Topic 5 Flashcards
What is a wave’s amplitude?
The amplitude of a wave is its maximum displacement form its equilibrium position
What is a wave’s frequency?
The frequency of a wave is the number of complete waves that pass a point per second
What is meant by the period of a wave?
The period of a wave is the length of time taken for one complete wave to pass a given point
How are frequency and period linked?
Frequency = 1/Period
They are reciprocals of each other
What is wavelength?
The distance between the same point on two adjacent waves (for example peak to peak or trough to trough)
State the equation used to calculate a wave’s speed
v=fλ
Speed = Frequency x Wavelength
Describe a longitudinal wave
Particle oscillations are parallel to the direction of the wave’s motion
Consists of rarefractions (low pressure regions), and compressions (high pressure regions)
Decribe a transverse wave
Particle oscilations are perpendicular to the direction of the wave’s motion
Consists of peaks (maximum positive displacement) and troughs (maximum negative displacement)
What is a progressive wave?
A progressivewave is 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 transferring 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 that wave is. it is usually measured in radians, where a complete cycle is 2π
What is the phase difference between two waves at a given point?
The phase difference is the difference between the phases of the two waves - in other words, it is a measure of the difference between how far each wave is through its cycle.
What happens when two waves meet?
The two waves will interfere with each other
What happens when two waves meet in phase?
They will interfere and undergo constructive interference
What happns when two waves meet in antiphase?
They will interfere and 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 node is a point of minimum displacement - there is no movement from the equilibrium position
What is an antinode?
An antinode is a point of maximum displacement
What two factors does the speed of a transverse wave on a strign depend on?
Time period
Mass per unit length of the string
State the equation used to calculate the speed of a transverse wave on a string.
V=√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
refractive index = speed of light / speed
State the equation linking the refractive indexes and angles at an interface between two mediums
n₁sinθ₁ = n₂sinθ₂
What is the critical angle?
The angle of incidence for which the angle of refraction is 90° 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
sinC = 1/n
What is total internal reflection?
Total Internal reflection is where all the light is reflected 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 the parallel rays of light entering the lens converge to
What is the focal length of a lens?
The distance between the centre of 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?
A real image is 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 consequence 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?
Plane polarisation is when the oscilations of a wave are restricted to a single plane.
What is diffraction?
Diffraction is 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 must be of 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 will be reflected
State the diffraction grating equation
nλ = dsinθ
What does electron diffraction provide evidence for?
The wave nature of electrons. It suggests that particles can demostrate wavelike properties
Describe the diffraction pattern produced by electrons
Concentric circles of bright and dark fringes from a central bright point
If electrons didn’t 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 a de Broglie wavelength
λ = h/p
De Broglie Wavelength = Planck’s constant / momentum
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
Which 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 = Planck’s constant x Frequency
Explain how a photon can liberate an electron
One photon interacts with one electron and transfers all 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 energy 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 dephs 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 kenetic energy of the electron
State the equation for the maximum kinetic energy of a photoelectron
1/2mv^2 = hf - Φ
Kinetic energy = planck’s constant x frequency - the metal’s work function
What is the conversion factor between eV and J?
1eV = 1.6x10^-19J
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 can 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
