4 Waves Flashcards
What are electromagnetic waves?
oscillating magnetic and electric fields that progress through space without needing a medium
What is a longitudinal wave?
A wave in which the direction of vibration of the particles is parallel to the direction of travel
What is a transverse wave?
a wave in which the direction of vibration is perpendicular to the direction of travel
What is plane-polarisation?
when the vibrations of a wave stay in one plane only
what does unpolarised mean?
when the vibrations of a wave change from one plane to another
what type of waves can be polarised?
transverse
what does displacement mean?
the distance and direction of a vibrating particle from its equilibrium position
what is the amplitude of a wave?
the maximum displacement of a vibrating particle
what is the wavelength of a wave?
the least distance between two adjacent vibrating particles with the same displacement and velocity at the same time
what is the cycle of a wave?
fro maximum displacement to the next maximum displacement
what is the period of a wave?
the time for one complete wave to pass a fixed point
what is the frequency of a wave?
the number of cycles of vibration of a particle per second, or the number of complete waves passing a point per second
what equation links frequency and period?
T = 1/f
what is the phase of a vibrating particle?
the fraction of a cycle it has completed since the start of the cycle
what is phase difference?
the fraction of a cycle between the vibrations of the two particles vibrating at the same frequency
what is the formula for phase difference?
phase difference (in radians) = 2 π d / λ (lambda)
what are wavefronts?
lines of constant phase (lines which connect particles in phase)
what is reflection?
when a wave hits a hard, flat surface the wave is reflected at the same angle
what is the rule for reflection?
angle of incidence = angle of reflection
what is refraction?
the bending of light due to a change in wave speed. this happens as the wavelength changes. this causes the wave to change direction
when does refraction occur?
when the wave is aimed at the boundary not along the normal
What is diffraction?
When waves spread out after passing through a gap or round an obstacle
what increases diffraction?
- a narrower gap
- a longer wavelength
how are satellite dishes designed to use reflection and diffraction?
larger dishes:
- reflect more radio waves to the aerial
what is the downside of larger satellite dishes?
a bigger dish reflects waves into a smaller focus as they diffract less. this means they have to be aligned more carefully
what is superposition?
when waves meet, pass through each other, combine and then move apart
what does the principle of superposition state?
when two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point
what is a supercrest or supertrough?
supercrest - when a crest meets a crest and the two waves reinforce each other
supertrough - when a trough meets a trough and the two waves reinforce each other
what happens when a crest meets a trough of the same amplitude?
the resultant displacement is 0
what are progressive waves?
a wave that transfers energy from one point to another without transferring the medium itself
what are stationary waves?
when two progressive travel through each other and combine. they then form fixed points with no displacement called nodes
what is an antinode?
the point on a stationary wave with maximum displacement
what is interference?
when two waves interact and they have identical phase, frequency and amplitude
what is constructive interference?
when two waves have displacement in the same direction (eg. two crests or two troughs)
what is destructive interference?
when one wave has positive displacement and the other has negative displacement
when does total destructive interference occur?
when the two waves have equal but opposite displacements
what does coherent mean?
when the waves the same phase or constant phase difference
what are stationary waves?
wave pattern with nodes and antinodes formed when two or more progressive waves of the same frequency and amplitude pass through each other.
what are some properties of stationary waves?
- stationary waves that vibrate freely do not transfer energy to their surroundings
- distance between adjacent nodes = 1/2 wavelength
- the phase difference between two vibrating particles:
➢ zero if the two particles are between adjacent nodes or separated by an even number of nodes
➢ 180 degrees or 𝜋 radians if the particles are separated by an odd number of nodes - all particles except those at nodes vibrate at the same frequency
- the amplitude varies from zero at the nodes to a maximum at the antinodes
How are stationary waves formed on a string?
- A progressive wave is sent out by the vibrator.
- the crest reverses its phase when it is reflected at the fixed end of the string.
- it then travels back along the string as a trough
- It is reflected and reverses phase again once it reaches the vibrator
- this wave can be reinforced by crests from the vibrator. it can also deconstructively interfere with a trough that has been reflected
what is the first harmonic pattern of vibration?
the pattern seen at the lowest possible frequency that gives a pattern.
what is an osciloscope?
a specially made electron tube and associated control circuits.
(an electron gun ad glass tube emits an electron beam towards the fluorescent screen)
what are oscilloscopes used for?
displaying the current/voltage of what it is connected to so it can be measured
in an oscilloscope, what is the displacement of a spot proportional to?
the applied pd
what are the X-plates connected to and what do they adjust?
the oscilloscope’s time base circuit
they make the spot move at a constant speed left to right across the screen
what is connected to the Y-plates and what do they adjust?
the pd to be displayed
they make the spot move up and down as it moves left to right across the screen
how do you work out the frequency of a wave from an oscilloscope?
- look at the time base
- count how many squares it takes for however many complete waves are displayed on the screen.
- multiply the number of squares by the time base
- divide this measurement by the number of waves measured
- use f = 1/T to find the frequency
