Chapter 4.4 - Waves Flashcards
Progressive wave
A wave in which the peaks and troughs move through the medium as energy is transferred.
Sound Waves, P-Waves
Displacement
Distance of a point from its rest position
Amplitude
Maximum magnitude of the displacement
Wavelength
The length of one full wave cycle
Period
Time taken for a whole cycle to complete
Frequency
Number of full cycles passing a point per second
Phase
A measurement of position of a certain point along a wave cycle
180deg = 1/2wavelength 360deg = 1 wavelength
Think of a circle i.e 180 degrees = half circle = half wavelength
Phase difference
Difference in phase between two points
Equation for wave speed
v=fλ
How an oscilloscope works
y axis is voltage, x axis is time and it plots a wave
Transverse waves
A wave in which the medium is displaced perpendicular to the direction of energy transfer - the oscillations of medium particles are perpendicular to the direction of travel.
Longitudinal waves
A wave in which the medium is displaced in the same line as the direction of energy transfer - oscillations of the medium particles are parallel to the direction of wave travel.
Intensity in terms of waves
The rate of flow of energy per unit area at right angles to the direction of travel
Intensity = Power / Area
4pir^2
How intensity is related to amplitude
Intensity is proportional to amplitude squared
Polarisation
To restrict vibrations of a transverse wave to a single direction
How visible light can be polarised
Using a polarising filter
How microwaves can be polarised
Using a metal grille. When the plane of oscillation is parallel to that of the grille, all the of energy will be absorbed and no polarised light will pass through, although some will be emitted in random directions. Most of the light makes it through when the plane is perpendicular (seems counter intuitive)
Diffraction
When waves spread out when they go through a gap
When will maximum diffraction occur
When the gap width is equal to the wavelength
How a ripple tank works and can be used to view wave effects
Ripple tanks are shallow tanks of water, where an oscillating paddle generates horizontal waves. Objects can be placed in the tank to create barriers
How intensity of a polarised wave is related to angle
cos^2 of the angle
Main principle of reflection of waves
angle of incidence = angle of reflection (where they are measured from the normal)
What happens when light is shone through a single slit
You get a diffraction pattern
Refraction
The change in direction of a wave when it enters a different medium
Does a wave entering a denser medium bend towards or away from the normal
Towards the normal
Refractive index of a material
The ratio between c and the speed of light in that material
n = c / v
How to work out the new angle when light is refracted
n1sinθ1 = n2sinθ2
1 is a material and 2 is the other material
How to investigate refraction
Use a raybox to shine a beam of light through a glass block and look at the light
When total internal reflection occurs
when θ2 is equal to 90
therefore sin C = n2/n1 (n2 is usually air and is therefore 1)
Total internal reflection
When light would enter a less dense medium but instead refracts so much that it is instead reflected off the inner surface
How to investigate total internal reflection
Use a lightbox to shine a beam at light into a semi-circular glass block (shine it so it would hit the centre of where the full circle would be). Vary the angle of incidence. (The light should not be refracted when entering or leaving the block which allows you to just observe the total internal reflection)
Principle of superposition of waves
When multiple waves cross, the resultant displacement equals the vector sum of the individual displacements
How in-phase waves interfere
Constructively
How waves that are nπ out of phase interfere
constructive if n is even
destructive if n is odd
Coherent
Same wavelength, frequency and a fixed phase difference
Path difference
The distance that one wave has moved further than another one
How to observe interference with sound waves
Connect two speakers to the same oscillator and place them in line. Walk across the room parallel to them and there will be spots of loud and quiet
Monochromatic
Only one wavelength present
Young’s double slit experiment
A coherent, monochromatic light is shone through two small slits onto a screen. The light then interferes and creates light and dark minima and maxima on the screen. Wavelength is calculated by λ = ax/D where λ is wavelength a is slit seperation x is the spacing between adjacent maxima D is the distance to the screen
How to investigate interference of microwaves
Two microwave transmitter cones are connected to the same signal generator. A microwave receiver probe can then be moved along perpendicular to the direction of the waves and it should see alternating patterns of strong and weak signals
Requirement of sizes for youngs double slit experiment
a «_space;D
What is a diffraction grating
Basically loads of slits
Advantage of a diffraction grating over using two slits
The maxima are brighter and narrower and therefore easier to measure
How to calculate wavelength of light from a diffraction grating
nλ = dsinθ where λ is wavelength n is the order d is the slit seperation θ is the angle of incidence
What pattern is produced when white light is shone through a diffraction grating
The central maxima is white, but all they other maxima are spectra of visible light
Stationary wave
The superposition of two progressive waves with the same wavelength, moving in opposite directions
How to create a standing wave in a string
Fix one end and attach the other to an oscillator
Node
Point on a standing wave where there is no movement
Antinode
Point on a standing wave with maximum amplitude
The main distinction between progressive and stationary waves
Progressive waves transfer energy whereas standing waves store energy
How stationary waves work in air columns
Nodes at closed ends and antinodes at open ends
How to produce stationary waves with microwaves
Set up a microwave transmitter and place a metal plate in front of it to reflect the microwaves. Between these you can place a microwave receiver to observe the stationary wave
Distance between adjacent nodes
λ/2
Distance between adjacent node and antinode
λ/4
Resonant frequency
A frequency of wave that for a given system will produce an exact number of half wavelengths along the system
Length of the system when vibrating at the third harmonic with nodes at both ends
3λ/2
Fundamental mode of vibration
The 1st harmonic - the lowest possible resonant frequency
Experiment to determine speed of sound using stationary waves
Set up a measuring cylinder with some water inside of it. Place a hollow plastic tube inside the measuring cylinder and get a tuning fork and record its frequency (it will be labelled on it). Sound the tuning fork above the cylinder and move the plastic tube up and down. At certain heights, a loud sound should be emitted, which means that you have found a harmonic. To find the 1st harmonic, find the smallest distance between the top of the water and the top of the plastic tube that creates this sound. This distance will then be λ/4 and the speed of sounds can then be calculated
