3. Waves Flashcards
Waves
Oscillations in a medium that transfer energy not matter. Examples are water waves, electromagnetic waves and sound waves.
Can be shown as a series of wavefronts moving away from the source of vibration, transferring energy
Peaks
High points / Crests
Troughs
Low points
Medium
The material through which a wave passes. The plural of medium is ‘media’. Waves travel at different speeds in different media.
Electromagnetic waves
Transerve waves that are vibrations in electric and magnetic fields. These are the waves that transfer energy across the Universe. Electromagnetic waves of different wavelengths make up the electromagnetic spectrum.
Wavefront
A line showing where all the points of the wave are being disturbed in the same way – for example, where all of the crests (peaks) of the waves are at a particular time.
Wavefronts are parallel to the propagation of the wave (direction the waves are travelling)
Direction of propagation
The direction that a wave travels in, this is the direction that energy is transferred.
Transverse wave
The particle oscillates perpendicular (at right angles) to the direction of propagation of the wave
(traditional wave form)
(Particle stays the same length from the source of the wave. It only oscillates (moves) up an down, forming a wave)
Longitudinal waves
The oscillation of the particles is parallel (in the same direction) to the direction of propagation of the wave.
(Particles move back and forth but always end up back where they began. eg. to the right, then to the left etc.)
Wave made of compressions and rarefactions.
Compression - Particles are closer together than normal due to a sound wave passing through it.
Rarefaction - Particles are further apart than normal due to a sound wave passing through it.
(Drawn as a transverse wave where compression = peak and rarefaction = trough)
Examples of transverse waves
- Ripples along water surface –> water moves up and down but wave motion is outwards, along water surface.
- Waves formind in a shaken rope –> wave moves along the length of the rope while the vibrations are from side to side (or up and down)
- Visible light + other electromagnetic radiation –> However, no vibrating particles. Instead there are vibrating electric and magnetic fields –> Vibration in these fields = perpendicular to the derection the radiation travels.
Wave speed (info)
- Measured in m/s
- Moves at the same speed throughout a medium (doesn’t differ between peaks and troughs)
Frequency of a wave
- Number of complete waves (1 wavelength –> from peak to peak/trough to trough) that pass a point each second
- Frequency measured with a unit called hertz (Hz)
- 1 Hz = one complete vibration per second
- 1 kHz (kilohertz) = 1000 Hz
Hertz
The unit of measurement for frequency. Symbol Hz. 1 Hz equals one complete wave/oscillation passes per second.
Wavelength
- The distance between one wave crest (peak) and the next.
- Or the distance between one trough and the next.
- As wavelength is a distance, it is measured in metres (m).
Amplitude
- The distance between the wave crest (peak) and the centre of the wave
- This is the greatest distance a particle moves from its rest position.
- The higher the amplitude a wave has, the more energy it transfers as it moves.
Wave speed (formula)
wave speed (m/s) = frequency (Hz) × wavelength (m)
v = f λ
λ = lamba (greek letter) –> represents wavelength in the formula.
Speed of a wave through a certain medium is fixed. All waves have the same speed through air.
Earthquake waves
Earthquakes are caused by sudden movement of the Earth’s crust. Enormous forces build up as continental plates push against each other and large amounts of energy are released as the pressure becomes too much and the plates slip past each other. These events cause two different types of a waves:
Primary wave and Secondary wave
Primary wave (earthquakes)
Primary waves are longitudinal waves. The ground is compressed in the same direction as the wave travels.
This can shake buildings from side to side as the wave passes through the ground beneath them.
Secondary waves (earthquakes)
Secondary waves are transverse waves. The ground rises and falls as the wave passes through it.
This will shake buildings up and down.
Reflection
A change in direction of a wave as it reaches a boundary.
‘Wave bounces back off the boundary’
Refraction
When a wave changes direction due to it changing speed when it moves from one material to another.
Refraction causes a change in direction unless the light enters along the normal.
Diffraction
The spreading of a wave as it moves through a gap or past the edge of an obstacle.
Reflection - Incident wave / wavefront
The wave / wavefront before it reaches the boundary
Reflection - Reflected wave / wavefront
Wave / wavefront after it has hit the boundary and bounced off.
- If the wavefront is parallel to the surface when it hits, the wavefront will be reflected back in the direction it came from.
- If the wavefront hits the surface at an angle, then its direction of travel will change.
- During reflection, the wave speed, frequency and wavelength does not change. Only the direction in which the wavefront is travelling changes.
Diffraction - Complex explanation
- Diffraction affects all waves
Diffraction through a gap….
- The effect is small unless the size of the gap and the wavelength of the wave are of a similar size.
- Why we don’t see light waves diffracting as they pass through a doorway –> Wavelength of the light is much smaller than the size of the gap.
- Soun waves have a wavelength of approx a few cm to a metre –> enough to significantly diffract through a doorway –> Why we can hear people approaching.
- Light waves can be diffracted by passing light through gaps a fraction of a mm. –> light sreads with wave fronts becoming circular and spreading out from the gap.
Diffraction around the edge of an object…
- Diffraction also happens when waves pass by the edge of an object
- The waves spread around the edge and can travel behind it.
- This amount of diffraction increases as the wavelength of the wave increases.
- Waves with long wavelengths diffract more than waves with short wavelengths.
- Sound waves have much longer wavelength than light waves –> much more strongly diffracted by edges.
- Allows sound waves to spread around corners in a way that is not possible for light.
Electromagnetic waves (complex)
- Transverse waves with unusual properties not typical to transverse.
- Can travel through a vacuum (don’t rely on vibration of any kind of particle to travel)
- Electromagnetic waves are vibrations in linked electric and magnetic fields moving through space.
- All electromagnetic waves travel at the same very high speed in a vacuum – much faster than any other type of wave.
Interaction between electric and magnetic fields to form electromagnetic waves
(READ EXTRA ON)
- Electromagnetic waves do not have oscillating particles – they have oscillating fields.
- Electromagnetic waves have a wide range of wavelengths and frequencies.
- There are two linked oscillating fields which are at right angles to each other and at right angles to the direction of propagation.
- In a vacuum the speed of propagation is 3.0 × 10⁸ m/s for all electromagnetic waves.
Speed of propagation of electromagnetic waves in a vacuum
3.0 × 10⁸ m/s
(Fastest anything can possibly travel)
Speed of propagation
Fancy phrase for wave speed
Electromagnetic spectrum
The complete set of electromagnetic waves, including radio waves, microwaves, infrared radiation, visible light, ultraviolet, X-rays and gamma rays.
Lowest frequency / Longest wavelengths
Radio waves
Microwaves
Infrared radiation
Visible light
Ultraviolet
X-rays
Gamma rays.
Highest frequency / Shortest wavelengths
Electromagnetic spectrum - Ionising radiation
Ultraviolet
X-rays
Gamma rays
Electromagnetic spectrum - Non-ionising radiation
Radio waves
Microwaves
Infrared radiation
Visible light
Radio Waves
A type of transverse wave that consists of vibrating electric and magnetic fields.
Radio waves have the longest wavelength of the electromagnetic waves.
They are produced when electrons are moved back and forth inside wires by varying electric currents.
Different wavelengths are produced by making the electrons oscillate at different frequencies.
Radio waves - TV / Radio Signals (Uses)
TV / Radio signals
- Used to transmit TV and radio signals.
- Transmitted from large radio transmission towers (Can produce powerful radio signals so detected over great distances)
- Transmission type = terrestrial broadcasting
- Most radio signals = Refracted (ASK MR. GARDINER) back by
upper atmosphere (thus can travel past curvature of the earth to reach distant places) - TV signals = shorter wavelength radio waves –> don’t refract as much –> escape the atmosphere.
Radio waves - Radio Astronomy
- Many objects emit radio waves that can pass through dust clouds in space + atmosphere –> can be detected on earth.
- Exceptionally large radio wave detectors designed to detect weak radio signals coming from stars and black holes –> Detect radio waves.
- Helpful bc not all objects in space emit visible radiation + dust/ clouds of dust obstruct view.
Radio waves - Radio frequency identification (RFID)
Radio frequency identification (RFID)
- Small reciever, containing a tiny computer chip + aerial is attatched to an object (with unique ID no. + other info in it)
- Transmitter send out radio waves + reciever absorbs them.
- Radio wave provides enough power for the RFID chip to send back a signal containing its data.
- Contactless payments using credit cards use RFID chips.
- When card is placed near a pay point –> it sends out a signal (requesting details in the form of a radio pulse eg. How much to pay and who is requesting the payment.)
- The absorbed radiation powers the RFID chip on the card.
- The credit card sends back the details of the user.
- The pay point checks the details, usually in a fraction of a second, and accepts payment.
- Mobile phones also commonly have active RFID chips, which can send out requests of their own.
Microwaves
Electromagnetic waves are a type of electromagnetic wave.
They are similar to radio waves and are also produced by oscillating electrons in wires. However, they have a shorter wavelength, usually a few centimetres or millimetres.
Microwaves - Microwave oven
- Microwaves are absorbed strongly by some of the molecules present in food and living tissue, causing a heating effect.
- This effect is used to cook food in microwave ovens.
- Microwaves are produced by a small transmitter inside the oven.
- They are absorbed by the food which heats up rapidly.
- The oven is made of metal so that the microwaves reflect back inside the chamber and heat the food instead of escaping.
Microwaves - Satellite and phone communication
- Microwaves = used to transmit signals to satellites in orbit around the Earth.
- Many radio waves cannot penetrate the upper atmosphere easily; however, microwaves can and so make satellite communication possible.
- Low-power microwaves are used in mobile phone networks.
- The mobile phones produce weak microwave signals which are detected by dishes on towers throughout towns and cities
- The towers send signals back to the mobile phones.
- The aerials needed to transmit and receive microwaves are much smaller than those needed for radio waves.
- A mobile phone will have a microwave receiver which is only a few centimetres long.
- Sometimes, if you hold your phone the wrong way or put it inside a badly designed case, you can block this receiver and make it difficult for the phone to receive a signal.
- Many houses and businesses have wireless local area networks (LANs) connected to the internet.
- These use microwaves to relay signals between devices, like laptops or smart TVs, and a ‘router’ usually connected to a wired phone network.
- The microwaves allow communications through the air and can even pass through walls, although this will weaken the signal.
- Mobile phone signals can also pass through some walls; this also reduces their signal strength so you may lose reception indoors.
Microwaves - Dangers
- Heating effect caused by microwaves = dangerous
- This could easily damage living tissue.
- As mobile phones use microwaves, some people were concerned that they would cause brain damage as the phones were placed next to ears, close to our brains.
- While the brain will absorb some of the radiation, the power level of the phone is far too low to produce a damaging effect.
Infrared radiation
Electromagnetic waves used for cooking (grilling) and in optical fibres for communications. Emitted by all objects but more radiation is emitted by hotter objects.
Infrared - Remote Controls
- Remote controls for TV use infrared radiation.
- To change channel/ volume, a diode on the remote control emits pulses of infrared radiation which is detected by a sensor on the television set.
- The radiation can travel a few metres through the air before being completely absorbed, and so the control has a limited range –> remote also has to be pointed directly at TV.
- Some television sets now operate on Bluetooth radio signals.
- These signals can pass through walls and so you could operate a TV, or music player, from another room.
- They have a fairly short range, up to 50 m + signal is weakened as it passes through walls –> but powerful enough for most houses.
Infrared - Cooking
- Infrared radiation has a heating effect when it is absorbed.
- An electric grill, or toaster, heats metal wires to very high temperatures so they emit large amounts of radiation
- Absorbed by surface of food palced nearby + cooks it
- Similar effect achieved with flames (emit large amounts of infrared.
Infrared - Thermal imaging + alarms
- As the temperature of an object increases, the amount of infrared radiation it emits increases too
- This effect is used in thermal imaging.
- Infrared cameras can produce images of hot or warm objects, even when there is no visible light.
Uses include…
- Night vision
- Identifying energy losses from houses due to poor insulation
- Finding hotspots indicating faults in electronic equipment
- Medical diagnosis.
- Some burglar alarms use simple infrared systems where only temperature change needs to be detected.
- When a burglar moves past an infrared sensor, it detects the radiation they are emitting and sets off the alarm system.
Infrared - Communications system
- Infrared radiation cannot travel far through air without being absorbed –> not of much use in transmitting information through the atmosphere.
- Can however travel very large distances though materials like glass –> often used in fibre optic networks to transmit information.
Infrared - Dangers
- Infrared radiation has a heating effect when it is absorbed –> an cause skin burns. (felt when standing near fire)
- The closer you are to the source, the more infrared radiation you will absorb and the more likely you are to get burned.
- Firefighters sometimes wear silver-coloured suits to prevent burns from working near intensely hot objects.
- The silver surface reflects the radiation instead of absorbing it.
Visible light
Electromagnetic waves with a wavelength that we can see.
- Used in any optical instrument where we want to see objects directly, such as microscopes, telescopes and binoculars.
- It is also used in photography where the light is detected by sensors in the camera or by photographic film.
Ultraviolet radiation
- Ultraviolet radiation (UV) is produced by the sun and some fluorescent tubes.
- Most of the UV radiation emitted by the sun is absorbed by our atmosphere before it reaches the surface of the Earth.
- Some parts of the UV spectrum can pass through.
UV - Clothes/ Make-up
- Some chemicals can absorb UV radiation and then emit visible light instead.
- Clothing which has been washed in biological washing powder is a good example.
- When the clothes are exposed to UV light, they appear to glow and this effect is used in fairgrounds and discos
- Some clothes are designed using UV-sensitive dyes to increase this effect.
UV - Money
- Paper produced from wood pulp is bleached to make it very white.
- This bleaching process causes the paper to glow when exposed to ultraviolet light.
- Most paper banknotes are produced from unbleached cotton which does not glow when exposed to UV
- Fake banknotes will glow under an UV lamp.
- Many new notes are made from plastics with some sections designed to glow in UV and other sections designed not to glow, depending on which inks have been used to dye the plastic.