6.1 Electromagnetic Spectrum
Electromagnetic radiations are electric and magnetic disturbances. They travel as waves and move energy form place to place. All electromagnetic waves can travel through a vacuum at the same speed (300 million m/s) but they have different wavelengths and frequencies.
The spectrum is continious. The frequencies and wavelengths at the boundaries are approximate as the different parts of the spectrum are not precisely defined.
Different wavelengths of electromagnetic radiation are reflected, absorbed or transmitted differently by different substances and types of surface. The higher the frequency of an electromagnetic wave the more energy it transfers.
We can link the speed of the waves to their wavelength and frequency using:
v = f x
· v = wave speed (m/s)
· f = frequency (Hz)
· x = wavelength (m)
6.2 Light, infared, microwaves and radio waves
Visible light is the part of the electromagnetic spectrum that is detected by our eyes. We see different wavelengths within it as different colours.
The wavelength increases across the spectrum from violet to red. a mixture of all the colours is seen as white light. Visible light can be used for photography.
Infrared radiation is given out by all objects. The hotter the object, the more IR it emits. Remote controls for devices such as TV's use infrared.
Microwaves are used in communications. 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.
Radio waves transmit radio and TV programs and carry mobile phone signals. Microwave radiation and radio waves penetrate your skin and are absorbed by body tissue. This can heat internal organs and may damage them.
Infrared radiation is absorbed by skin; too much will burn your skin.
An alternating voltage applied to an aerial emits radio waves with the same frequency as the alternating voltage. When the waves are recieved they produce an alternating current in the aerial with the same frequency as the radiation recieved.
The radio and microwave spectrum is divided into different bands, which are used for different communication purposes.
The shorter the wavelength the more information they carry, the shorter their range, the less they spread out.
Mobile phones communicate with a local mobile phone mast using wavelengths just on the border between radio waves and microwaves. Some scientists think that the radiation from mobile phones may affect the brain.
Opitical fibres are very thin glass fibres. We use them to transmit signals carried by visible light or infrared radiation. The signals travel down the fibre by repeated total internal reflection.
6.4 Thye expanding Universe
Optical fibres carrying visible light or infrared are useful in communications because they carry much more information and are more secure than radio wave and microwave transmissions.
Imagine a wave source is moving relative to an observer. The wavelength and frequency of the waves detected by the observer will have changed (shifted) from the original produced by the source. This is called the 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. - 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 the red-shift and means that the frequency has decreased and the wavelength increased.
6.5 The Big Bang
A blue-shift would indicate that a galaxy is moving towards us. We are able to see these effects by observing dark lines in the spectra from galaxies.
The Red-shift gives us evidence that the universe is expanding out 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 very small initial point. This is known as The Big Bang Theory.
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. This is cosmic microwave background radiation (CMBR), the radiation produced by the Big Bang.
The Big Bang theory is the only theory so far, to explain CMBR.