The Electromagnetic Spectrum

Year 11 Physics

What is an Electromagnetic Wave?• Electromagnetic waves are transverse waves.• They consist of alternating electric and magnetic

force fields at 90 degrees to one another and in the direction of energy transfer.

• These forces are generated by changes in the speed or direction of moving electric charge.

Some properties of Electromagnetic Waves• All electromagnetic waves can pass through a vacuum.• Electromagnetic waves travel the vacuum of space at the

common speed of light of 300 million metres per second (3.0 x 108 ms-1)

• Almost all of the energy that reaches the Earth from the Sun is in the form of electromagnetic radiation.

• It takes 8 minutes for light from the Sun to reach the Earth.

What is the Electromagnetic Spectrum?• The Electromagnetic Spectrum is a continuum of electromagnetic waves with

artificial divisions based on the frequency and wavelengths of the waves.• There is no distinct point at which the frequency changes and no special

change in properties at particular waves boundaries (looking at a rainbow illustrates this).

• Simply oscillating electrons in a wire or aerial can produce low frequency electromagnetic waves, like radio and television waves.

• Light waves oscillate too rapidly in this way and are produced by the outer electrons changing energy levels (shells) in atoms.

• X-rays are produced when the inner electrons change energy levels.

• Gamma rays, which have extremely high frequencies, are produced by energy changes in the atomic nucleus.

Radio Waves• Wavelengths ranging from 10cm to 1000m.• Lowest energy waves in Electromagnetic Spectrum.• Radio Waves include: AM radio, FM radio, TV, Microwaves and

Radar.• Detected by aerials connected to tuned electric circuits in radios• Variety of uses – depends upon frequency (see below):

AM and FM Radio• In AM (Amplitude Modulation) the audio signal

changes the amplitude of the carrier wave. • In FM (Frequency Modulation) the audio signal

changes the frequency of the carrier wave.

• AM radio waves have longer wavelengths than FM and can be received at greater distances.

• FM radio waves are less affected by electrical interference and hence provide a higher quality transmission of sound

Television• Television signals are transmitted on two

separate carrier waves– Visual signal is added onto one carrier wave

using Amplitude Modulation (AM)– Audio signal is carried on a separate carrier

wave using Frequency Modulation (FM)

• When you select a particular channel, you are selecting the respective visual and audio carrier waves for that channel.

• Your TV then completes the task of ‘stripping’ the carrier waves to produce the desired picture and sound.

Microwaves

• Wavelengths ranging from 1 millimetre to 30 centimetres.

• Were first used in World War 2 in Radar.• Used in microwave ovens (frequency of 2450 MHz) for

cooking. Produced by a magnetron when cathode rays (a beam of electrons) rotate past an electric field.

• Also, used in mobile phone communications at frequencies of around 900 MHz. Transmission can be across distances of up to 100 km, but there must be a direct ‘line of sight’

• Detected in the same way as radio waves and television signals

Infra-red Radiation• Wavelengths ranging from 700 nanometres (0.0007 millimetre) to 1

millimetre.• Emitted by hot objects• Detected by special photographic film and semiconductor devices • Variety of uses including:

– Remote controls– Security and burglar alarms– Medical treatments for soft tissue injury.– Thermal imaging applications.

Visible Light

• Wavelengths ranging from 400 to 700 nanometres.

• We see light of different frequencies as different colours.

• White light is light that contains all the colours of the spectrum

• Detected by the eyes, photographic film and photo cells

• A variety of applications including:– fibre-optic communications– Photography– Laser technology

Ultraviolet (UV) Radiation• Wavelengths ranging from 10-400 nanometres.• Small doses beneficial to humans as it encourages production of

vitamin D.• Larger doses can lead to cell and tissue damage – possibly causing

skin cancer or eye cataracts.• Most types of glass absorb UV rays but clouds do NOT absorb UV

(that is why you can get sunburnt on cloudy days)• Detected by photographic film, photo cells and fluorescent chemicals• Variety of uses including:

– Photo-initiator chemicals in polymerisation– Astronomical observations– Sterilisation of hospital equipment

X-rays

• Wavelengths ranging from 0.01-10 nanometres.

• Have energy enough to pass through human flesh

• Detected by photographic film and fluorescent screen

• Variety of uses including:

– Cancer treatment by focussing the rays to kill cancer cells

– Finding weakness in metals and analysing structures of complex chemicals.

– Imaging applications in medicine.

X-ray Images

Gamma Rays

• Wavelengths less than 0.01 nanometres.• Highest energy waves in Electromagnetic Spectrum.• Produced when energy is lost from the nucleus of an atom

during radioactive decay.• Detected by photographic film or a Geiger-Müller counter.• Highly destructive to human tissue.• Can be used to kill cancer cells.• Also used in finding fractures and weaknesses in metals.

Atmospheric Filtering

• Only a small range of the frequencies in the electromagnetic spectrum reach the Earth’s surface

• The Earth’s atmosphere and ionosphere absorb the rest

• Very little ultraviolet, X-ray or gamma radiation penetrates the atmosphere (a good thing)

• The ionosphere is the upper layer of the atmosphere in which the gaseous atoms and molecules have become ionised (gained or lost electrons)

• The ionosphere itself can be divided into three layers: D, E and F

• D: 50 – 80 km above Earth’s surface, absorbs short wavelength (hard, high energy) X-rays

• E: 80 – 105 km above Earth’s surface, absorbs long wavelength (soft, low energy) X-rays

• F: 145 – 300 km above Earth’s surface, absorbs short wavelength UV-rays