 10–4 m
Just next to visible red light in the electromagnetic spectrum
Infrared

7. Applications of infrared radiation
Thermography
A thermograph shows regions of different temperatures in different colours.
a small child
a tropical storm

8. Infrared viewers enable firefighters to see through the smoke
enable us to see things in complete darkness

9. Infrared remote control
Infrared lamp
Infrared remote control
infrared transmitter — sends out signals using infrared radiation.
dries car paint

10. infrared pulses sent out from a transmitter
Auto-focus camera
infrared pulses sent out from a transmitter
pulses reflected by the object
Time difference between sending and receiving the pulses
is used to calculate
Distance between the object and the camera

11. Ultraviolet radiation
Typical wavelength: 10–8 m
emitted by mercury lamps, the Sun, etc.
a mercury lamp

12. Most UV radiation from the Sun is absorbed by the ozone layer of the atmosphere.
Due to the depletion of the ozone layer, more UV radiation is reaching the Earth’s surface.

13. i.e. they glow (fluoresce) under UV radiation.
Some chemicals (e.g. phosphor) absorb UV radiation and release the energy in the form of visible light
i.e. they glow (fluoresce) under UV radiation.
Applications of ultraviolet radiation
Production of vitamin D
UV radiation in sunlight can cause a suntan and produce vitamin D in our skin.

14. Sterilization
UV radiation of shorter wavelength can kill germs and bacteria.
It can be used to sterilize
water
air
equipment in hospitals and laboratories

15. Checking of banknotes
The numbers on each banknote are printed with fluorescent chemicals.
They become visible under an UV lamp.
Washing powders
contain fluorescent chemicals which makes the clothes look brighter.

16. Radio waves wavelength: 0.1 m — 10 km, longest in the EM spectrum
divided into different types according to their wavelengths
Radio waves
Wavelength range
Frequency range
Applications
Long waves
(low frequency)
Radio communications
1 km – 10 km
30 kHz – 300 kHz
Radio communications, AM radio broadcasting
Medium waves (medium frequency)
100 m – 1 km
300 kHz – 3 MHz

17. Short waves (high frequency) Radio communications 10 m – 100 m
Radio waves
Wavelength range
Frequency range
Applications
Short waves (high frequency)
Radio communications
10 m – 100 m
3 MHz – 30 MHz
Radio communications, FM radio broadcasting, TV broadcasting
VHF
(very high frequency)
1 m – 10 m
30 MHz – 300 MHz
UHF
(ultra high frequency)
Radio communications, TV broadcasting
0.1 m – 1 m
300 MHz – 3 GHz

18. Applications of radio waves
Radio communications
Long waves and medium waves
repeater station
Repeater stations are used to extend the range of transmission.

19. Short waves
ionosphere
Radio waves are reflected by the ionosphere in the upper atmosphere to a receiver on the Earth’s surface.

20. Radio broadcasting Amplitude modulation (AM) Frequency modulation (FM)
Medium waves
VHF
amplitude of radio waves varies in accordance with the sound signals
frequency of radio waves varies in accordance with the sound signals

21. TV broadcasting
The sound and video signals are carried by VHF or UHF.
cannot diffract around tall buildings or hills
short wavelengths
∴ TV antennas must point towards the transmitting station

22. Microwaves Typical wavelength: a few centimetres Applications:
Microwave ovens
The microwaves cause the water molecules in the food to vibrate, thereby heating up the food.

23. Satellite communications2
1. signals sent to satellite in space 1 3
2. The signals are amplified.
3. The signals are re-transmitted to other parts of the world.
We can watch live TV broadcast at home!

24. Radar – radio detection and ranging
How does radar work?
2. The pulses are reflected by a distant object.
1. A transmitter sends out microwave pulses.
4. The range and direction of the object is determined.
3. A receiver detects the reflected pulses a short time later.

25. Demonstrating wave properties
receiver – receives microwaves and coverts them to electrical signals
transmitter – emits 3 cm microwaves
millammeter – measures the amount of signals

26. Microwaves of wavelength 3 cm are used
∵ easy to demonstrate their diffraction and interference properties

27. X-rays very short wavelengths, typically 10–10 m
produced by bombarding a heavy metal target with fast-moving electrons in an X-ray tube
heavy metal target
electron beam
X-ray

28. very penetrating – can pass through plastic, cloth, wood, living tissues, etc
Applications of X-ray
Making images of bones or teeth on photographic film
X-rays pass more readily through flesh than through bones and teeth.
Checking for cracks on steel or concrete structure

29. Gamma rays shortest wavelengths in the electromagnetic spectrum
typical wavelength: 10–12 m
highly dangerous
∵ (i) carry the greatest energy
(ii) the most penetrating
among all electromagnetic waves

30. Applications of gamma rays
Sterilizing food
Killing cancer cells

31. That’s the end of Section 12.3
Check Point
Key Ideas
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