Flashcards in Waves Deck (114)
What do waves do?
Transmit energy not matter
What are the three pairs of words used to classify waves?
- Mechanical/ Electromagnetic
- Progressive/ Stationary
What is a mechanical wave?
A wave which requires a medium (particles) through which to transfer energy
What is an electormagnetic wave?
A wave which does not require a medium through which to transfer energy
What is a progressive wave?
A wave where there is a net transfer of energy from the source to a receiver
What is a stationary wave?
A wave where there is no net transfer of energy, the energy is stored in the wave
What is a transverse wave?
A wave where the direction of disturbance is perpendicular to the direction of energy transfer
What is a longitudinal wave>
A wave where the direction of disturbance is parallel to the direction of energy transfer
What do longitudinal waves consist of?
Compressions and rarefactions
What is amplitude?
The maximum dispacement of a wave from rest/normal/equilibrium position
What is wavelength?
The distance between two identical points on a wave ie. the distance for one complete wave cycle
What is time period?
The time taken for one complete wave cycle
What is frequency?
The number of wave cycles per second
What does frequency equal?
1/time period or wave speed/wavelength
What does time period equal?
What is phase?
A measure of how far through a cycle a wave has got e.g. 1/2 a cycle is 180 degrees and 1 cycle is 360 degrees
What is constructive superposition?
If waves meet in phase, they construct to produce increased amplitude
What is destructive superposition?
If waves meet out of phase, they destruct to produce zero amplitude
What does wave speed equal?
frequency x wavelength
What does wavelength equal?
What speed do all elctromagnetic waves travel at in a vacuum?
3.0 x 10^8 m/s
At around what speed do sound waves travel?
3.0x10^2 m/s in air at sea level
What is the wavelength of the different parts of the em spectrum?
10^3 , 10^-3, 10^-5, 10^-7, 10^-9, 10^-11, 10^-13
What are the frequencies of the different parts of the EM spectrum?
10^5, 10^11, 10^13, 10^15, 10^17, 10^19, 10^21
What is the use of radio?
What is the use of microwaves?
What is the use of infrared?
heating and cooking
What is the use of visible light?
seeing, visual imaging
What is the use of ultraviolet?
sun beds, security marking and forensics
What is the use of x-rays?
What is the use of gamma?
sterlization and imaging
What is radio detected by?
What are microwaves detected by?
What is infrared detected by?
What is visible light detected by?
What is ultraviolet detected by?
what are x-rays detected by?
CCDs (x-ray camera)
What is gamma detected by?
CCDs (gamma camera)
What are the dangers of radio?
What are the dangers of microwaves?
soft tissue damage
What is the danger of infrared?
What is the danger of visible light?
What is the danger of ultraviolet?
What is the danger of x-ray?
deep tissue cancers
What is the danger of gamma?
deep tissue cancers
WHat are radio and microwaves generated by?
Electrons in metals
What are Infrared, Visible and Ultraviolet generated by?
Electrons in atoms
How are x-rays generated?
Through the rapid deceleration of electrons
How is gamma generated?
Through nuclear energy changes
Explain how EM radiation is produced by atoms:
- electrons exist at discrete energy levels in atoms
- when the electrons gain energy, they move to higher energy levels
- but they instaneously fall back to the lower energy levels and emit photons of energy equal to the difference between levels
-photons are particles of EM radiation
What increases the frequency of EM radiation?
The bigger the energy difference when the electrons move between energy levels
What affect does the frequency of e.m. radiation have on the energy of the photons?
The higher the frequency of the radiation, the greater the energy of the photons associated with the EM radiation
What happens to the danger of the photon as it increases in energy? Why?
The danger of it to human tissue increases as its additional small wavelenght makes it likely to interact with matter on a cellular and atomic level.
How are ultraviolet photons produced in relation to electrons in atoms?
If electrons jump to the un-excited ground state
How are visible photons produced in relation to electrons in atoms?
If electrons jump to the 1st excited state
How is infrared produced in relation to electrons in atoms?
If electrons jumpt to the 2nd excited state
What produces radiowaves?
the oscillation of conduction electrons in circuits
How are radiowaves recieved?
- the transverse radio waves
- interact with electrons in a metal aerial or receiving circuit
- causing them to oscillate at the frequency of the radio wave
- producing an alternating current
What em radiation causes ionisation?
high frequency, high energy
What is a sievert?
A measure of the risk of harm resulting from body's exposure to radiation in a dosage
What is a black body?
- a perfect absorber and emitter of EM radiation
- emits and absorbs over the range of wavelengths of the EM spectrum
- emit and absorb most, a wavelength linked to their temperature
What are the key features of a cool object on an intensity-wavelength graph?
- peaks at a longer wavelength
- lower intensity
What are the key features of a hot object on an intensity-wavelength graph?
- peaks at shorter wavelength
- higher intensity
What is the best emitter and absorber of infrared radtion?
What is the worst emitter and absorber of infrared radiation?
Describe the experiment used to investigate the effect of colour on heating/infrared radiation:
- fill a Leslie's cube with hot water
- monitor the temperature at each face using a thermometer
- ensure the thermometer is a fixed distance from the cube and not actually touching it as temperature of the same metal will otherwise be measured
What would be the expected results for the Leslie's cube (highest to lowest relative temperature)?
matt black, shiny black, matt white, shiny silver
What is the control variable in the Leslie's cube experiment?
the material used for each face
How could the equipment be improved for the leslie's cube experiment?
using a IR sensitive camera/detector instead of a thermometer
What are the laws of reflection?
- Angle of incidence = angle of reflection
- object to mirror distance = mirror to image distance
What are the properties of a reflected image?
- it is the right way up
- but laterally inverted
- it is virtual
What does virtual mean?
It cannot be projected onto a screen (ie. real)
How do parabolic reflects reflect incident rays?
All to a single focus point
WHat are parabolic reflectors used in?
Satellite dishes and telescopes
Define light refraction:
- a change in the speed of waves
- when they cross the boundary
- between mediums of different optical density
- this results in a change in the direction of the propagation of the waves
What happens to the velocity and wavelength of a wave as it goes crosses the boundary of an object with greater optical density?
- the velocity and wavelength both reduce
How are wave fronts and incident rays related?
They are perpendicular to one another
What happens to the frequency of a wave during refraction?
stays the same
What is the refractive index?
The ration of the speed of light in air to the speed of light in an optically more dence medium . It is equal to sini/sinr
What is a convex (converging lens)?
A lens which is thicker in the middle than at the edges
What is focal length?
The distance between the lens and where the principle focus is
On what line does the principle focus focuss on?
The principle axis
What happens to light hitting a convex lens?
It is refracted to a single principle focus point
What must be true of an object for its image to be the same size?
it must be located two focal lengths away from the lens
What happens to the image if the object is placed in between 1 and 2 focal lengths from the lens? Why is it not a magnifying glass?
It is magnified and inverted and real
What has to occur for a lens to be a magnifying glass?
The object has to be located less than 1 focal length from the lens so that the rays diverge and their cross over is where the new larger image is formed.
What are the properties of the image with a magnifying glass?
What is a concave lens?
A lens which is thinner in the middle than at the edges. It causes light rays to diverge
What are the properties of an image of an object that has light passing trough a concave lens?
- not inverted
- diminished (not magnified)
What are the two types of reflection?
What is specular reflection?
Reflection from mirrors which form an observable image
What is diffuse reflection?
When parallel waves are scattered into different directions by a rough surface. This does not form a reflected image.
Why can't we see a clear image when light is reflected on a wall for example?
- the wall has bumps on its surface
- so the approaching light rays are reflected at many different angles in different directions by the wall
- this causes a blur of light
- this is diffuse reflection
Why do some objects appear white?
Because they reflect every wavelength of light equally
What objects reflect each wavelength equally?
- white objects
- a mirror
What is observed colour the result of?
Diffuse reflection and selective absorption
What will happen to white light hitting a red filter?
- the red part of the white light will be transmitted
- the orange, yellow, green, blue and violet will be absorbed
What will happen to white light hitting red surface?
- the red part of the white light will be reflected
- the orange, yellow, green, blue and violet will be absorbed
What does each colour in the visible spectrum consist of?
A wide range of wavelengths. A colour is not the result of the reflection of a single wavelength
What is the range of wavelengths for visible light?
400nm to 700nm continuously
What is the difference between a water wave and a sound wave?
water waves are transverse and sound waves are longitudinal
Describe how to calculate wavelength, frequency and wavespeed with a ripple tank:
- count the number of waves passing a fixed point in a fixed amount of time. Time using a stopclock.
- divide the number of waves by the time (s) to calculate the frequency of the wave
- Measure the wavelength from a frozen image which is enabled by a photograph from a camera or stroboscope
- do this by measuring the distance between e.g. 10 waves and divide this by the number of waves measured e.g. 10
- repeat measurements and calculate a mean
- calculate wave speed using: wave speed = frequency x wavelength
What does the frequency of a string vibration depend on?
- the length of the string that's free to vibrate
- the tension in the string (ie. the force to hold the string straight)
- the thickness of the string
Describe the wave speed through a solid required practical:
- connect string to a vibration generator connected to a signal generator , adjusting length using a wooden bridge and maintaining tension with masses on a hanger linked to the pivot
- adjust the frequency or wavelength of string until it looks like the waves are not moving and clear standing waves can be seen
- measure the length of one wavelength
- record the frequency on the signal generator
- calculate the speed using wave speed = frequency x wavelength
What is the relationship between wavelength and frequency of a string and why?
As wavelength increases, f decreases in proportion as wave speed in a string is constant for constant tension and thickness
What are the two types if seismic waves?
p-waves and s-waves
What are the properties of a p-wave?
- propagate through solids and liquids
- wave speed increases with density in earth
- propagate through the earth rather than just on the surface
What are the properties of an s-wave?
- propagate in the crust
- responsible for the aftershock in an earthquake
Why do p-waves arrive before s-waves?
as p-waves are longitudinal, they take a straight line path through the earth so it's a shorter distance
Describe the distribution of particles in a sound wave:
- there are compressions where particles are closer together and at a higher pressure
- there are rarefactions where particles are further apart and at a lower pressure
What determines the amplitude of a sound wave?
- the extent of the compressions and rarefactions
- a higher pressured compression and lower pressured rarefaction gives a higher amplitude
How can the speed of sound be measured?
- place the visible sound source and timer a certain distance away from eachother
- set of the soure and start timing on the visual signal given and stop timing on hearing the sound
- the speed of sound will be the distance/time measured
What is the greatest source of uncertainty in the speed of sound measurement? Why?
timing errors as unless the distance is very large, any timing error will be a large fraction of the measured time