4.4 Waves Flashcards
Frequency
Number of waves that occur in 1 second (Hz)
Period
time for a wave peak to travel a distance of one wavelength
progressive waves
Waves that transfer energy away from a source without transfering material
longitudinal
Oscillation moving parallel to the direction the wave is travelling in
Transverse
Oscillation moving perpendicular to the direction the wave is travelling in
Displacement
how far a point on the wave has moved from its undisturbed position
amplitude
maximum displacement of a wave from the rest position
wavelength
Distance between 2 points of identical displacemnt on consecutive cycles of a wave
phase difference
difference in degrees/radians/angle between points on the same wave or similar points on two waves
phase
a measurement of the position of a certain point along the wave cycle.
What’s a CRO and what dose it do?
A cathode ray oscilloscope which measures voltage.
It displays waves from a single generator as a function of voltage over time.
What do you call a displayed wave on a CRO?
Trace
What is the screen of a CRO split into?
Divisions
Whats on the vertical axis on a CRO?
Vertical: Volts. Volts per division is controlled by the GAIN DIAL (in Volts/div)
Whats on the horizontal axis on a CRO?
Horizontal: Seconds. aka the time base. seconds per division controlled by TIME BASE DIAL (in ms/div)
How do you work out what direction a point on the wave is travelling in from a snapshot?
- Look at which direction the wave is travelling in
- if its travelling left to right…
- look at the displacemnt just left to the point
- if its greater then the point..
- The point will need to move UPWARDS to have that displacement
Intensity
the rate of flow of energy per unit area at right angles to the direction of travel of the wave. Measured in Wm^-3
What can waves do?
Refract, reflect, diffract and undergo interference
What can all EM waves do?
Carry energy
be polarised
travel at a speed of 3x10^8 in vacuum
Radio waves
Approx wavelength (m): 10^-1 to 10^6
Penetration: Pass through matter
Uses: Radio transmission
Microwaves
Approx wavelength (m): 10^-3 to 10^-1 (10cm)
Penetration: Mostly pass through matter but cause some heating
Uses: Radar, microwave cooking, tv transmission
Infrared
Approx wavelength (m): 7x10^-7 to 10^-3 (1mm)
Penetration: Mostly absorbed by matter causing heating
Uses: Heat detector, night vision cameras, remote controls, optical fibres
Visible light
Approx wavelength (m): 4x10^-7 to 7x10^-7
Penetration: Absorbed by matter causing some heating
Uses: Human sight, optial fibres
UV
Approx wavelength (m):10^-8 to 4x10^-7
Penetration:Absorbed by matter, slight ionisation
Uses: Sun beds, security marks that show up under uv
X-rays
Approx wavelength (m):10^-13 to 10^-8
10^-10m
Penetration: mostly pass through matter, but causes ionisation as they pass
Uses: To see damage bones and teeth, airprt security scans, kill cancer cells
Gamma rays
Approx wavelength (m): 10^-16 to 10^-10
Penetration: mostly pass through matter, but causes ionisation as they pass
Uses: irradiation of food, sterilisation of med instruments, kill cancer cells
Diffraction
The spreading out or bending of a wave as they pass through a gap or around an obstacle. Diffraction is greatest when the wavelength is the same as the width of the gap
Refraction
Change of direction a light ray undergoes when it enters a medium with a diffrent optical density.
Optics
The study of the nature and behaviour of a visible lightalong eiht its interaction with matter
EM waves
travel as oscillations in electrical and magnetic fields, and can transmit energy
Mono chromatics
Emmited with only one wavelength
Coherant
All waves are either exactly in phase, or display a constant phase difference
collimated
narrow, approximately parallel beam.
1 wavelenth (landa) is = to how many radians and what degree?
1 wavelength = 2pie radians = 360 degrees
1 pie radians =
1/2 wavelegth = 180 degrees
What’s snell’s law of refraction?
n1 sin (theta) = n2 sin (theta)
n = c/v
What do they stand for
n is the refractive index of the substance
C is the speed of light in a vacuum
v is the speed of light in substance
Polarised wave
Transverse wave with Oscillations in one direction only
How can we prove light is transverse?
We can polarise it
What’s a polarising filter?
It’s used to polarise light and other waves, it only transmits light in one direction.
If you have two polarising filters at right angles to eachother, then no light will get through
Names some applications of plane polarised light?
TV signals
Radio waves
How do you find the critical angle?
Sin(theta)c = n2/n1
What’s intensity proportional to?
Intensity is proportional to amplitude squared
Why don’t polarising filters work on microwaves? What do we have to use instead?
Their wavelength is too long. So we use metal grilles
How do you investigate the polarisation of microwaves?
- Place a metal grille between the microwave transmitter and reciever
- The intensity of microwaves passing through the grille is at a maximum when the direction of the vibration of the microwaves and the wires on the grille are at right angles to each other.
- As you rotate the grille, the intensity of polarised microwaves able to pass through the grille decreases, so the reading on the voltmeter decreases
- When the wires of the metal grille are aligned with the direction of the polarised waves, no signal will be shown on the voltmeter
When does the intensity of a microwave drop to zero?
When the wires are aligned with the direction of the microwaves, because the grille is absorbing their energy.
Describe what happens in terms of polarisation for microwaves
- The vibrating electric field of the microwaves excited electrons in the metal grille
- The energy of the incoming microwaves is absorbed and re-emitted in all directions
- Only a few of those re-emitted waves are vibrating in the direction of the microwave reciever
- The microwave receiver only receives microwaves in one plane, so even if the re-emitted wave travels towards to receiver, it might not be picked up
- When the wires and vibrations of the waves are aligned, more electrons are excited than when the grille and vibrations of the wave are rapt right angles to each other- causing the drop in intensity.
What happens when the gap is a lot bigger then the wavelength?
diffraction is unnoticeable
What happens if the gap is the same size as wave length?
Diffraction is the most it can be
What happens when the gap is way to small?
Waves then reflected back on themselves
How do you demonstrate diffraction in light?
Shining a laser through a narrow slit
Vary diffraction by changing width of slit
Or
Use white light and set of coloured filters
Size of lit is constant, wave length varies with colour filters
2 points with a phase difference of zero or a multiple of 360 degrees are…
In phase
Points with a phase difference of odd-number multiples of 180 degrees (pie radians) are
Exactly out of phase
What is a refractometer and how do you use it?
It accurately measures the refractive index of a material. The machine shines a beam of light at the sample. You then view the refracted beam through a microscope and measure it’s angle of refraction.
What is total internal reflection?
If we increase angle of incidense above critical angle, no refraction will take place and all light will be reflected-totally internally reflected
How do you find the critical angle?
SinC=n2/n1
What’s the principle of superposition?
When 2 or more waves of the same type meet, the resultant wave can be found by adding the displacments of the individual waves.
What would the amplitude of the resultant wave be for: two waves A and B travelling in phase with the same amplitude? and what type intefernce is this?
Double that of the indiviudal wave.
Constructive interference
What would the amplitude of the resultant wave be for: two waves A and B travelling in antiphase with the same amplitude? and what type intefernce is this?
amplitude of 0 as they cancel out
destructive interference
What type light is coherant, filament bulb or laser beam?
laser beam
path difference (referred to in relation to wavelength)
the difference between the distance travelled by two or waves arriving at the same point.
What type path diffence would contructive interference have?
whole number of wavelengths
like:
0, 360, 720 in degrees
0, 2pie 4pie in radians
What type path difference would destructive interference have?
Odd number or half wavelengths
like:
180, 540, 900 in degrees
pie, 3pie, 5 pie in radians
How do you investingate interference with sound waves?
2 loud speakers connected to same signal generator.
walk in front of speakers.
you hear a loud sounds where the waves reinforce one another.
hear a quiet sound where waves partially cancel out each other
(cover one ear for clearer variation)
During investigating interference with sound waves:
The distance between the loud and quiet regions is shorter for low frequencies
T or F
False
During investigating interference with sound waves:
The distance between the loud and quiet regions is longer for low frequencies
T or F
True
wavelength= aX/D
What does a, X, and D stand for?
where a<
a=slit seperation in m
X=Distance from centre point to the first bright frindge (m)
D=Distance of the sceen from slit
What has a longer wavelength red or violet?
RED
What has a larger frequency red or violet?
VIOLET
For diffraction grating questions, if they want the maximum number of order what do you do?
Take the angle as 90 degrees
Whys diffraction grating better then young s double slit
Since there are more slits interference patters are sharper as there are so many beams reinforcing the pattern
so its:
More precise
Easier to measure
Why do astronomers and chemist often need to study spectra to help identify elements?
to identify elements, diffraction gratings more accurate then prism
Stationary waves
superposition of two progressive waves with the same wavelength, moving in opposite directions
Node
amplitude of vibration 0
Antinode
Amplitude of vibration is maximum
What are the similarities and differences between progressive and stationary waves?
A progressive wave transfers energy/information (in
the direction of the wave)/all points have (the same)
amplitude
In a stationary wave there is no net energy
transfer/energy is stored/has points which are always
zero amplitude/or points have different amplitudes
What happens at resonant frequency’s on a stationary wave in terms of wavelength
Exact number of half wavelength fit on a string
Stationary waves using stretched string
Attach vibration transducer to end of stretched string with other end fixed.
Alter signal generator so it produces exact number of waves in the time it takes to get to end and back and then the original and reflected waves reinforce each other.
Generated wave reflects back and forth
Whats the fundamental mode of vibration also known as?
First harmonic, It is the lowest frequecny at which resoance can occur
(Shown as half a wavelength, one loop
NAN)
Stationary waves using microwaves
Microwave transmitter produce microwaves
Reflected of metal reflecting plate
Reflected and incoming waves set up stationary wave
Find nodes and anti nodes by setting microwave receiver between transmitter and reflecting plate
If node detected meter will read min value
Where do longitudinal stationary waves form?
Wind instuments or other air collumns
What forms at close end of instrument?
Node
How do you use stationary waves to measure the speed of sound?
- Create closed end pipe (put tube in water measuring cylinder
- Get tuning fork and note frequency of it
- Tap fork and hold above tube (Sound wave travels down tube and reflects at water boundary forming node)
- Move tube up down till you find shortest distance between top of tube and water level that the sound from the fork resonates at
- The distance is 1/4 of wavelength
- use v= frequency x wavelength
Interference
Superposition of coherent waves so there’s a change in overall displacement
Between two nodes all parts of the wave vibrate in phase
T OF F
T
f is proportional to root T
True or false
f=frequency
T=Tension
T
Explain the term critical angle (3 marks)
The angle of incidense at which the angle of refraction is 90 degrees and the refracted ray travels along the boundary between the 2 materials. Above the critical angle light does not leave the first material but is entirely reflected at the boundary
Explain why only monochromaticsources of light can be coherent? (2 marks)
Monochromatic sources consist of light of a single colour ie. frequency which is a nessisary condition for the sources to be coherant. The realtvie phase of two different frequency sources would be constanlty changing and so the light could not be coherant.
d x sin(theta)=?
n x wavelength
