Waves Flashcards
define a progressive wave
a progressive wave transfers energy from one place to another
define transverse waves
In a transverse wave, the particles oscillate perpendicular to the direction of the wave.
Consists of peaks (maximum positive displacement) and troughs (maximum negative displacement).
give examples of a transverse wave
water waves, EM waves
define longitudinal waves
In a longitudinal waves, the particles oscillate parallel to the direction of propagation of the wave
Consists of rarefactions (low pressure regions), and compressions (high pressure regions)
give example of longitudinal waves
sound waves, P-waves in earthquakes
define displacement
displacement is the distance from a point on the wave to the equilibrium
define amplitude
amplitude is the maximum particle displacement from the equilibrium
define wavelength
wavelength is the minimum distance between two points in phase, crest to crest/trough to trough
define time period
time period is the time taken for one complete oscillation
define frequency
frequency is the number of oscillations per unit time
define wave speed
wave speed is the distance travelled by the wave per unit time
what does phase difference state
phase difference describes how far out of sync two points on a wave are
equation for phase difference
phase difference = (x/λ) * 360°(or 2 pi)
where x = distance between two points on the waves
squares on an oscilloscope have what length
1cm
what does each horizontal space represent on an oscilloscope
a time interval
what does the height represent on an oscilloscope
voltage
How are frequency and period linked?
Frequency = 1/Period
They are reciprocals of each other.
equation for wave speed
v = fλ
Speed = Frequency x Wavelength
What is a standing wave?
A wave that stores energy rather than transferring it from one place 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 2π.
What is path difference a measure of?
Path difference is a measure of how far ahead one wave is compared to another.
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 happens 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 string depend on?
- Tension
- Mass per unit length of the string
equation used to calculate the speed of a transverse wave on a string
v = √(T/μ)
v = speed
T = tension in the string
μ = mass per length of the string
equation for intensity
I = P/A
P is the power
A is the area
What is the unit for intensity?
Wm^-2
What is the refractive index of a material through which light travels a speed of ‘v’?
n = c/v
Where c is the speed of light in a vacuum.
State the equation linking the refractive indexes and angles at an interface between two mediums.
n₁ sinθ₁ = n₂ sinθ₂
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.
equation for the 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 lens?
P = P₁ + P₂ + P₃ +…
Unit for power
m^-1
What is a real image?
A real image is one that can be projected onto a screen and i 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 oscillations 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
n = no. order of maxima
λ = wavelength
d = space in between slits
θ = angle between two maximas
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 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 absorption spectrum?
The frequencies at which a certain element absorbs photons at a higher rate as these frequencies correspond to the spacings between energy levels in the element’s atoms.
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 and 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
State the equation used to calculate a de Broglie wavelength.
λ = h/mv
h is Planck’s constant
What can ‘mv’ be replaced with in the de Broglie equation?
p, momentum
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
h is Planck’s constant
f is the frequency of light
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 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 electron.
State the equation for the maximum kinetic energy of a photoelectron.
½ mv^2 [max] = hf - ɸ
ɸ is the metal’s work function
What is the conversion factor between eV and J?
1eV = 1.6 x 10⁻¹⁹J
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.
