Chapter 5 Waves Flashcards
Define amplitude
A wave’s maximum displacement from the equilibrium position
Define frequency
The number of complete oscillations passing through a point per second
Define period
The time taken for one full oscillation
Define wave speed
The distance travelled by the wave per unit time
Define wavelength
The length of one whole oscillation - the distance between a point on the wave and the same point on the following one
What is the wave equation?
- v = fλ
- wave speed = frequency x wavelength
Describe a longitudinal wave and what they are made up of.
The oscillation of particles is parallel to the direction of energy transfer. They are made up of compressions (high pressure regions) and rarefactions (low pressure regions). They cant travel in a vacuum
Which direction are particles moving in a longitudinal wave?
Particles are always trying to move to the nearest compression and so move away from rarefactions
Describe a transverse wave.
- The oscillation of particles is perpendicular to the direction of energy transfer.
- All EM waves are transverse and travel at 3x10^8 ms-1 in a vacuum
What can displacement-distance graphs of waves tell you?
You can calculate the wavelength
What can you calculate from a displacement-time graph of a wave?
The time period
Define a wave’s phase.
- The phase of a point on a wave relative to the origin can be defined as the fraction of the wave cycle that has elapsed since the origin
- The position of a certain point on a wave cycle. Can be measured in degrees, radians or fraction of a cycle
Define phase difference.
How much a particle / wave lags behind another. Same units as phase
Define path difference
The difference in the distance travelled by two waves
Define superposition
Where the displacements of two waves are combined as they pass each other. The resultant displacement is the vector sum of each wave’s displacement
Define coherence
Coherent waves must have the same frequency and wavelength and a fixed phase difference
Define wavefront
A surface / line connecting points on a wave that have the same phase
What are the two types of interference?
- Constructive interference: occurs when the waves are in phase and so their displacements are added
- Destructive interference: occurs when the waves are completely out of phase and so their displacements are subtracted
When are two waves in phase?
Two waves are in phase if they are both at the same point of the wave cycle: the same frequency and wavelength.Their phase difference is a multiple of 360 degrees / 2pi
What is a progressive wave?
One that transfers energy from one point to another without any transfer of matter
What is a standing wave?
A wave that stores energy rather than trasnferring it from one place to another
What is path difference a measure of?
Path difference is a measure of how far ahead one wave is compared to another
What is a wave’s phase?
A wave’s phase at a given point is a measure of how far through its cycle the wave is. It is usually measured in radians, where a complete cycle is 2pi
What happens when two waves meet in phase?
They will undergo constructive interference
What happens when two waves meet in antiphase?
They will undergo destructive interference
How is a standing wave formed on a string?
A wave reflects from a closed end meaning two identical waves are travelling in opposite directions down the same string
- At points where the waves meet in phase, constructive interference occurs and an antinode is formed
- At points where the waves meet in antiphase, destructive interference occurs and a node is formed
What is a node?
A point of minimum displacement - there is no movement from the equilibrium position
What is an antinode?
A point of maximum displacement
What two factors does the speed of a transverse waves on a string depend on?
- Tension
- Mass per unit length of the string
State the equation used to calculate the speed of a transverse wave on a string
v = sqrt(T/μ)
State the equation used to calculate intensity
I = P / A
Intensity = Power / Area
What is the refractive index of a material through which light travels a speed of ‘v’ ?
n = c/v = sin(i) / sin(r)
where c is the speed of light in a vacuum
i is the angle of incidence, r is the angle of refraction
State the equation linking the refractive indexes and angles at an interface between two mediums
n1 sin θ1= n2sinθ2
What is the critical angle?
The angle of incidence for which the angle of refraction is 90 degrees and all the light passes along the boundary between the mediums. Beyond this angle all light will be reflected
State the equation used to calculate a critical angle
sin C = 1/n
(if in air)
What is total internal reflection?
Total internal reflection is where all the light is reflectde back at the boundary between two mediums. It occurs when light is incident at an angle greater than the critical angle
What is the focal point of a converging lens?
The single point where all the parallel rays of light entering the lens converge to
What is the focal length of a lens?
The distance between the lens and the focal point
What is the equation used to calculate the power of a lens?
Power = 1 / focal length
How do you calculate the total power of a combination of thin lenses?
P = P1+P2+P3+…
What is a real image?
One that can be projected onto a screen and is always inverted. Real images are the consequence of light meeting at a focal point
What is a virtual image?
virtual images are the consequences of rays of light appearing to meet at a point. They cannot be projected onto a screen
State the equation used to calculate the magnification of an image?
Magnification = Image Height / Object Height
What is plane polarisation?
When the oscillations of a wave are restricted to a single plane
What is diffraction?
The spreading out of a wave as it passes through a gap
What criteria must be met for maximum diffraction to occur?
The size of the gap (the aperture) must be the same magnitude as the wavelength of the wave
What happens if the gap is much smaller than the wavelength of the wave?
The wave willl be reflected
State the diffraction grating equation.
nλ =d sinθ
What does electron diffraction provide evidence for?
The wave nature of electrons. It suggests that particles can demonstrate wavelike properties
Describe the diffraction pattern produced by electrons.
Concentric circles of bright and dark fringes from a central bright point
If electrons didnt have a wave nature, describe the pattern that would be produced when they pass through a slit.
The electrons would be unaffected by the gap and pass straight through. A single bright region would be formed
What is the name given to the wavelength of a particle?
De Broglie wavelength
What two factors does the de Broglie wavelength depend on?
Mass
Velocity
State the equation used to calculate the de Broglie wavelength.
λ = h/mv
lambda = plancks constant / mass x velocity
What is the basic process of a pulse-echo technique?
A wave pulse is emitted. It is transmitted and reflected at the boundary between two media. The returning wave (echo) is detected. The speed and time taken are used to calculate the distance to the object.
Suggest two things that may limit the amount of information that can be obtained by a pulse echo technique
The wavelength of the radiation
The duration of the pulse
What are the two models that can be used to describe electromagnetic radiation?
The wave model
the particle model
What model does the photoelectric effect provide evidence for?
The particle model
Outline the photoelectric effect.
Light is shone on a metal plate. If the light has a high enough frequency, electrons are emitted from the metal surface. If the frequency is too low, no electrons are emitted
What are the particles of light used to explain the photoelectric effect called?
Photons
How do you calculate the energy of a photon?
E = hf
Energy = plancks constant x frequency
Explain how a photon can liberate an electron.
One photon interacts with one electron and transfers all of its energy to it. If this energy is greater than the metal’s work function, the electron will have sufficient energy to be released
What is threshold frequency?
A metal’s threshold frequency is the minimum frequency that a photon requires to liberate an electron from its surface
If the intensity of light being shone on a metal increases, how does the enrgy of the photoelectrons change?
The energy remains unaffected. An increase in intensity means more photons per area and so more photoelectrons are emitted
Why are photoelectrons emitted with a range of kinetic energies?
The electrons are at different depths in the metal and so require different amounts of energy to be liberated. The excess energy from a photon once an electron has been liberated, is the kinetic energy of the photon
State the equation for the maximum kinetic energy of a photon.
hf = ϕ + 1/2mv^2
ϕ is the metal’s work function
What is the conversion factor between eV and J?
1eV = 1.6x10^-19 J
What happens when electrons transition between energy levels?
If electrons move to a higher energy level, radiation must be absorbed
If electrons move to a lower energy level, radiation is emitted
Why can only certain frequencies of radiation be absorbed by an atom to cause an electron transition?
the electrons only exist in discrete energy levels. The energy of the photon absorbed must be the exact amount of energy required to cover the difference between two discrete energy levels
What are EM waves emitted and absorbed by and what is the exception?
Electrons, except gamma which is emitted by nuclei
What is the range of wavelengths of visible light?
400-750nm
Blue light has the lowest wavelength at 400nm and red light has the largest
For a sound wave, what is the impact of a higher frequency?
It leads to a higher pitch
For a sound wave, what is the impact of a greater amplitude?
The sound will be louder
What is the relationship between amplitude and intensity?
Intensity is proportional to Amplitude^2
(if amplitude doubles, intensity quadruples)
Define refraction
When waves change direction and speed as they enter a new medium
How can you remember which way a ray refracts when going into a new medium?
FAST
Faster Away Slower Towards (the normal)
Why does a prism disperse light?
in reality, different wavelengths of light have slightly different refractive indexes eg blue light has the highest n, so it refracts more than red
What conditions must be necessary for TIR?
- The light must be IN the medium with the HIGHER refractive index(n)
- The angle of incidence (i) must be greater than the CRITICAL ANGLE
What is an application of TIR?
Optic fibres
What is multiplexing?
Sending multiple signals using different wavelengths down same fibre
What are the two problems with optic fibres?
Material Dispersion - blue light travels slower than red light
Multipath Dispersion - light of the same wavelength taking slightly different routes and therefore time to reach the end, results in pulse broadening
How do fibre optics mitigate their problems?
- Thinner core
- Repeaters
- Graded fibre
How does graded fibre work?
n decreases closer to the edge, so rays taking the longer route travel faster and curve back, meaning they arrive at the same time
What is the method for drawing a ray diagram?
Draw a line straight through the centre of the lens
Draw a line parallel to the principle axis, until the lens and then through the principle focus
Draw the image where they meet
What are the two ways of calculating magnification?
M = i / o = v / u
= image height / object height
= image distance / object distance
What is the lens equation?
1/f = 1/u + 1/v
What is the image produced by a convex lens at a distance from the lens that is greater than 2f?
- real image
- inverted
- diminished
What is the image produced by a convex lens at a distance from the lens that is between f and 3f?
- real image
- inverted
- magnified
Describe the image produced by a convex lens closer than f to the lens?
virtual image
upright
magnified
Describe the image produced by a concave lens at any distance
virtual image
upright
diminished
Which type of waves cannot be polarised?
Longitudinal
How do you calculate phase difference?
Δd/λ x360 or 2 pi
Δt /T x 360 or 2pi
What can points on a stationary wave only be?
In phase or in antiphase
What is the length of the string when there is a first harmonic on a stationary wave?
L = 0.5λ
Describe how stationary waves can be produced in tubes?
A node must be made at a closed end of a tube and an antinode at an open end
What are the conditions for coherence?
Same frequency, same wavelength and kept at a constant phase difference
What is Young’s double slit equation?
w = λD /s
where w is fringe spacing, D is slit-screen distance, s is slit separation
Why is Young’s double slit equation limited?
It only applies if D»_space; s
What happens when white light passes through the double slit experiment?
It results in coloured fringers as red light diffracts more than blue light because it has a larger wavelength
How is the number of orders related to the wavelength from the diffraction grating equation?
They are inversley proportional
How does a flourescent tube work?
A free electron from a cathode excites a low pressure mercury gas aotm.
A UV photon is produced, this is absorbed by the coating which excites an electron in the coating
The electron de-excites and a visible light photon is emitted
What happens if you put a sheet of metal with light incident on it, in a circuit?
A small current will flow
What is ‘stopping potential’
the pd required in the opposite direction to the sheet of metal, stopping the current
What piece of equipment is needed for both the photoelectric effect and electron diffraction?
An evacuated tube
How is momentum related to kinetic energy?
Ek = p^2 /2m
