Waves Flashcards
Define a Transverse wave
A wave in which the direction of vibration is perpendicular to the direction in which the wave travels.
State the condition required for a transverse wave to be polarised
It’s vibrations must stay in one plane only
Explain how longitudinal waves become polarised
They can’t
Define the plane of polarisation of an electromagnetic wave
The plane in which the electric field oscillates
Define a longitudinal wave
A wave in which the direction of vibration of the particles is parallel to the direction in which the wave travels
Define the displacement of a wave
A particles distance and direction from its equilibrium position
State an equation to calculate speed (c) given f
C = fλ
Define the phase difference of 2 vibrating particles
The fraction of a cycle between the vibrations of the two particles
Where 1 cycle = 360° = 2π
∴ phase difference (in radians) = 2πd / λ
(Where d = distance along the wave)
State the principle of superposition
When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point
What is the distance between adjacent nodes in a stationary wave?
λ/2
State the 3 conditions required for the formation of a standing wave
1) 2 waves travelling towards each other
2) Same frequencies
3) Superpose
On a standing wave, what are the names given to:
i) points of no amplitude
ii) points of maximum amplitude
i) nodes
ii) antinodes
State a difference between energy transfer of a progressive wave and a stationary wave
Stationary waves do not transfer energy
Describe the phase of all the particles between adjacent nodes
They are all in phase
Describe the phase of the particles either side of a node
They are out of phase
Define electromagnetic waves
Vibrating electric and magnetic fields that progress through space without need for a substance
The vibrating electric field creates a vibrating magnetic field, which generates a vibrating electric field further away, and so on
State the types of electromagnetic waves in order of lowest to highest frequency
- Radiowaves
- Microwaves
- Infra Red
- Visible Light
- Ultra Violet
- X-Ray
- Gamma Ray
Define amplitude
The maximum displacement of a vibrating particle
For a transverse wave it is the height of a wave crest or the depth of a wave trough from the middle
Define wavelength
The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time (e.g. distance between adjacent crests)
Define complete cycle
From the maximum displacement to the next maximum displacement (e.g. from one wave peak to the next)
Define wave period (Time period)
The time for one complete wave to pass a fixed point
Define frequency and give its units
The number of cycles of vibration of a particle per second, or the number of complete waves passing a point per second
Units: Herts (Hz)
Give the equation for the time period of a wave
T = 1 / f
State and describe the set up of a piece of apparatus to view the interference of light by waves
A ripple tank
An electronic ‘dipper’ is set up just above a shallow, transparent tank of water with sloping sides. A stroboscope is used in accordance with a lamp so that the wavefronts are projected onto a surface below
State the significance of the sloped sides of a ripple tank
It prevents the waves from reflecting off the sides of the tank, which would make it more difficult to see the wavefronts projected.
Describe the reflection of a straight wave on a hard flat surface
Straight waves directed at a certain angle to a hard flat surface reflect off at the same angle.
Therefore, the direction of the reflected wave is at the same angle to the reflector as the direction of the incident wave
Define refraction
When waves pass across a boundary at which the wave speed changes, the wavelength also changes
If the wavefronts are at a non-zero angle to the boundary, they change direction as well as speed
State when the refraction of light is observed
Refraction of light is observed when a light ray is directed into a glass block non-normally
The light ray changes direction when it crosses the glass boundary because light waves travel more slowly in glass than in air
Define diffraction
Diffraction occurs when waves spread out after passing through a gap or round an obstacle
State when diffraction is observed in water
When straight waves in a ripple tank are directed at a gap
What 2 factors affect the amount of diffraction for waves passing through a gap
- The narrower the gap, the more the waves spread out
- The longer the wavelength. the more the waves spread out
In which direction do satellite dishes in Europe need to point and why?
South, because the satellites orbit the Earth directly above the equator
Explain the significance of the size of a satellite dish on the signal it receives
The larger the satellite dish, the stronger the signal it can receive, because more radiowaves are reflected by the dish onto the aerial.
Define superposition
When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point
Where waves interact, what happens when:
i) A crest meets a crest?
ii) A trough meets a trough?
iii) A crest meets a trough?
i) A supercrest is created
ii) A supertrough is created
iii) The resultant displacement is 0; the two waves cancel each other out
Give 2 examples of superposition
1) Stationary waves on a rope
2) Water waves in a ripple tank
What must the 2 sources of waves be to produce a interference between the 2 waves and why is this effect shown?
Coherent
An interference pattern where they overlap is produced, because they vibrate at the same frequency with a constant phase difference
Describe an experiment to demonstrate the reflection, refraction, interference and polarisation of microwaves
The transmitter produces microwaves of wavelength 3.0cm. The reciever can be connected to a suitable meter, which gives a measure of the intensity of microwaves at the reciever
1) Place the receiver in the path of the microwave beam from the transmitter. As you move it further away from the transmitter the signal decreases, showing that microwaves become weaker at further distances from the transmitter
2) Place a metal plate between the transmitter and receiver to show that microwaves cannot pass through metal
3) Use 2 metal plates to make a narrow slit and show that the receiver detects microwaves that have been diffracted as they pass through the slit. Show that if the slit is wider, less diffraction occurs
4) Use a narrow metal plate with 2 plates either side to make a pair of slits. Direct the transmitter at the slits and use the receiver to find points of cancellation and reinforcement, where the microwaves from the 2 slits overlap
Describe how stationary waves are formed
A stationary wave is formed when 2 progressive waves pass through each other.
This can be achieved on a string in tension by fixing both ends and making the middle part vibrate, so progressive waves travel towards each end, reflect at the ends and then pass through each other
Define a node
A fixed point of no displacement
State the distance between adjacent nodes
Distance between adjacent nodes = ½λ
Define antinode
The point midway between 2 adjacent nodes which has maximum amplitude
Define fundamental mode of vibration
The simplest stationary wave pattern on a string, consisting of a single loop that has a node at either end
Describe the energy transfer for stationary waves vibrating freely
They do not transfer energy because the nodes and antinodes are at fixed positions
Describe stationary waves in terms of the progressive waves that cause them
Considering the 2 progressive waves passing through each other:
- When they are in phase, they reinforce each other to produce a large wave
- A quarter of a cycle later, the 2 waves have each moved one-quarter of a wavelength in opposite directions, thus are now in antiphase, so cancel each other out
- After a further quarter cycle, the 2 waves are back in phase. The resultant is again a large wave except reversed
For 2 vibrating particles in a stationary wave, describe:
i) The amplitude
ii) The phase difference
i) The amplitude of a vibrating particle varies with position from 0 (at a node) to maximum (at an antinode)
ii) The phase difference between 2 vibrating particles is:
- 0 if the two particles are between adjacent nodes or separated by an even number of nodes
- 180° (= λ radians) if the 2 particles are separated by an odd number of nodes
Give the similarities between the frequency of stationary and progressive waves
Stationary: All particles except at the nodes vibrate at the same frequency
Progressive: All particle vibrate at the same frequency
Give the differences between the amplitude of particles of stationary and progressive waves
Stationary: The amplitude varies from 0 (at the nodes) to a maximum (at the antinodes)
Progressive: All particles have the sane amplitude
Describe the phase difference between 2 particles for stationary and progressive waves
Stationary: Equal to mπ, where m is the number of nodes between the two particles
Progressive: Equal to 2πd/λ, where d is the distance apart and λ is the wavelength
State the resonant frequency for the standing waves in an air-filled pipe closed at one end
In a pipe closed at one end, the resonant frequencies occur when there is an antinode at the open end and a node at the other end
Describe how standing waves can be detected using microwaves
A microwave transmitter is directed at a metal plate which reflects them back towards the transmitter.
When a detector is moved along the line between the transmitter and the metal plate, the detector signal is found to be 0 at equally spaced positions (half wavelengths apart) because the reflected waves interfere with the waves from the transmitter and form a stationary wave pattern
Define fundamental pattern of vibration
The lowest possible frequency produced by a vibrating system. Has an antinode at the middle as well as a node at either end.
Because the length L of the vibrating section of the string is between adjacent nodes and the distance between adjacent nodes is ½λ - the wavelength of the waves that form this pattern, the fundamental wavelength is:
λ₀ = 2L
Give the equation for the fundamental frequency from the fundamental wavelength
λ₀ = 2L
f₀ = c / λ₀ = c / 2L
where c is the speed of the progressive waves on the wire
Define first overtone
The next stationary wave pattern from the fundamental pattern of vibration, where there is a node at the middle, so the string is in 2 loops
Therefore the waves that form this pattern:
λ₁ = L
Because each loop has a length of half-a-wavelength
Give the equation for the frequency of the first overtone, and its relation to the fundamental frequency
λ₁ = L f₁ = c / λ₁ = c / L = 2f₀
Define second overtone
The next stationary wave pattern from the fundamental pattern of vibration, where there is an antinode in the middle and the nodes are distance ⅓L from each other
The wavelength of waves that form this pattern are:
λ₂ = ⅔L
State the condition for the formation of a stationary wave in terms of the time take for a wave to travel along the sting and back
The time taken for a wave to travel along the string and back must be equal to the time taken for a whole number of cycles of the vibrator
f = mc /2L = mf₀
where m is a whole number
For a string, how would you:
i) raise the pitch?
ii) lower the pitch?
i) Increasing the tension or shortening the length
ii) Decreasing the tension or increasing the length
