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.

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.

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

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.

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.

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

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

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

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

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

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

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.

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.

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

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.

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

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

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

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

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

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

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.

Satellite communications 2 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!

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.

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

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

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

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

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

Applications of gamma rays Sterilizing food Killing cancer cells

That’s the end of Section 12.3 Check Point Key Ideas Previous Page Exit