4.4 - Waves Flashcards
Define progressive wave
An oscillation that travels through matter (or in some cases a vacuum), transferring energy from one place to another, but not transferring any matter.
What are the two types of progressive wave?
Transverse, Longitudinal
Define transverse wave.
A wave with oscillations perpendicular to the direction of energy propagation.
Give an example of transverse wave.
Electromagnetic waves
Define longitudinal wave.
A wave with oscillations parallel to the direction of energy propagation. They cannot travel through a vacuum.
Give an example of a longitudinal wave.
Sound waves.
Define displacement
The distance from the equilibrium position in a particular direction.
Define amplitude.
The maximum displacement from the origin.
Define wavelength.
The minimum distance between two adjacent points on a wave oscillating in phase.
Define period.
The time taken for a full wavelength to pass a given point.
Define frequency.
The number of complete oscillations passing a given point per unit time.
Define wave speed.
The distance travelled by a wave per unit time.
Define phase difference.
The difference in phase between two points on a wave.
What is phase difference typically expressed in? What can also be used?
Radians. Degrees can also be used.
What are the two key areas in a longitudinal wave?
Compressions and Rarefactions.
What are two key areas in a transverse wave?
Peaks and troughs.
What does polarisation do?
It restricts the oscillations of a wave to one plane.
What does polarisation prove about EM waves?
They are transverse waves. If they were longitudinal, the forwards and backwards motion wouldn’t be stopped by crossed pieces of polaroid.
Define reflection.
When a wave rebounds from a barrier, changing direction but remaining in the same medium.
Define refraction.
When waves change direction when they travel from one medium to another due to a difference in the wave speed in each medium.
How is frequency changed during refraction?
It doesn’t. It remains constant.
Define diffraction.
The spreading out of a wave front as it passes through a gap.
When does maximum diffraction occur?
When the gap the wave passes through is the same size as the wavelength of the incident wave.
Why can longitudinal waves not experience polarization, compared to transverse?
The direction of energy transfer is already in one plane only.
With transverse, the oscillations occur in many planes, at right angles to the direction of travel.
What are two ways of demonstrating wave effects?
Ripple tank, polarizing filters.
Define the intensity of a progressive wave.
The radiant power passing at right angles through a surface per unit area.
Why do we use surface area of sphere to calculate intensity?
Light from a point source spreads out in a sphere.
What are EM waves?
Transverse waves consisting of magnetic and electric fields which oscillate at right angles to each other.
What speed do EM waves travel at?
Speed of light - 3.0x10^8ms
Define refractive index.
A material property equal to the ratio between the speed of light in a vacuum, and the speed of light in a given material.
A light ray is incident at 90 degrees to a material, and so it is refracted. How is this ray different in the new material?
It changes speed, but doesn’t change direction.
How is refractive index related to speed and density?
Higher refractive = slower speed
Higher refractive = Denser material.
What is total internal reflection?
When all of the light incident on the boundary is reflected back into the original medium.
What two conditions must be met for total internal reflection to occur?
Light must be travelling from a material with a higher refractive index, to a material of lower refractive index.
Angle of incidence of the ray to the normal must be above the critical angle.
Define critical angle.
The angle of incidence in an optically denser medium at which the angle of refraction in the less dense medium is 90 degrees.
sinC = 1/n
For what situation is this formula only true?
For refraction occurring where the original material is air.
What does it mean for two progressive waves to ‘superpose’?
It means they have met and overlapped to produce a single wave.
State the principal of superposition.
At the point where two or more waves meet, the resultant displacement is equal to the sum of the individual displacements of the wave.
Define coherence.
Waves with the same frequency and constant phase difference.
What are all the types of EM waves (from biggest wavelength to smallest wavelength)?
Radio, Micro, IR, Visible, UV, XRay, Gamma
How can we investigate superposition for sound waves?
Two audio signal generators will emit coherent waves in all directions, which overlap and form an interference pattern.
Connect a microphone to an oscilloscope, and move it parallel to the speakers. You will detect regions of loud and quiet noise.
What can we investigate using the young double slit experiment?
Superposition in light, and wavelength of light source being used.
You could also investigate superposition in microwaves with this.
How does young double slit work?
Laser producing monochromatic light is placed behind a sheet with two small slits in it, of separation distance ‘a’ apart.
The two coherent waves produced by the slits overlap and superpose, creating alternating bright (maxima) and dark (minima) fringes on a screen.
The distance between two adjacent maxima = x
The distance between the double slits and screen = d
What equation can we use as a result of the young double slit?
λ= ax / d.
λ: Wavelength
a: Distance between slits
x: Distance between two adjacent maxima
d: Distance between double slits and screen
What can diffraction grating be used to investigate?
The wavelength of a light source.
What is a diffraction grating?
A piece of transparent material with many opaque lines scratched into it.
How does light pass through diffraction gratings?
By passing through the transparent slits between the scratches.
When light passes through a diffraction grating, what is produced?
An interference pattern with bright and dark maxima and minima.
Diffraction gratings have the number of slits written on them. How is the number of slits usually given?
Slits per centimetre. You will need to convert this to metres between each slit.
What formula is used in diffraction grating to determine the wavelength of a light source?
dsin(Θ) = nλ
d = distance in metres between slits Θ = angle between 0th and nth maxima n = Order of maxima (whether it is original, first second etc. bright maxima)
What is monochromatic light?
Light of a single wavelength.
Define stationary wave.
A wave formed by the interference of two progressive waves (with the same frequency) travelling in opposite directions.
What are nodes?
Points which always have zero amplitude.
What are antinodes?
Points which always have maximum amplitude.
In terms of wavelength, how far apart are two adjacent nodes?
Half a wavelength
The frequency of a stationary wave is the same at all points except for where?
The node where it is 0.
How can you get the phase difference between two points of a stationary wave?
180 degrees X n
n = number of nodes between the two points.
How are stationary waves different from progressive waves in terms of energy?
They don’t transfer energy, but store it instead.
What is the process for producing a stationary wave in a stretched string?
The string is held taught over a pulley. A vibration generator is used to oscillate the string in a coherent manner, with the frequency being adjusted until a stationary wave is produced.
The initial wave produced is reflected at the pulley and, producing two waves with the same frequency, travelling in opposite directions.
Describe the arrangement of nodes for a stationary wave on a stretched string.
Pulley and transmitter ends will be nodes, with a node-antinode pattern along the string.
Describe the procedure for producing a stationary wave with sound in an air filled tube.
A tuning fork is used to produce a loud sound, and is then held over the end of the tube.
The length of the tube can be adjusted by placing one end in water until a stationary wave is produced.
A stationary wave is formed with sound in an air filled tube. How are nodes and antinodes formed here?
If the column is open at both ends, there is an antinode at each end.
If the column is open at one end, the open end has an antinode and the closed end has a node.
What does the number of nodes produced in a stationary wave depend on?
The frequency of the vibration.
What is the fundamental frequency of a stationary wave?
The lowest frequency of vibration for a given arrangement.
When a stationary wave vibrates at the fundamental frequency, what is this called?
The first harmonic.
Are stationary waves formed at frequencies other than multiples of the fundamental frequency?
No.
Stationary waves: As frequency increases (so moving to higher harmonics), what decreases?
Wavelength
