Waves Flashcards
What is frequency
- the number of complete waves passing a certain point per second
- measured in hertz (Hz)
- 1 Hz is one wav per second
What is amplitude
- the maximum displacement of a point on the wave from its undisturbed position
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It’s the gap from the middle line to the crest of the wave
Crest and trough on a wave
The top point and bottom point
What is wavelength
- the distance between the same point on two adjacent waves
- basically length of one wave (one sideways S)
Transverse waves
- the oscillations are perpendicular (at 90 degrees) to the direction of energy transfer
- vibrations go up and down
TRANSVERSE WAVES: - all electromagnetic waves
- ripples an waves in water
- a wave on a string
Longitudinal waves
- vibrates are parallel to the direction of energy transfer
- have rarefactions and compressions
- example: sound waves
Frequency equations
Wave speed=frequency x wavelength
Time period= 1/frequency
What is time period
The amount of time it takes for a full cycle of the wave
Using an oscilloscope to measure the speed of sound
- Set up oscilloscope so the detected waves at each microphone are shown as separate waves
- Start with both microphones next to the speaker, then slowly move one away until 2 waves are aligned on the display, but have moved exactly one wave length apart
- Measure the distance between the microphones to find one wavelength
- Use formula to find wave speed passing through air. Frequency is whatever you eat the signal generator to (around 1kHz)
- The speed of sound in air is around 330 m/s, so results should roughly agree with this
Ripple tank experiment
- Using a signal generator attached to the dipper of a ripple tank you can create water wave at a eat frequency
- Dim the lights in the lab and turn on the lamp. You should see the wave crests as shadows on the screen below the tank
- The distance between each shadow line is equal to one wavelength. Measure the distance between shadow lines that are 10 wavelengths apart, then divide by 10 to find the average wavelength
- Use v = f x wavelength to find wave speed
Why is ripple tank experiment a good experiment
- good for measuring wavelength of moving waves or small wavelengths
- suitable because it allows you to measure the wavelength without disturbing the waves
Experiment for wave equation for waves on strings
- Turn on signal generator and vibration transducer. String will star to vibrate
- Adjust frequency of signal generator until there’s a clear wave on the string. The frequency needed will depend on the length of the spring between the pulley and the transducer and the masses used
- You need to measure the wavelength of these waves. Measure 4 or 5 wavelengths and divide to get mean half-wavelength. Then double this mean to get a full wavelength
- The frequency of the wave is whatever the signal generator is set to
- Use formula to find speed of wave
What 3 things happen when a wave meets a boundary between 2 materials
- Wave is ABSORBED by second material
- Wave is TRANSMITTED through second material
- Wave is REFLECTED
What happens when a wave is absorbed
The wave transfers energy to the material’s energy stores. Often, the energy is transferred to a thermal energy store which leads to heating
What happens when a wave is transmitted
The wave caries on travelling through the new material
- This often leads to refraction
- This can be used in communications as well as in the lenses of glasses and cameras
What happens when a wave is reflected
- this is where the incoming wave is neither absorbed or transmitted, but its sent back away from the second material. This is how echoes are crated
EM waves
- travel at same speed through air or vacuum
- they aren’t vibrations of particles, they’re vibrations of electric magnetic
- travel at different speeds in different materials
Wavelength an frequency in EM waves
- long wavelength: low frequency
- short wavelength: high frequency
Order of EM spectrum with decreasing wavelength
Radio waves, micro waves, infrared, visible light, ultra violet, x rays, gamma rays
What is refraction
- When a wave hits a boundary at an angle and it changes direction
What happens if a wave slows down or speeds up
- slow down=bends towards normal
- speeds up= bends away from normal
What is optical density of a material
- a measure of how quickly light can travel through it
- the higher the density, the slower the wave travels through it
What happens to wavelength during refraction
It changes
What happens to frequency in refraction
Stays the same
What is a ray
- straight lines that are perpendicular to wave fronts
- they show direction a wave is travelling in
What is the normal
An imaginary line that’s perpendicular to the point where the incoming wave hits the boundary
Wha is the angle of incidence
Angle between the incident ray and the normal
What is the angle of refraction
Between refracted ray and normal
What does a ray diagram tell us
- refracted ray bends towards normal and angle of refraction is smaller tan the angle of incidence so the second material is more optically dense
- if refracted angle is bigger than incidence angle then the second material is less optically dense
What is a wave front
It’s a line showing all of the points on a wave that are in the same position as eachother after a given number of wavelengths
How do wave front diagrams show refraction
- when a wave crosses a boundary at an angle, only part of a wave front crosses the boundary at first. If its travelling into a denser material, that part travels slower than the rest of the wave front
- so by the time the whole wave front crosses the boundary, the faster part of the wave front will have travelled further than the slower part of the wave front
- this difference in distance travelled by the wave front causes the wave to refract
How are low frequency waves harmful to people
- radio waves
- don’t transfer much energy
- mostly pass through soft tissue without being absorbed
How are high frequency waves harmful to people
- uv, x rays an gamma rays
- all transfer lots of energy so can cause lots of damage
How does UV radiation cause damage
- damages surface cells
- can lead to sunburn ad cause skin to age prematurely
- moe serious effects are blindness and an increased risk of skin cancer
How do x rays and gamma rays cause damage
- they’re both types of ionising radiation
- they carry enough energy to knock electrons off of atoms
- this can cause gene mutation or cell destruction and cancer
What is the radiation dose
- measured in sieverts (1000mSv=1Sv)
- its a measure of the risk of harm from the body being exposed to radiation
- the risk depends on the total amount of radiation absorbed and how harmful the type of radiation is
What are radio waves made by
- radio waves can be produced using an alternating current in an electrical circuit
- the object in which electrons oscillate to create the radio waves is called a transmitter
- when transmitted radio waves reach a receiver, the radio waves are absorbed
- the energy carried by the waves is transferred to the electrons in the material of the receiver
- this energy causes the electrons to oscillate and, if the receiver is part of a complete electrical circuit, it generates n alternating current
- this current has the same frequency as the radio wave that generated it
How are frequency of waves and ac related
In EM waves, the frequency of the waves produced will be equal to the frequency of the alternating current
EM waves and AC
- EM waves are made up of oscillating electric and magnetic fields
- AC is made up of oscillating current
- as the charges oscillate, they produce oscillating electric and magnetic fields i.e. electromagnetic waves
Long-wave radio
- Long-wave radio (1-10km) can be transmitted from London and be received halfway around the world
- This is because long wavelengths diffract around the curved surface of the earth
- they can also diffract around hills, into tunnels
- this makes it possible for radio signals to be received even if the receiver isn’t in line of sight of the transmitter
Short-wave radio
- wave lengths of about 10m-100m
- they can be received at long distances from the transmitter
- this is because they’re reflected from the ionosphere - an electrically charged layer in the earths upper atmosphere
- Bluetooth uses short wave radio waves to send data over short distances between devices without wires
- these waves can also reflect from the ionosphere, depending on atmospheric conditions and time of day
Ha type of wavelength do TV and FM radio transmissions use
- very short wavelengths
- to get reception, you must be in direct sight of the transmitter- the signal doesn’t bend or travel far through buildings
Microwaves and satellites
- microwaves can pass easily through earth’s watery atmosphere
How does satellite TV work
- the signal from a transmitter is transmitted into space
- where its picked up by the satellite receiver dish orbiting 1000s of kilometres above the earth. The satellite transmits the signal back to earth in a different direction
- where its received by a satellite dish on the ground
- there’s a slight time delay between the signal being sent and received because of the long distance the signal has to travel
How are microwaves used in microwave ovens
- the microwaves are absorbed by water molecules in food
- the microwaves penetrate up to a few cm of food before being absorbed and transferring the energy they are carrying to the water molecules in the food causing the water to heat up
- the water molecules then transfer this energy to the res of the molecules in the food by heating - which quickly heats the food
What is infrared radiation used for
- To increase or monitor temperature
- infrared radiation is given out by all objects, the hotter the object, the more IR radiation it gives out
How does electric heaters work
- they contain a long piece of wire that heats up when a current flows through it
- this wire then emits lots of infrared radiation (and a little visible light)
- the emitted IR radiation is absorbed by the objects and the air in the room
- energy is transferred by the IR waves to the thermal energy stores of the objects, causing their temperature to increase
How do infrared cameras work
- they can be used to detect infrared radiation and monitor temperature
- the camera detects IR radiation and turns it into an electrical signal, which is displayed on a screen as a picture
- the hotter the object is, the brighter it appears
What is visible light used for
- optical fibres are thin glass or plastic fibres that can carry data over long distances as pulses of visible light
- they work because of reflection
- visible light is used in optical fibres because it is easy to refract light enough so that it remains in a narrow fibre
- light is also not easily absorbed or scattered as it travels along a fibre
What is UV radiation used for
- sun tan- exposure
- fluorescent lights generate UV radiation, which is absorbed and re emitted as visible light by a layer of phosphor inside of a light bulb, makes them energy efficient
- security pens, under UV light the ink will glow
- tanning salons
What are x-rays and gamma rays used for
- x-ray photographs: x-rays easily pass through flesh but not so easily through denser material like bones or metal. So its the amount of radiation thats absorbed that gives you an x ray image
- x-rays and gamma rays are used to treat people with cancer: this is because high doses of these rays kill all living cells
- gamma radiation can also be used as a medical tracer: this is where a gamma emitting source is injected into the patient and its progress is followed around the body
- x rays and gamma rays are harmful so radiographers wear lead aprons and stand behind a lead screen or leave the room to keep their exposure to a minimum
Investigating absorption of different materials with melting wax trick
- Two ball bearings are each stuck to one side of a metal plate with solid pieces of candle wax. The other sides of these plates are then faced towards the flame
- The sides of the plates that are facing towards the flame each have a different surface colour - one is Matt black and the other is silver
- The ball bearing on the black plate will fall first as the black surface absorbs more infrared radiation - transferring more energy to the thermal energy store of the wax
- This means the wax on the black plate melts before the wax on the silver plate
What is a Leslie cube
- a hollow, watertight, metal cube made of eg aluminium, whose 4 vertical faces have different surfaces
Investigating emission with a Leslie cube
- Place an empty Leslie cube on a heat-proof mat
- Boil water in a kettle and fill the Leslie cube with boiling water
- Wait a while for the cube to warm up, then hold a thermometer against each of the 4 vertical faces of the cube. All 4 faces should be the same temperature
- Hold an infrared detector a set distance away from one of the cube’s vertical faces, and record the amount of IR radiation detects it
- Repeat this for each of the cube’s vertical faces
- There should be a result of more IR from the black surface than thee white one, and more from the Matt surfaces than thee shiny ones
- Repeat experiment to make sure results are accurate
