Electromagnetic Waves

Electromagnetic Waves

1. The Electromagnetic Spectrum

   • Electromagnetic radiations are electric and magnetic disturbances
   • They travel as waves and move energy from place to place
   • All electromagnetic waves travel through space (a vacuum) at the same speed but they have different wavelengths and frequencies
   • Gamma Rays = shortest wavelength & highest frequency
   • The spectrum is continuous
   • The frequencies and wavelengths at the boundaries are appropriate as the different parts of the spectrum are precisely defined
   • Different wavelengths of electromagnetic radiation are reflected, absorbed or transmitted differently by different substances and types of surfaces
   • The higher the frequency of an electromagnetic wave = the more energy is transfers
   • All electromagnetic waves travel through space at a wave speed of 300m/s

2. Light, Infrared, Microwaves and Radio Waves

   • Visible light is the part of the electromagnetic spectrum that is detected by our eyes
     • Visible Light
   • We see the different wavelengths within it as different colours
   • The wavelength increases across the spectrum from violet to red
   • We see a mixture of all colours as white light
   • Visible light can be used for photography
   • Infrared radiation is given out by all objects
     • Infrared Radiation
   • The hotter the object is = The more IR it emits
   • Remote controls for devices such as TV's and CD players use IR
   • Microwaves are used in communications
     • Microwaves
   • Microwave transmitters produce wavelengths that are able to pass through the atmosphere
   • They are used to send signals to and from satellites and within mobile phone networks
   • Radiowaves transmit radio and TV programs and carry mobile phone signals
     • Radiowaves
   • Microwave radiation and radio waves penetrate your skin and are absorbed by body tissue - Can heat body organs and damage them
   • Infrared radiation is absorbed by skin - Too much will burn your skin

3. Communications

   • An alternating voltage applied to an aerial, emits radio waves with the same frequency as the alternating voltage
   • When wave are received they produce an alternating current in the aerial with the same frequency as the radiation received
   • The radio and microwave spectrum is divided into different bands
   • The different bands are used for different communication purposes
   • The shorter the wavelength of the wavs = The more information they can carry, the shorter their range, the less they spread out
   • Mobile phones communicate with a local mobile one mast using wavelengths just on the border between radio waves and microwaves
   • Some scientists think that he radiation from mobile phones may affect the brain - Especially in children
   • Optical fibres are very thin glass fibres used to transmit signals carried by visible light or infrared radiation
   • The signals travel down the fibre by repeated total internal reflection
   • Optical fibres carrying visible light or infrared are useful in communications because they carry much more information and are more secure than radio waves and microwaves transmission

4. The Expanding Universe

   • Doppler Effect
     • When the source moves away from the observer, the observed wavelength increases and the frequency decreases
     • When the source moves towards the observer, the observed wavelength decreases and the frequency increases
     • The Doppler Effect can be demonstrated with sound waves
     • e.g. an ambulance siren will sound differently depending on whether it is moving away from you (lower pitch) or towards you (pitch is higher)
   • Red Shift
     • Galaxies are large collections of stars
     • Light observed from distant galaxies has been shifted towards the red end of the spectrum
     • This is known as 'Red Shift' and means the frequency has decreased and the wavelength has increased
     • A blue-shift would indicate that the galaxy is moving towards us
     • We can see these effects by observing dark lines in the spectra from galaxies
     • Further away the galaxy = Bigger the red shift
     • Suggests that the distant galaxies are moving away from us and the most distant galaxies are moving the fastest

5. The Big Bang

   • Red-shift gives us evidence that the universe is expanding outwards in all directions
   • We can imagine going back in time to see where the universe came from, if it is now expanding outwards
   • This suggests that it started with a massive explosion at a vey small initial point
   • If the universe began with a Big Bang, then high energy gamma radiation would have been produced
   • As the universe expanded, this would have become lower energy radiation
   • Scientists discovered microwaves coming from every direction in space
   • Cosmic Microwave Background Radiation (CMBR)
   • The Big Bang Theory is so far the only theory which can explain CMBR