The Electromagnetic Spectrum
It is a continuous spectrum of waves. At one end, the waves have a low frequency with a long wavelength, and at the other end the waves have a very high frequency and very short wavelengths.
All of the waves in the EM spectrum have these properties:
- They transfer energy
- They are all transverse waves
- They all travel at the speed of light when in a vacuum
- They can all be reflected, refracted and diffracted
Although the EM spectrum is continuous, it can be easily divided into sections that all have different properties and therefore are used for different things.
Radio Waves and Microwaves
- Typical frequency (Hz) -> 105-1010
- Typical wavelength (m) -> 103-10-2
- Sources -> Radio transmitters and TV transmitters
- Detectors -> Radio and TV aerials
- Uses -> Long, medium and short wave radio, TV
They have the longest wavelength and lowest frequency of all the EM spectrum, and are used mainly for communication. The waves are given out by a transmitter and when they cross an aerial, the information they carry is received.
- Typical frequency (Hz) -> 1010-1011
- Typical wavelength (m) -> 10-2-10-3
- Sources -> microwave transmitters and ovens
- Detectors -> Microwave receivers
- Uses -> Mobile phone and satellite communication and cooking
For cooking, the water molecules in food absorb the heat from microwaves, cooking the food throughout rather than from the outside. They are dangerous to human body tissue as they can cook it. They pass easily through the atmosphere.
Infra-red Radiation and Visible Light
- Typical frequency (Hz) -> 1011-1014
- Typical wavelength (m) -> 10-3-10-6
- Sources -> Hot objects
- Detectors -> Skin, blackened thermometer, special photographic film
- Uses -> Cookers and heaters, remote controls, night vision
All objects emit infra-red. The hotter an oject is, the more is emits. Overexposure to it can cause our skin to burn. They are unlikely to interfere with other waves due to their low penetrating power.
- Typical frequency (Hz) -> 1014-1015
- Typical wavelength (m) -> 10-6-10-7
- Sources -> Luminous objects
- Detectors -> The eye, photographic film
- Uses -> Seeing, communication (optical fibres), photography
Visible to the human eye - we use it to see. There are 7 colours in the spectrum (ROYGBIV) with red having the lowest frequency and longest wavelength.
Ultraviolet Light and X-Rays
- Typical frequency (Hz) -> 1015-1016
- Typical wavelength (m) -> 10-7-10-8
- Sources -> UV lamps and the sun
- Detectors -> Skin, photographic film and some fluorescent chemicals
- Uses -> Fluorescent tubes and UV tanning lamps
Harmful to human eyes and skin - cause of sunburn. Some chemicals glow in UV light and it is used to aid chemical rection.
- Typical frequency (Hz) -> 1016-1018
- Typical wavelength (m) -> 10-8-10-10
- Sources -> X-ray tubes
- Detectors -> Photographic film
- Uses -> X-radiography to observe the internal structures of objects
They pass easily through soft body tissue but connot pass through bones, allowing us to see the structure of skeletons. Overexposure can cause cancer. They are used for security to see inside luggage and ship containers etc.
- Typical frequency (Hz) -> 1018-1021
- Typical wavelength (m) -> 10-10-10-14
- Sources -> Radioactive materials
- Detectors -> Geiger-Muller tube
- Uses -> Sterilising equipment and food, radiotherapy
Gamma rays are highly penetrating and can cause damage to living cells. They can cause DNA to mutate, causing cancerous cells. They can also kill cells when concentrated, meaning that they are also used to kill cancerous cells in the body.
Communicating Using Waves
When we talk we use sound waves. We use light waves to signal to people (sign language) and radio waves to communicate over large distances.
- To send a message using digital signals, the information is converted into a series of 1s and 0s called binary code. It looks like this:
- The information is converted into electrical voltages or currents that vary continuously.
Advantages of Digital Signals:
- All signals become weak and need to be regenerated. Digital signals regenerate well so they are accurate copies, whereas analogue signals are still weak when regenerated
- Digital systems are easier to design and build and deal with data that is easily processed.