Module 4.4 - Waves Flashcards
Define progressive wave
A wave that transfers energy away from a source
Define wavelength
The smallest distance between one point on a wave and the identical point on the next wave
Define period
Time taken for one complete pattern of oscillation
Define frequency
The number of oscillations per unit time
Define displacement
The distance travelled in a particular direction by a wave from its mean or rest position
Define amplitude
The maximum displacement of a wave from its mean or rest position
Define phase difference
The difference (in radians) in the phases of two waves of the same frequency
Define path difference
The difference (in metres) between the distances travelled by two waves arriving at the same point, in relation to the wavelength
Define intensity
The rate at which energy is transferred from one location to another as the wave travels through space, perpendicular to the direction of wave travel
Define reflection
When waves rebound 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
Define diffraction
When a wave spreads out after passing around an obstacle or through a gap
Define interference
The addition of two or more waves that results in a new wave pattern due to the principle of superposition
Define electromagnetic wave
A self-propagating transverse wave that does not require a medium to travel through
Define polarisation
The process of turning an unpolarised wave into a plane-polarised wave
Define plane-polarised wave
A transverse wave oscillating in only one plane
Define polarising filter
Produces a plane-polarised wave of light by selective absorption of one component of the incident oscillations
The filter transmits only the component of light polarised perpendicular to that direction
Define refractive index
A value that indicates how quickly or slowly EM radiation will travel through a medium
Define total internal reflection
When EM radiation travels from a material of a higher refractive index to one of a lower refractive index and the angle of incidence is larger than the critical angle
Define critical angle
The angle of incidence when the angle of refraction is 90°
What is the principle of superposition?
It states that when two or more waves of the same type meet, the resultant wave can be found by adding the displacements of the individual waves
Define constructive interference
Occurs when two waves meet in phase
Define destructive interference
Occurs when two waves arrive at a point out of phase
Define coherent
Two waves with a constant phase relationship
Define diffraction grating
A piece of optical equipment made from glass, onto which many thousands of very thin, parallel and equally spaced grooves have been accurate,y engraved using a diamond
Define order
The number of the pattern (n) on either side of the central maximum
How to calculate period from an oscilloscope
Distance between peaks x time base setting
What does it mean if two waves are in phase?
Exact same pattern of oscillation
No phase difference
What does it mean if two waves are in antiphase?
Exact opposite pattern of oscillation
Half a cycle phase difference
How to covert from degrees to radians
Divide by 180
Multiply by pi
How to convert from radians to degrees
Multiply by 180
Divide by pi
How to derive the wave equation
Speed = distance / time
In one period, T, wave travels one wavelength: v = wavelength / T
F = 1 / T, so substitute f for 1 / T
V = wavelength x frequency
Why is intensity proportional to the amplitude squared?
The amplitude of a wave decreases as the wave spreads out from a source
Energy of a wave is proportional to the amplitude squared
Therefore intensity is also proportional to the amplitude because intensity relates to energy transfer
When is diffraction shown the most clearly?
Wavelength of diffracted wave is equal to gap that it is travelling through
Law of reflection
Angle of incidence = angle of reflection
Snell’s Law
n1 x (sin x incident angle) = n2 x (sin x refracted angle)
Formula for critical angle
sin x critical angle = n2 / n1
n1 = refractive index of more dense medium
n2 = refractive index of less dense medium
What happens when the angle of incidence is less than the critical angle?
The ray is refracted with very little reflection
What happens if the angle of incidence is equal to the critical angle?
The ray emerges along the edge of the block
What happens if the angle of incidence is greater than the critical angle?
The ray is totally internally reflected
Conditions for total internal reflection
Angle of incidence must be greater than the critical angle
Incident substance must have larger refractive index
Applications of total internal reflection
Fibre optics to send information (often telephone calls or internet access) at the speed of light through a fibre optic
Quicker and can carry more information
Diffraction if the gap is smaller than the wavelength?
No diffraction
Waves are reflected by barrier
Diffraction if width of gap is similar to wavelength?
Diffraction occurs
Diffraction if width of gap is equal to wavelength?
Maximum diffraction
Diffraction if width of gap is larger than wavelength?
Slight diffraction at waves’ edges
Similarities between progressive and stationary waves
Neither transfers matter
Both can be longitudinal or transverse
Differences between progressive and stationary waves
Progressive transfers energy in direction of propagation - for stationary there is no transfer of energy
For progressive, all points are out of phase along one wavelength (e.g. crest and trough by 180°) - for stationary, points between 2 nodes or an even number of nodes are in phase, points between an odd number of nodes are in antiphase
For progressive, all particles have the same amplitude - for stationary, amplitude varies according to position (0 at nodes)
What does the frequency of the vibrations on a string depend on?
Mass per unit length
Tension
Length of the string
Define the fundamental mode of vibration
The length of the string is half the wavelength, producing the lowest possible frequency (first harmonic)
Define harmonics
Whole number multiples of the fundamental frequency of a stationary wave
Describe an experiment to determine the speed of sound in air
Use a tuning fork of a known frequency and a tube of water held by a clamp so its length can be altered
When the tuning fork is struck above the tube of water, a stationary wave will be set up in the tube
The fundamental frequency can be obtained by listening carefully and finding the minimum length at which the sound is loudest (when the length of the tube is equal to a quarter of the wavelength)
Lengthen the tube and the sound will decrease in volume before reaching its second maximum loudness
The difference between these two lengths is half a wavelength, so multiply by 2 to find the speed of sound in air
