4.4 WAVES Flashcards
what is a progressive wave?
It transfers energy from one point to another through a medium without any net movement of the medium itself. This movement of energy is characterized by oscillations or vibrations that travel through the medium, allowing the wave to progress or move forward.
what are the two types of waves?
transverse and longitudinal
define longitudinal waves
waves have oscillations that are parallel to the direction of energy transfer
define transverse waves
waves which have oscillations perpendicular to the direction of the energy transfer
define displacement
Distance from the equilibrium position in a particular direction
define amplitude
the maximum displacement from the equilibrium position
define wavelength
minimum distance between two points in phase on adjacent waves, for example, the distance from one peak to the next or from one compression to the next
define period
the time it takes for one complete oscillation to occur at any point
define phase difference
The measure of how much one wave is out of step (or phase) with another wave. It is expressed in degrees (from 0° to 360°) or radians (from 0 to 2π), where 360° or 2π radians represent a full cycle.
When are waves in phase?
A phase difference of 0° or 0 radians means the waves are in phase
When are waves out of phase?
a phase difference of 180° or π radians means the waves are in complete opposition, or out of phase.
For example when one particle reaches its maximum displacement at the same time as the other reaches its maximum negative displacement
define frequency
the number of complete oscillations 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)
what is the phase difference formula
Phase difference = x (distance between two points) / wave length x 360° (or 2π)
Examples of transverse waves
Waves on the surface of water
all EM waves
Waves on a stretched string
S - waves produced in earthquakes
Examples of longitudinal waves
P - waves from earthquakes
Sound waves
outline an experiment using an oscilloscope to determine the frequency of a wave
An oscilloscope is used to determine the frequency of a wave
use a microphone to generate a graph of p.d. against time (microphone converts sound waves into electrical signals)
-voltage on y axis (controlled by GAIN DIAL, volts per division), time on the x axis (controlled by TIMEBASE DIAL, seconds per division)
-each horizontal square represents a certain time interval. This is called timebase
-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?
rarefaction = 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 changes direction at a boundary between two different media, remaining in the original equilibrium.
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
when is diffraction effect the most significant and what happens when the wavelength is smaller than the gap size
when the size of the gap is about the same size as the wavelength of the wave
Small wave length will not diffract
when waves reflect, refract and diffract what happens in terms of speed, wave length and frequency
reflect: frequency and wavelength do not change
Refract: has affect on wavelength, but not frequency
Diffract: speed, wavelength and f do not change
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, c=fλ as all EM waves travel at speed of light
outline an experiment using a ripple tank to investigate wave effects (reflection, diffraction and refraction)
Set up the apparatus as shown (look it up) and fill the ripple tank with water to a depth of no more than 1 cm
Turn on the power supply and the light source to produce a wave pattern on the screen
The wavelength of the waves can be determined by using a ruler to measure the length of the screen and dividing this distance by the number of wavefronts
The frequency can be determined by timing how long it takes for a given number of waves to pass a particular point and dividing the number of wavefronts by the time taken
Record the frequency and wavelength in a table and repeat the measurements
Speed => v=fλ
what is Malus’ law and what does it tell you?
I = Io x cos^2θ
I = final intensity after passing through second filter
Io = initial max intensity
θ = angle between the first and second filter
-it tells you the intensity of plane polarised light that passes through a filter
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
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^2
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 at a boundary, reflected at a surface, diffracted through a gap, polarised and can undergo interference
What is interference?
Interference occurs when two or more waves overlap and combine to form a new wave pattern. This can result in areas of increased amplitude (constructive interference) when the waves are in phase, or decreased amplitude (destructive interference) when the waves are out of phase.
what is the order of the EM spectrum?
Radio, Micro, Infrared, Visible, Ultraviolet, X-Ray, Gamma
order of magnitude for radio (m)
10^6 - 10^-1 m
order of magnitude for micro (m)
10^-1 - 10^-3 m
order of magnitude for infrared (m)
10^-3 - 7x10^-7 m
order of magnitude for visible (m)
7x10^-7 - 4x10^-7 m
order of magnitude for ultraviolet (m)
4x10^-7 - 10^-8 m
order of magnitude for x-rays (m)
10^-8 - 10^-12 m
order of magnitude for gamma (m)
10^-12m - 10^-16 m
frequency of radio
<(3 × 10⁹ Hz)
frequency of micro
(3 × 10⁹ Hz) to (3 × 10¹⁰ Hz)
frequency of infrared
(3 × 10¹⁰ Hz) to (4.3 × 10¹⁴ Hz)
frequency of visible
(4.3 × 10¹⁴ Hz) to (7.5 × 10¹⁴ Hz)
frequency of UV
(7.5 × 10¹⁴ Hz) to (3 × 10¹⁶ Hz)
frequency of X-rays
(3 × 10¹⁶ Hz) to (3 × 10¹⁹ Hz)
frequency of gamma
> (3x10¹⁹ Hz)
what is the formula for refractive index?
n = c/v
c = speed of light in a vacuum
v = speed of light in the medium
where
what does the refractive index tell us?
it tells us the ratio of the speed of light in the medium compared to the speed of light in a vacuum (measures how much the material slows down light)
if something has a high refractive index what does that mean about the speed of light in the medium?
slow speed
if something has a low refractive index what does that mean about the speed of light in the medium?
high speed
how does light bend when it moves into a medium with a high refractive index (slows it down)?
light bends TOWARDS the normal
how does light bend when it moves into a medium with a low refractive index (speeds it up)?
light bends AWAY from the normal
what is snell’s law?
n1sinθ1 = n2sinθ2
n1 = refractive index of first medium
θ1 = angle of incidence
n2 = refractive index of first medium
θ2 = angle of refraction
nsinθ = CONSTANT at a boundary where θ is the angle to the normal
what is total internal reflection and when does it occur?
when the light is reflected back into the material and it occurs when the angle of refraction is above 90 degrees and therefore the angle of incidence is more than CRITICAL (when it is equal to critical is will travel along the boundary of the material at 90 degrees)
what is the critical angle equation?
sinC = 1 / n
(n = refractive index of the material)
this comes from snells law as n2 = 1 because its in air and θ2 is 90 degrees (angle of refraction) making sin90 = 1
outline an experiment to investigate total internal reflection
shine a light ray into the curved face of a semi-circular glass block so that it always enters at right angles to the edge (this means the ray won’t refract as it enters the block, just when it leaves from the the straight edge)
vary the angle of incidence until the light beam refracts so much that it exits the block along the straight edge, this angle of incidence is critical
if you increase the angle of incidence so its greater than C, you’ll find the ray is reflected from the straight edge of the block (total internal reflection occurs)
