Waves and Optics Flashcards
Progressive wave def
A wave that transfers energy from one point to another without transferring the medium itself
displacement def
the distance of a point on the wave from its equilibrium position
It is a vector quantity; it can be positive or negative
amplitude def
the maximum displacement of a particle in the wave from its equilibrium position
wavelength def
the distance between points on successive oscillations of the wave that are in phase
(time) period def
time taken for one complete oscillation or cycle of the wave
frequency def and unit
the number of complete oscillations per unit time. Measured in Hertz (Hz) or s-1
What do the components of the wave speed equation mean
What does the wave speed equation show about frequency and wavelength
- As the wavelength increases, the frequency decreases
- As the wavelength decreases, the frequency increases
What is phase difference
how much a point or a wave is in front or behind another
How can waves phase difference be found
- from the relative positive of the crests or troughs of two different waves of the same frequency
- When the crests or troughs are aligned, the waves are in phase
- When the crest of one wave aligns with the trough of another, they are in antiphase
What is phase difference measured in
in fractions of a wavelength, degrees or radians
How many degrees/ radians correlate to in or out of phase
In phase is 360o or 2π radians
In anti-phase is 180o or π radians
traverse wave def
A wave in which the particles oscillate perpendicular to the direction of the wave travel (and energy transfer)
What kind of wave are electromagnetic waves
transverse waves
How can transverse waves be shown and can they be polarised
Transverse waves can be shown on a rope
Transverse waves can be polarised
Longitudinal wave def
A wave in which the particles oscillate parallel to the direction of the wave travel (and energy transfer)
Describe the compressions and rarefactions of longitudinal waves in terms of pressure
Compressions are regions of increased pressure
Rarefactions are regions of decreased pressure
How can longitudinal waves be shown and can they be polarised
Longitudinal waves can be shown on a slinky spring
Longitudinal waves cannot be polarised
Examples of transverse waves
Electromagnetic waves e.g. radio, visible light, UV
Vibrations on a guitar string
Waves on a string
Seismic (S) waves
Describe how energy is transferred through a longitudinal wave
- The particles in the medium vibrate as they are given energy
- The compressions cause the nearby particles to also vibrate with more energy
- This produces a compression further along in the medium
Examples of longitudinal waves
Sound waves
Ultrasound waves
Waves through a slinky coil
Seismic (P) waves
What is polarisation
Particle oscillations occur in only one of the directions perpendicular to the direction of wave propagation
Why can polarisation only occur in transverse waves
because transverse waves oscillate in any plane perpendicular to the propagation direction
What does it mean when transverse waves are polarised
Vibrations are restricted to one direction
These vibrations are still perpendicular to the direction of propagation / energy transfer
Draw a diagram showing the difference between an unpolarised wave and a vertically polarised wave
Why can’t longitudinal waves be polarised
because they oscillate parallel to the direction of travel
How does a polariser or a polarising filter work
They only allow oscillations in a certain plane to be transmitted
How can waves be polarised
- Through a polariser or polarising filter
- Light can also be polarised through reflection, refraction and scattering
Draw a graph showing how the intensity of a transmitted beam varies with the angle between the transmission axes of two polarisers
What are the practical applications of polarisation
- Polaroid sunglasses are glasses containing lens with polarising filters with transmission axes that are vertically oriented. This means the glasses do not allow any horizontally polarised light to pass through. When sunlight reflects off a horizontal reflective surface, such as water, the light becomes horizontally polarised. This is where polaroid sunglasses come in useful with their vertically aligned filter. objects under the surface of the water can be viewed more clearly
- Polaroid cameras work in the same way as polaroid sunglasses. They are very useful for capturing intensified colour and reducing glare on particularly bright sunny days
- radiocasting towers always transmit either vertically or horizontally polarised signals. This is why aerials must be positioned accordingly otherwise they won’t pick up the TV signal correctly
How are standing waves formed
by the superposition of two waves of the same frequency and amplitude travelling in opposite directions
What is the difference between stationary and progressive waves
Stationary waves store energy, unlike progressive waves which transfer energy
Compare stationary and progressive waves
What are nodes and antinodes definitions
Node = A point of minimum or no disturbance
Antinode = A point of maximum amplitude
Describe waves being in/ out of phase, in terms of odd/even nodes
- Points that have an odd number of nodes between them are out of phase
- Points that have an even number of nodes between them are in phase
What is the principle of superposition
When two or more waves with the same frequency arrive at a point, the resultant displacement is the sum of the displacements of each wave
Describe how constructive and destructive interference occur
When two waves with the same frequency and amplitude arrive at a point, they superpose either:
- In phase, causing constructive interference. The peaks and troughs line up on both waves and the resultant wave has double the amplitude
- In anti-phase, causing destructive interference. The peaks on one wave line up with the troughs of the other. The resultant wave has no amplitude
When is a stationary wave formed
Two waves travelling in opposite directions along the same line with the same frequency superpose
How is sound made through stationary waves
By making vibrations on stretched waves
What happens when a resonant frequency occurs on a string
standing waves with different numbers of minima (nodes) and maxima (antinodes) form
What are harmonics
Different stationary wave patterns
What is the equation linking the harmonic frequency, n and frequency of the first harmonic
nth harmonic frequency = n × frequency of first harmonic
What do the components of the speed of a wave travelling along a string equation mean
What do the components of the first harmonic equation mean
When does interference occur
when waves overlap and their resultant displacement is the sum of the displacement of each wave
When is a wave said to be ‘coherent’
When two waves are neither in phase or antiphase, and have:
- The same frequency
- A constant phase difference
What is coherence necessary for
to produce an observable, or hearable, interference pattern
Examples of the applications of coherence
- Laser light filament lamps produce coherent light waves
- When coherent sound waves are in phase, the sound is louder because of constructive interference
Path difference def
The difference in distance travelled by two waves from their sources to the point where they meet
Safety precautions of handling lasers
- Never look directly at a laser or its reflection
- Don’t shine the laser towards a person
- Don’t allow a laser beam to reflect from shiny surfaces into someone else’s eyes
- Wear laser safety goggles
- Place a ‘laser on’ warning light outside the room
- Stand behind the laser
What factors must be present for two-source interference fringes to be observed
- Coherent (constant phase difference)
- Monochromatic (single wavelength)
What is path difference
The difference in path length which the two waves emerging from a double slit take before hitting the same boundary
Describe what the path difference of constructive interferences and destructive interferences are
For constructive interference (or maxima), the difference in wavelengths will be an integer number of whole wavelengths
For destructive interference (or minima) it will be an integer number of whole wavelengths plus a half wavelength
What do the components of the fringe spacing equation mean
What makes light fringes more coherent
A monochromatic light source
Why is the central fringe always the same colour as the source
Because, at that position, the path difference for all wavelengths present is zero, therefore all wavelengths will arrive in phase
Why does blue light appear closer to the centre than red light
Since blue light has a shorter wavelength than red light, the path difference will be smaller, so the blue maximum will appear closer to the centre
What are the key development theories of electromagnetic radiation
- Newton proposed that visible light is a stream of microscopic particles called corpuscles
However, these corpuscles could not explain interference or diffraction effects, therefore, the view of light as a wave was adopted instead - Huygens came up with the original Wave Theory of Light to explain the phenomena of diffraction and refraction. This theory describes light as a series of wavefronts on which every point is a source of waves that spread out and travel at the same speed as the source wave. These are known as Huygens’ wavelets.
- Young devised the famous double-slit experiment
This provided experimental proof that light is a wave that can undergo constructive and destructive interference - Maxwell showed that electric and magnetic fields obeyed the wave equation. This means that light was simply waves made up of electric and magnetic fields travelling perpendicular to one another
Later, Maxwell and Hertz discovered the full electromagnetic spectrum - Einstein discovered that light behaves as a particle, as demonstrated by the photoelectric effect. He described light in terms of packets of energy called photons. Later the scientific community came to understand that light behaves both like a wave and a particle. This is known as wave-particle duality
What is diffraction
the spreading out of waves when they pass an obstruction
This obstruction is typically a narrow slit known as an aperture
What does diffraction depend on and when is it the highest
- the width of the gap compared with the wavelength of the waves
- Diffraction is the most prominent when the width of the slit is approximately equal to the wavelength
What is the only property of a diffracted wave that changes and why
amplitude, because some energy is dissipated when a wave is diffracted through a gap
What can the diffraction pattern of light be represented by
a series of light and dark fringes which show the areas of maximum and minimum intensity
What are the features of the single slit diffraction pattern
- A central maximum with a high intensity
- Subsidiary maxima equally spaced, successively smaller in intensity and half the width of the central maximum
What happens when the gap in single slit diffraction gets larger
diffraction gradually gets less pronounced until, in the case that the gap is much larger than the wavelength, the waves are no longer spread out
What happens when the gap in single slit diffraction gets smaller
The intensity would decrease
The fringe spacing would be wider
What is a diffraction grating
A plate on which there is a very large number of parallel, identical, close-spaced slits
What do the components of the diffraction grating equation mean
Equation linking d and lines per m (N)
What is the equation used for calculating the highest order of maxima visible
Show how the first order maxima (and later the diffraction grating) equation is derived
What are diffraction gratings used for
Analyse light from stars
Analyse the composition of a star
Chemical analysis
Measure red shift / rotation of stars
How do x-ray crystallographies word
- X-rays are directed at a thin crystal sheet which acts as a diffraction grating to form a diffraction pattern
- This is because the wavelength of x-rays is similar in size to the gaps between the atoms
- This diffraction pattern can be used to measure the atomic spacing in certain material
Suggest an experiment that could demonstrate the wave nature of sound
diffraction through a door / out of a pipe
How is the value of a quantity accepted in the scientific community
- The method and value are published
- other scientists repeat the experiment using the same method
What combination of slit spacing and wavelength gives the greatest possible number of interference maxima
the smallest wavelength and greatest slit spacing
Is the refractive index for blue light higher or lower than for red light, and what effect does this have on the light’s speed
- The refractive index of core for blue light is greater than the refractive index for red
- Blue light has a lower speed
