P13 Electromagnetic waves Flashcards
Electromagnetic spectrum (from longest to shortest wavelength)
- Radio waves
- Micro waves
- Infrared
- Visible light
- Ultraviolet (UV) light
- X-rays
- Gamma
describe the penetration power of radio waves
- Can pass easily though air
- Can pass through many solids and can also diffract around objects similar in size to, or smaller than, their wavelength
- The ionosphere refracts longer wavelength radio waves rather than letting them pass through
Practical uses of radio waves
- Radio and television transmission (across long distances)
Microwaves - penetration
- Pass easily though air, and can pass right through the Earth’s atmosphere
- Good for transmitting information from one place to another because microwave energy can penetrate light rain, snow, clouds, and smoke
Microwave ovens
- Microwave ovens can be used to cook food quickly
- Microwaves are absorbed by water molecules in the food, and these molecules then vibrate more, increasing their kinetic energy
- This heats the food
Microwaves - hazards
- Microwaves can penetrate the body
- Can heat up water, and our bodies contain a lot of water
- The microwaves will cause water molecules in the body to vibrate more, raising the body’s temperature and heating the person from the inside
- So microwaves can cause heating of body tissues
Infrared waves - penetration
- Can easily penetrate air, but are stopped by most solids
- Can penetrate glass
X-rays - Penetration
- Quite penetrating
- Pass through air very easily, as well as low-density solids such as body tissue
- Partially stopped by bone and thin metal
Uses of x-rays
X-ray machines: these are used to make pictures of bones, to reveal fractures.
This works because x-rays pass fairly easily though most body tissue, but not through bone. So in an x-ray image, silhouettes of the bones are left on the film, while the rest of the tissue appears almost transparent
Electromagnetic waves
- They are all transverse waves
- They all travel at the same speed in the same material (3 x 10^8 m/s)
- They can travel through a vacuum because they are vibrations of electric and magnetic fields, not particles
- Like all waves, they obey the wave equation
- These waves are produced naturally by stars and living things, and by man-made objects like electrical appliances and machinery
- The only electromagnetic waves that can be detected by the human eye are visible light waves
What type of spectrum do electromagnetic waves form?
A continuous spectrum
How do the speeds of EM radiation differ in a vacuum and in air?
Electromagnetic waves all travel at the same speed in a vacuum and in air, 3 x 10^8 m/s
What property of waves in different mediums causes refraction?
- Velocity
- Wave speed is slower in denser materials, causing refraction
In which direction (relative to the normal) do waves refract when entering a denser material?
- They bend towards the normal
- The angle of refraction is less than the angle of incidence
What type of waves can be produced by oscillations in an electrical circuit?
Radio waves
Where do gamma rays originate from?
They originate from changes in the nuclei of atoms
What health effects can ultraviolet waves cause?
- They can cause the skin to age prematurely
- They can increase the risk of developing skin cancer
What health effects can X-rays and Gamma rays cause?
- They are ionising radiation so can cause mutations in genes
- They can lead to increased risk of developing various cancers
Practical uses for infrared radiation
- Electrical heaters
- Cooking food
- Infrared cameras
- Remote controls
Practical uses for microwave radiation
- Satellite communications
- Cooking food
What happens when EM waves meet materials?
Waves are either absorbed, transmitted or reflected
- If transmitted, the wave carries on travelling, but the speed and direction may change, this is refraction
- If absorbed, energy is transferred to the material. This can cause:
- an increase in the thermal store of the material
- electrons in the wires of a receiver to oscillate, creating a current
What happens depends on the type of wave and the type of material
- energy increases along the EM spectrum and the higher the energy of the wave, the more able it is to penetrate materials
Wavelength (m) equation
Wave speed (m/s) / frequency (Hz)
Infrared radiation
- Infrared is a type of electromagnetic wave
- All objects emit and absorb infrared radiation, it is felt as heat
- The hotter an object is, the more infrared radiation it is emitting
- The cooler an abject is, the less infrared radiation it is emitting
- Infrared radiation is also called thermal radiation
Gamma - penetration
- The most penetrating of all types of electromagnetic radiation
- Can pass straight through the body
Uses of gamma rays
Sterilising food and medical equipment: because gamma rays can kill living cells, they can be used to destroy bacteria, mould or insects on food or medical equipment
Treatment of cancers: cancer cells can be killed by gamma rays. This treatment is called radiotherapy. However, gamma rays also kill healthy cells, so the treatment has to be carefully designed to target the cancer cells only
Hazards of gamma rays
- Reasonably safe in low doses, as they pass straight through the body without being absorbed much
- High exposures can kill living cells
Protection from gamma rays
- Takes a lot to stop gamma rays
- Several centimetres of lead, or thick concrete is needed
Hazards of x-rays
Too much exposure to x-rays can damage the cells of your body and may result in cancer later
Protection from x-rays
Takes a sheet of lead several millimetres thick to stop x-rays completely
X-rays
High energy and short wavelength waves
Gamma
Very high energy and very short wavelength waves
Communications
- Communications can use microwaves and radio waves
- Shorter wavelengths carry more information and spread out less, but they have shorter range because of increasing absorption by the atmosphere
Microwaves (and near-microwave radio waves)
- Spread out less than radio waves, so the signs doesn’t weaken as much
- Uses: mobile phones, satellite phones, satellite TV
Radio waves (wavelength less than 1m)
- Carry more information than longer wavelength radio waves
- Uses: TV broadcasts
Radio waves (wavelength between 1m-100m)
- Range is limited to the area around the transmitter
- Uses: Local radio stations, Emergency services
Radio waves (wavelength greater than 100m)
- Longest range
- Uses: National and International Radio stations
Carrier waves
- When sound waves reach a microphone, they induce an alternating current called an audio signal
- An oscillator provides a carrier wave in the form of an alternating current
- The transmitter uses the audio signal to modulate the carrier wave (changing its amplitude and/or frequency)
- The varying alternating current in a transmitter aerial emits radio waves, which carry the audio signal
- The radio waves induce a current in a receiver aerial, which causes oscillations with a frequency equal to the radio wave frequency
- The audio signal is separated from the carrier waved by the receiver. It is sent to a loudspeaker which then produces sound waves
Why do we need carrier waves?
- So the frequency of the oscillations in the circuit is high enough for radio waves to be emitted by the transmitter
- The frequency of an audio signal would be too low
Optical fibres
- Visible light and infrared waves can be used to transmit information down a hollow transparent glass fibre
- The waves are reflected along the fibre, rather than transmitted to the air outside
- This is called total internal reflection
- Visible light and infrared have shorter wavelengths than radio waves, so more information can be carried in more pulses of waves
- The signal stays in the fibre.
Why is it beneficial that the signal stays in the fibre?
The information sent along the fibre is more secure compared to a radio wave. Radiowaves can be intercepted by any receiver
Describe the relationship between the energy being transferred by an electromagnetic wave and the frequency of the wave
As frequency increases, wave energy also increases
Describe the relationship between the energy being transferred by an electromagnetic wave and its wavelength
As wavelength increases, wave energy decreases
Which electromagnetic wave has the highest energy transfer?
Gamma
what are the practical applications of radio waves?
television and radio
what are the practical applications of microwaves?
satellite communications and cooking food
what are the practical applications of infrared radiation?
electrical heaters, cooking food and infrared cameras
what are the practical applications of visible light?
fibre optic communications
what are the practical applications of ultraviolet?
energy efficient lamps and suntanning
what are the practical applications of x-rays and gamma rays?
medical imaging and treatments
