4.4 Waves Flashcards
what is a progressive wave?
a progressive wave carries energy from one place to another without transferring any material, the transfer of the material is in the same direction as the wave
what are the two types of waves?
transverse and longitudinal
define longitudinal waves
waves which have oscillations parallel to the direction of the wave (or wave propagation)
define transverse waves
waves which have oscillations perpendicular to the direction of the wave (or wave propagation)
define displacement
the distance a point on a wave has moved from its undisturbed/equilibrium/rest position, can be positive OR negative, measured in metres
define amplitude
the maximum displacement, the distance from a peak or trough to its rest position, measured in metres
define wavelength
the distance between two successive identical points on a wave that have the same pattern of oscillation
define period
the time it takes for one complete oscillation to occur at any point
define phase difference
the difference in phase angle by which one wave lags behind another, measured in radians or degrees
define frequency
the number of complete oscilations that pass a point per unit time
define wave speed
the distance the wave travels per unit time
what is the equation for frequency?
f = 1/T (T = period)
outline an experiment using an oscilloscope to determine the frequency of a wave
- an oscilloscope displays a voltage-time signal from either a microphone or a signal generator (microphone converts sound waves into electrical signals)
- the screen is split into divisions, voltage on y axis (controlled by GAIN DIAL, volts per division), time on the x axis (controlled by TIMEBASE DIAL, seconds per division)
- use frequency = 1/t (t=period of wave in seconds) to calculate frequency
(note if the time base is turned off, the wave no longer moves across the screen making it easy to look at the intensity of the wave)
in a graphical representation of a longitudinal wave, what parts are the peaks and troughs?
rarefraction = trough (where the lines far apart) compression = peak (where the lines bunch up)
what is reflection and what is the key rule?
reflection occurs when a wave bounces off a surface and changes direction when it hits a boundary, the angle of incidence is ALWAYS equal to the angle of reflection
what is refraction?
refraction occurs when a wave changes speed and direction as it travels through a different medium, this depends on how optically dense the medium is (bending of a wave)
what is diffraction?
the spreading of a wave through a small gap or around an obstacle, the effect is most significant when the gap width is equal to the wavelength, noticeable effects when the gap width is several wavelengths wide
what is plane polarisation? what types of waves can be polarised?
plane polarisation is when a wave is restricted so that it only oscillates in one direction, only TRANSVERSE waves can be polarised
what is the wave speed equation?
v = fλ
with questions concerning EM radiation, all EM waves travel at c = 3 x10^8
outline an experiment using a ripple tank to investigate wave effects (reflection, diffraction and refraction)
First, set up a ripple tank of water with a light directly above it and a straight bar motor connected to a paddle, the wave fronts will be visible on the screen below the tank.
REFLECTION:
-to investigate reflection place a barrier in the tank at an angle to the wave fronts, the waves will reflect off the barrier and travel in a different direction to the way they arrived, remember angle of incidence θi = angle of reflection θr
DIFFRACTION:
-to investigate diffraction place an object in the ripple tank to create a barrier with a gap in the middle of it, this gap can be varied to see the effects this has on how the waves spread through the tank, remember when the gap is close to or the same size as the wavelength = most diffraction
REFRACTION:
-to investigate refraction a glass sheet can be used to decrease the water depth and produce a region with a different wave speed
outline an experiment using polarising filters to polarise visible light
place a light source in front of two polarising filters (unpolarised light is in all directions), keep the first filter in a fixed position and rotate the second to change the intensity from maximum light to no light, rises and falls as the angle is changed remember MALUS’ LAW
how does polarisation work?
just like vectors, you can think of the transmission axis of the rotating filter as having a vertical and horizontal component, the larger the vertical component, the more vertically polarised light will pass through the filter.
as the second filter is rotated horizontally, less light will get through it as the vertical component of the second filter’s transmission axis decreases which means the intensity through the second filter decreases
what is Malus’ law and what does it tell you?
I = Io x cos^2θ
-it tells you the intensity of plane polarised light that passes through a filter
I = final intensity after passing through second filter
Io = initial max intensity
θ = angle between the first and second filter
outline an experiment to observe polarisation of microwaves
- place a metal grille in between a microwave transmitter and receiver (opposite sides of grille), connect the receiver to an ammeter
- the microwave transmitter produces vertically plane-polarised radiation
- the metal grille absorbs radiation of the same plane as the radiation meaning when the metal grille is horizontal very few of the microwaves will be absorbed so the ammeter will show a high output, when the metal grille is vertical all of the microwaves will be absorbed meaning the ammeter will show no output
what is the formula for intensity of a progressive wave and what is the relationship between intensity and amplitude?
intensity = power/area
intensity is directly proportional to amplitude squared
this comes from the fact that intensity is proportional to energy and the energy of a wave depends on the square of the amplitude
what are the main properties of EM radiation?
- they all travel at c (3x10^8m/s)
- they are transverse
- they consist of an electric and magnetic filed that are at right angles to each other and the direction of wave travel
- they can be refracted, reflected, diffracted, polarised and can undergo interfernce
what is the order of the EM spectrum?
remember hammond’s mnemonic
Radio, Micro, Infrared, Visible, Ultraviolet, X-Ray, Gamma
order of magnitude for radio
10^6 - 10^-1 m
order of magnitude for micro
10^-1 - 10^-3 m
order of magnitude for infrared
10^-3 - 7x10^-7 m
order of magnitude for visible
7x10^-7 - 4x10^-7 m
order of magnitude for ultraviolet
4x10^-7 - 10^-8 m