Waves Flashcards
What is a PROGRESSIVE WAVE?
It moves energy from one place to another without transferring any matter
How does a WAVE work?
Either matter or a field oscillates at a source, which passes through a medium or field when it oscillates which takes energy away from the source
Describe a reflection
When a wave is bounced back when it hits a boundary
Describe a refraction
When the wave changes direction as it enters a different medium - as a result of the wave slowing down or speeding up
Describe a diffraction
When the wave spreads out as it passes through a gap or round an obstacle
What is displacement in terms of waves?
How far a point on a wave has moved from its undisturbed position
What is amplitude in terms of waves?
The maximum magnitude of displacement that a wave can be from an undisturbed position.
What is wavelength in terms of waves?
The length of one whole oscillation (length between two whole crests or two equal point on a wave)
What is a period in terms of waves?
The time taken for one whole wave cycle
What is frequency in terms of waves?
The number of whole waves passing a certain point per second
What is a phase in terms of waves?
A measurement of the position of a certain point along the wave cycle
What is phase difference in terms of wave?
The amount which one wave lags behind another wave
can be measure in degrees, radians, or fractions of a cycle
What is the equation for FREQUENCY?
frequency=1/time(or period)
f(Hz) = 1
—–
T(s)
What is the equation for WAVE SPEED
Wave speed = frequency x wavelength
or wave speed = distance/time
c(ms^-1)=f(Hz) x lambda(m)
What is the speed of an EM wave is a vacuum?
3 x 10^8 (ms^-1)
How could you measure the speed of a wave in water?
- Set up a ripple tank with a strobe light over it with a signal generator and a ripple tank dipper
- Measure the depth of the water, then set the dipper to a frequency with regular vibrations
- Dim the rooms light and match the strobe light frequency so that the waves appear to be standing still
- Record the frequency on the signal generator and the distance between adjacent peaks and then dividing it by the amount of troughs in between them
- Then use the equation c=f x wl to calculate the wave speed
What is a TRANSVERSE WAVE?
It is a wave that oscillates particles or the field perpendicular to the direction of travel
How could you draw a transverse wave on a graph?
Displacement against time
or
Displacement against distance
What is a LONGITUDINAL WAVE?
It is a wave that oscillates particles or the field parallel to the direction of travel
What is a polarising filter?
A filter that only lets light waves oscillate through it in one direction - so if you had two filters at right angles to each other no light could pass through
What are the uses for polarising filters?
To reduce glare in sunglasses
To improve TV and radio signals
When does superposition occur?
When two or more waves pass through each other - the displacements in each wave combine
What is constructive inference?
When waves meet and their displacements are in the same direction they combine to make a larger net displacement
What is destructive interference?
When waves meet and the positive displacement of one wave meets a wave with a negative displacement of another wave and cancel each other out
What is total destructive interference?
When two waves of equal and opposite displacements meet and completely cancel each other out
What is a stationary wave?
When two progressive waves superposition and they have the same frequency(or wavelength) and amplitude and are moving in opposite directions
Do stationary waves transmit energy?
NO
What is a resonant frequency?
When a source produces an exact number of waves in the time it takes a wave to get to be reflected
What does a resonant frequency cause on a string?
A stationary wave where the pattern does not move it just vibrates up and down
What is a node?
A node is a point on a wave where amplitude is zero
So total destructive interference
What is an anti-node?
An antinode is a point on a wave where the amplitude is at maximum
So constructive interference
What is the first harmonic?
When a stationary wave is vibrating at its minimum possible resonant frequency
2 nodes 1 antinode
What is the equation for wavelength for a standing wave?
wavelength =2 x total length / number of harmonics(or antinodes)
lambda = 2 x L
————
n
Explain what you would see in Kundt’s tube if you fired sound waves through it and there was powder in the tube?
You would see that gradually the powder would clump together
This is because the sound wave reflects back and superposes the fundamental wave, therefore they super pose and a stationary wave is made.
Where the powder gathers is where the nodes are as they dotn shake the powder at all.
What is the equation for the frequency of the first harmonic?
frequency = (1/2 x length of the vibrating string)xsqroot(tension/mass per unit length)
f= 1 T(N)
————– x —————-
2L(m) mu(kg/m)
Describe an investigation on resonant frequencies
- Measure the mass and lengths of several different strings that you will test - Then find mu by using
mu = mass (kg)
————
length (m)
- Then get a signal generator, transducer, string, clamp, pulley and masses and set the up accordingly
- Then measure the length between the transducer and the pulley this will be L. Also work out the tension in the string by using
T=mass (kg) x acceleration due to gravity (ms^-2)
- Then turn on the transducer and vary the frequency until the first harmonic is found
- Then start changing the variables such as masses on the end of the string - the string itself - or the length of the string
- Should find that the longer the string the lower the resonant frequency - the heavier the string the lower the resonant frequency - the lower the tension on the string the lower the resonant frequency
How is a stationary wave made?
(When a wave generated by the source is reflected back, and the amount of waves generated is the same as the time it takes for a wave to get reflected and received again (in phase))
The waves be travelling in opposite directions must have equal wavelength and frequencies - but only similar amplitude
What happens when a wave travels trough a gap that is a lot larger than its wavelength?
Unnoticeable diffraction
When does the most diffraction occur?
When the wavelength is the same as the gap the wave is travelling through
What happens to a wave when the gap it is trying to travel through is smaller than its wavelength?
The waves are mostly reflected back
What type of light should you use to observe a diffraction pattern?
Monochromatic/coherent light
What is monochromatic light?
It is light of a single wavelength and frequency, so a single colour
To get monochromatic light use a laser
Describe a diffraction grating pattern
There are bright fringes where the light waves constructively interfere
There are dark fringes where the light waves destructively interfere
What happens when white light diffracts?
All the different wavelength in the diffract by different amounts, so instead of getting clear bright and dark fringes - you get a spectra of colours
Describe a white light diffraction pattern
Dark fringes
white central fringe
the bright fringes would be blue on the side nearest the centre
(or red on the side away from the centre)
Define Intensity
The power per unit area
How can you change the width of the central maximum in diffraction?
Increasing the slit width decreases the amount of diffraction - central maxima is narrower, and more intense
Increasing wavelength - increases diffraction - central maxima wider but less intense
What is PATH DIFFERENCE?
The amount by which the path travelled by a wave is longer than the path travelled by the other wave
If a waves paths difference is half the wavelength what occurs?(Coherent light)
Total Destructive interference - so dark fringes
As waves arrive at the boundary out of phase
