Module 4.2 - Waves ~✓ Flashcards
What is Displacement? (waves)
Displacement - how far a point has moved from it’s equilibrium position
What is Amplitude? (waves)
Amplitude - the maximum magnitude of the displacement
What is wavelength? (waves)
Wavelength - the length of one whole wave cycle, from crest to crest
What is period? (waves)
Period - the time taken for one whole wave cycle to complete
What is phase? (waves)
Phase - a measurement of the position of a certain point along the wave cycle
What is phase difference? (waves)
Phase difference - the amount one wave lags behind another
What is the wave equation
wave speed = frequency * wavelength
Which way are the vibrations in transverse/longitudinal waves?
Transverse - Perpendicular to the direction of travel
Longitudinal - parallel to the direction of travel
What is intensity? (waves)
Intensity is the power transfer per unit area at right angles to the direction of travel of a wave
What is the equation for intensity?
Intensity = Power/Area
State three properties of EM waves
- Travel in a vacuum at 3*10^8 ms-1 (3sf)
- They are transverse waves consisting of vibrating electric and magnetic fields at right angles to each other
- They carry energy (like all progressive waves)
- They can be polarised
- They can be refracted, reflected and diffracted
Name the types of EM wave in order of wavelength (large > small)
Radio, Microwave, Infrared, Visible light, Ultraviolet, X-rays, Gamma rays
What is polarisation?
Polarisation is the filtering out of vibrations of a wave in other directions
How does a polarising filter work?
A polarising filter only transmits vibrations in one direction
Define diffraction
Diffraction is the spreading out of the wave fronts of a wave when passing through a gap or around objects
What is the law of reflection?
Angle of incidence = angle of reflection
Define refraction
Refraction is the changing of the direction of a wave as it enters a different medium
What is the absolute refractive index?
The absolute refractive index of a material is the ratio between the speed of light in a vacuum and the speed of light in the material
What is the equation for absolute refractive index?
n = c/v
n > absolute refractive index
c > speed of light in vacuum
v > speed of light in material
What is the Snell’s law equation? (for refraction)
n1 sin()1 = n2 sin()2
What transition in optical densities is needed for total internal reflection?
For total internal reflection to occur light must travel from more optically dense material to a less optically dense one
What is total internal reflection?
Total internal reflection is when all light is reflected back into the material at a boundary between two materials - happens at angles of incidence greater than C
What happens at the critical angle of incidence?
At the critical angle C, the angle of refraction is 90 degrees, so the light gets refracted along the boundary between materials.
What is Snell’s law equation for light hitting a material to air boundary?
sin(C) = 1/n
State the principle of superposition
When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements
How do you know if two points on a wave are in phase?
They should have the same displacement and velocity
What does it mean for two sources to be coherent?
Two wave sources are coherent if they have the same wavelength and frequency, and a fixed phase difference between them
What is path difference?
Path difference is the amount by which the path travelled by one wave is longer than the path travelled by another
At what path difference does constructive,destructive interference occur?
Constructive > whole number of wavelengths
Destructive > half number of wavelengths
State young’s double slit formula
Fringe Spacing x = (λ*D)/a
λ > wavelength
D > distance of slits to screen
a > spacing between slits
Why was young’s double slit experiment so important?
Young’s double slit experiment showed that light could diffract and interfere, providing evidence for the wave nature of light
What is the naming scheme for diffraction grating patterns?
the central spot is the zero order line, the next one out from each side is the 1st order, then 2nd and so on
What is the equation for finding the wavelength of light from a diffraction grating?
d sin(x/D) = nλ d > slit separation x > fringe width D > dist form screen to laser n > order of maximum observed
d sin(θ) = nλ (x/D = θ) > using small angle approximations
Why does shining white light through a diffraction grating produce spectra?
White light is a mixture of colours. If diffracted, the different wavelengths of light are diffracted by different amounts, so each order in the pattern becomes a spectrum (the zero order stays wight tho)
What is a stationary wave?
A stationary wave is the superposition of two progressive waves with the same wavelength, travelling in opposite directions.
What is the difference between progressive and stationary waves?
- Progressive waves transfer energy however stationary waves store it
- Progressive waves have a fixed amplitude throughout whereas stationary waves have different amplitudes along them
What is a Node?
Nodes are points on a stationary wave with an amplitude of zero
What is an Antinode?
Antinodes are points on stationary waves with maximum amplitude
What are “resonant frequencies”?
resonant frequencies are frequencies at which a stationary wave forms, at these an exact number of half wavelengths fit on a string
What is the fundamental mode of vibration?
The first resonant frequency/first harmonic. On a string it has a wavelength of ½
at what lengths do stationary waves form in closed instruments? Where are the nodes/anti-nodes located?
1/4 λ, 3/4 λ, 5/4 λ, etc…
A node always forms at the closed end and an antinode at the open end, more can form in between.
At what lengths do stationary waves form in open ended instruments? Where are the nodes/anti-nodes?
½λ, 1λ, 3/2λ, etc…
Anti-nodes will always form at the open ends, nodes in between
Where does intensity ∝ (amplitude)^2 come form?
Intensity is proportional to energy and the energy of a wave depends on the square of its amplitude, therefore:
Where does intensity ∝ (amplitude)^2 come form?
Why do polarising filters not work on microwaves?
Their wavelength is too long, metal grilles must be used instead
How can you tell if a light ray is slowing down or speeding up when refracted?
If the ray bends towards the normal it is being slowed down, if it bends away it is speeding up
What is an optically dense material?
A more optically dense material is one where light slows down more when travelling through it
Define interference
Interference is the combining of the displacements of two or more superposed waves
What general conclusions can be drawn from d sin(θ) = nλ?
- The larger the wavelength, the larger the spacing
- The coarser the grating the less spread out the pattern (d bigger > sin theta smaller)
Why does a stationary wave form when vibrating one end of a fixed string?
1) The wave generated by the vibrations at one end moves down the string
2) This wave is reflected at the fixed end of the string
3) If an exact number of waves is produced in the time it takes for a wave to travel to the end and back the waves superpose and form a stationary wave
This happens at resonant frequencies
A string has a mechanical oscillator at one end and is tensioned using massed and a pulley at the other. Explain how stationary waves form in the string?
1) The oscillator sends a progressive wave down the string
2) This wave gets reflected at the end of the string to produce an equal progressive wave moving in the opposite direction
3) The two progressive waves interfere to produce nodes and anti-nodes producing a stationary wave
