Waves Flashcards
What are mechanical waves?
Waves that require a medium to travel through
Give examples of mechanical waves
Sound waves
Seismic waves
What are electromagnetic waves?
Oscillating electric and magnetic fields that travel without the need of a medium
Give an example of electromagnetic waves
All electromagnetic waves
What are longitudinal waves?
Waves, whose direction of vibration is parallel to the direction of wave travel
Give examples of longitudinal waves?
Sound
Primary seismic waves
What are transverse waves?
Waves, whose direction of vibration is perpendicular to the direction of wave travel
Give examples of transverse waves
All electromagnetic waves
Secondary seismic waves
What is the amplitude?
The length from the trough/crest to the equilibrium, maximum displacement
What is the wavelength?
The distance in metres between the same point on two successive waves
What is frequency?
The number of cycles per second(Hz)
What is displacement?
The distance of a vibrating particle from its equilibrium position
What is a cycle?
The maximum displacement from one wave to the next
What is the time period?
The time for one complete wave to pass a fixed point
What is the equation for the time period?
T = 1/f
What happens to the wavelength when the frequency of a wave increases?
The wavelength shortens when the frequency increases
What is the equation for wave-speed?
Wave-speed = frequency * wavelength
What is polarisation?
The production of waves oscillating in one plane from a source of randomly oscillating waves
What does it mean when the waves are unpolarised?
Waves are unpolarised when the vibrations change from one plane to another
What happens when a light goes through a polaroid?
The light travelling through the polaroid has a reduced intensity as it is polarised due to the arrangements of the molecules in the polaroid
What is reflection?
The wave is bounced back when it hits a boundary
What is refraction?
The wave changed direction as it enters a different medium
What is the constant speed of all EM waves in a vacuum?
3.00*10^8m/s
How do X-rays and Gamma rays cause ionisation?
They knock electrons out of orbits
What is a progressive wave?
A wave that caries energy from one place to another without transferring any material
What evidence shows that electromagnetic waves are transverse?
Polarisation
In 1808, Etienne Louis Malus discovered that light was polarised by reflection
Why can light be polarised?
Light is a transverse wave consisting of vibrating electric and magnetic fields at right angles to the transfer of energy
When does superposition happen?
When two or more waves pass through each other
What is the principle of superposition?
When two or more waves cross and the resultant displacement equals the vector sum of the individual displacements
How can interference be described?
Constructive or destuctive
Give two examples of when constructive interference can occur
Crest + Crest = Supercrest
Trough + Trough = Supertrough
Give an example of destructive interference
Crest + Trough = Zero
What does it mean when waves are in phase?
The wave meet at the same point in the wave cycle
At the point, they have the same displacement and velocity
What is 360 in radians?
2Pi radians
How do you convert from degrees to radians and vice versa?
Degrees to radians, multiply by Pi/180
Radians to degrees, multiply by 180/Pi
What does it mean if sources are coherent?
The coherent sources have the same wavelength and frequency and a fixed phase difference between them
What factor affects interference?
Path difference
What is the path difference?
The amount by which the path travelled by one wave is longer than the path travelled by the other wave
When does constructive interference occur?
When there is an equal distance at a point from two sources that are coherent and in phase
Also when the path difference is a whole number of wavelengths
Path difference = n(wavelengths)
When does destructive interference occur?
The path difference is half a wavelength or one and a half e.t.c wavelengths out of phase
path difference = (n+0.5)wavelengths
What is diffraction?
The spreading out of waves as they go through a narrow gap or go around obstacles
What factors affect diffraction?
Wavelength
Size of the gap
When is the diffraction unnoticeable?
The size of the gap is bigger than the wavelength
When does most diffraction occur?
The size of the gap is the same as the wavelength
When are most of the waves reflected back?
The size of the gap is smaller than the wavelength
What is needed to observe a clear diffraction pattern?
A monochromatic, coherent light source
Give an example of a monochromatic and coherent light source
Lasers
What is the meaning of monochromatic?
All the light has the same wavelength, frequency, and therefore the same colour
What can be observed when a monochromatic, coherent laser light is shone through a narrow list?
A central bright fringe with dark and bright fringes alternating on either side
What type of interference causes the dark fringes?
Destructive interference
What type of interference causes the bright fringes?
Constructive interference
What is white light?
A mixture of difference colours, each with different wavelengths
What happens when white light is shone through a single narrow slit?
All the different wavelengths are diffracted by different amounts
You get a spectra of colours rather than clear fringes
What is the intensity of light?
The power per unit area
Why is the central maximum in a single slit light diffraction the brightest?
There are more photons per unit area hitting the central maximum per second than the other bright fringes
What factors affect the width of the central maximum?
Wavelength
Slit size
What happens when the slit width is increased?
The amount of diffraction decreases
The central maximum is narrower
The intensity of the central maximum is higher
What happens when the wavelength is increased?
The amount of diffraction increases
The central maximum is wider
The intensity of the central maximum is lower
What can you do to prevent damage when using lasers?
Never shine the laser towards a person Wear laser safety goggles Avoid shining the laser beam at a reflective source Have a warning sign on display Turn the laser off when it's not needed
Describe the Young’s Double Slit experiment?
A laser is shone through two slits onto a screen
The slits have to be about the same size as the wavelength so that the laser light is diffracted
You get a pattern of light and dark fringes
Describe a similar experiment to the Young’s slit experiment with microwaves
Replace the laser and slits with two microwave transmitter cones attached to the same signal generator
Replace the screen with a microwave receiver probe
If you move the probe perpendicular to the microwave generator, you’ll get an alternating pattern of strong and weak signals
What is the Young’s Slit formula?
w=(wavelength*D)/s
What is the meaning of fringe spacing?`
The distance between the centre of the one minimum to the centre of the next minimum or the centre from one maximum to the centre of the next maximum
What is ‘s’ in Young’s slit formula?
Spacing between slits
What is ‘D’ in Young’ slit formula?
The distance from the slits to the screen
What was Young’s slit experiment evidence for?
It showed that light could both diffract and interfere
What happens when light diffracts through more slits?
Interference patterns become sharper
Which order is the maximum brightness?
Zero-order
How can the wavelength be calculated from diffraction grating?
wavelength = d*sin(angle)/n
How would you calculate the slit spacing(d), if you are given the number of slits per metre?
d = 1/slits per metre
What general conclusions can be drawn from dsin(angle) = nwavelength?
If the wavelength is bigger, the bigger the angle.
If d is bigger, the smaller the angle.
Values of sin(angle) greater than 1 means the order doesn’t exist
How can diffraction grating be used?
It can be used to identify elements
It can be used to calculate atomic spacing
What is white light?
A mixture of colours with different wavelengths
What will happen if you diffract white light?
The colours in the light will spread out by different amounts because of the different wavelengths
Which colour will be on the inside and on the outside?
Why?
Inside = violet, has the shortest wavelength Outside = red, has the longest wavelength
What is a stationary wave?
The superposition of two progressive waves with the same frequency and wavelength moving in opposite directions
What don’t stationary waves transmit?
Stationary waves do not transmit energy
What is a node?
This is where the amplitude of the vibration is zero
What is an antinode?
This is where the point of maximum amplitude
Where do stationary waves occur?
They occur at certain special values of frequency and wavelength called harmonics
What causes progressive and stationary waves?
Fluctuation
Disturbance
What is the difference in energy transfer between stationary and progressive waves?
Progressive waves
-transfer energy and momentum from the source to
surroundings
Stationary waves
-no energy or momentum transfer as they are confined
to a specific region
What is the difference in occurrences between stationary and progressive waves?
Progressive waves
-occur at all frequencies
Stationary waves
-occur at harmonics
What is the difference in the amplitude of particles between stationary and progressive waves?
Progressive waves
-all particles will reach a maximum and minimum
amplitude
Stationary waves
-some points have a maximum amplitude and others
do not. The amplitude of the particle depends on the
position of the particle
What is the difference in the phase difference of particles between stationary and progressive waves?
Progressive waves
-only particles exactly one wavelength apart move in
phase with each other
Stationary waves
-within a one-half wavelength, all particles move in
phase
What is the first harmonic on a string?
Length = wavelength/2
What is the second harmonic on a string?
Length = wavelength
What is the general formula for the length of the string?
Length = n*wavelength / 2
Wha is the first harmonic in a tube with one closed end?
Length = wavelength/4
What is the second harmonic in a tube with one closed end?
Length = 3/4 * wavelength
What is the first harmonic in a tube with both ends open?
Length = wavelength/2
What is the second harmonic in a tube with both ends open?
Length = wavelength
What is the fundamental mode frequency?
f = 1/2L√(T/µ)
What is the second harmonic frequency?
f = 1/L√(T/µ)
What is the third harmonic frequency?
f = 3/2L√(T/µ)
What is refraction?
The change in direction that occurs when light passes at an angle across a boundary between 2 transparent substances
What happens to the light when it travels from a less dense material to a more dense material?
The light bends towards the normal
What happens to the light when it travels from a more dense material to a less dense material?
The light bends away from the normal
What is the refractive index of a material?
It is a measure of how much the material slows down light
What is the absolute refractive index of a material?
A measure of optical density
it is found from the ratio between the speed of light in a vacuum and the speed of light in that material
How do you calculate the refractive index of a substance?
n = C/Cs
What is the relative refractive index between two materials?
The ratio of the speed of light in material 1 to the speed of light in material 2
How do you calculate the relative refractive index?
n = C1/C2
What is the refractive index of air?
1
What is the angle of incidence?
The angle the incoming light makes to the normal
What is the angle of refraction?
The angle the refracted ray makes to the normal
What is Snell’s law?
n1 sin angle 1 = n2 sin angle2
