Flashcards in Waves and Optics Deck (51):
Define the term "mechanical waves"
Mechanical waves are vibrations which pass through a substance.
Define the term "EM waves"
EM waves are oscillating electric and magnetic fields that progress through space without the need for a substance.
Define the term "longitudinal waves"
Longitudinal waves are waves in which the direction of vibrations of the particles is parallel to the direction in which the wave travels (e.g. sound waves and primary (P) seismic waves).
Define the term "transverse waves"
Transverse waves are waves in which the direction of the vibration is perpendicular to the direction in which the wave travels (e.g. EM waves and secondary (S) seismic waves).
Define the term "plane-polarised wave"
A plane polarised wave is a transverse wave where the vibrations stay in one plane only.
The plane of polarisation of an EM waves is defined as the plane in which the electric field oscillates.
State how unpolarised light can be polarised.
If unpolarised light is passed through a polaroid filter, the transmitted light is polarised as the filter only allows through light which vibrates in a certain direction, according to the alignment of its molecules.
Describe what happens when unpolarised light is passed through 2 polaroid filters
If unpolarised light is passed through 2 polaroid filters, the transmitted light intensity changes if 1 polaroid is turned relative to the other.
Light intensity is at minimum when the second filter is rotated 90 degrees to the first filter - because the polarised light from the first filter can't pass through the second.
As you rotate the second filter further, light intensity increases until it reaches a maximum, where the 2 filters are parallel to each other.
How do polaroid sunglasses work?
Polaroid sunglasses reduce the glare of light reflected by water or glass. The reflected light is polarised and the intensity is reduced when it passes through the polaroid sunglasses.
Define the term "displacement"
The displacement of a vibrating particle is its distance and direction from its equilibrium position.
Define the term "amplitude"
The amplitude of a wave is the maximum displacement of a vibrating particle.
For a transverse wave, this is the height of a wave crest.
Define the term "wavelength"
The wavelength of a wave is the least distance between 2 adjacent vibrating particles with the same displacement and velocity at the same time.
Define the term "period"
The period of a wave is the time for one complete wave to pass though a fixed point.
Define the term "frequency"
The frequency of a wave is the number of cycles of vibrations of a particle per second, or the number of complete waves passing a point per second.
Define the term "phase"
The phase of a vibrating particle at a certain time is the fraction of a cycle it has completed since the start of the cycle.
Define the term "phase difference"
The phase difference between 2 particles vibrating at the same frequency is the fraction of a cycle between the vibrations of the 2 particles. One cycle = 360 degrees.
For 2 points at distance d apart along a wave:
Phase difference in radians = 2dpi / wavelength
Define the term "wave fronts"
Wave fronts are lines of constant phase.
Describe what happens during reflection
Straight waves directed at a certain angle to a hard flat surface reflect off at the same angle. The angle between the reflected wave front and the surface is the same as the angle between the incident wave front and the surface.
Explain what causes waves to refract when they pass across a boundary
When waves pass across a boundary at which both the wave speed and wavelength changes. If the wave fronts approach at an angle to the boundary, they change direction as well - this is refraction.
Describe the direction light waves bend when they travel into the glass from the air, and out of glass into the air
When a light ray is directed into a glass block at an angle, it changes direction because light waves travel more slowly in glass than in air. The light bends towards the normal.
When the light ray is directed out of a glass block at an angle, it changes direction away from the normal.
What is diffraction?
Diffraction occurs when waves spread out after passing through a gap, or round an obstacle.
How can you maximise diffraction?
- The narrower the gap, the more the waves spread out
- The longer the wavelength, the more the waves spread out
Why do satellite TV dishes need to be designed carefully?
The bigger the dish, the stronger the signal it can receive, because more radio waves are reflected by the dish onto the aerial.
A bigger dish reflects the radio waves to a smaller focus, because it diffracts the waves less. The dish needs to be aligned more carefully than a smaller dish, otherwise the radio waves will not focus onto the aerial.
Define the term "superposition"
Superposition is the effect of two waves adding together, when the waves combine for an instant.
What does the principle of superposition state?
The principle of superposition states that when two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point.
What happens during reinforcement?
When a crest meets a crest - a supercrest is formed.
When a trough meets a trough - a super trough is formed.
Maximum amplitudes are achieved when waves reinforce each other.
What happens during cancellation?
When a crest meets a trough of the same amplitude, the resultant displacement is 0 - since the 2 waves cancel each other out.
If the waves are not of the same amplitude, the resultant is called a minimum.
Define the term "progressive waves"
Progressive waves are waves which travel through a substance or through space, if electromagnetic.
Define the term "stationary waves"
Stationary waves are a wave pattern with nodes and antinodes formed, when 2 or more progressive waves of the same frequency and amplitude pass through each other.
Define the term "interference"
Interference is the formation of points of cancellation and reinforcement when 2 coherent waves pass through each other.
Since there is constant frequency and constant phase difference, these points happen at fixed positions.
Why are coherent sources of waves needed for interference?
Coherent wave sources produce an interference pattern where they overlap, because they vibrate at the same frequency with a constant phase difference.
If the phase difference randomly changed, the points of cancellation and reinforcement would move at random - no interference pattern seen.
Describe an experiment to test diffraction and interference using microwaves
- The transmitter produces microwaves of wavelength 3cm. The receiver is connected to a suitable meter.
- Place the receiver in the path of the microwaves beam from transmitter. Move the receiver gradually away from the transmitter.
- Place a metal plate between the transmitter and the receiver to show microwaves can't pass through metal
- Use metal plates to make two slits to allow diffraction. Direct the transmitter at the slits and use the receiver to find points of cancellation and reinforcement.
Define the term "first harmonic"
The first harmonic is a pattern of stationary waves on a string when it vibrates at its lowest possible frequency.
Why do stationary waves that vibrate freely not transfer energy to their surroundings?
The amplitude of vibration is 0 at the nodes so there is no energy at the nodes. The amplitude of vibration is a maximum at the antinodes, so there is maximum energy at the antinodes. Since nodes and antinodes are at fixed positions, no energy is transferred in a freely vibrating stationary wave pattern.
Explain why nodes are formed in fixed positions
- When 2 progressive waves are in phase, they reinforce each other to produce a large wave.
- A quarter of a cycle later, the 2 waves have each moved one quarter of a wavelength in opposite directions. They are in antiphase so they cancel each other.
- After a further quarter cycle, the 2 waves are back in phase, and the resultant is a large wave.
- The points where there is no displacement are in fixed positions .
Describe a general stationary wave pattern
- The amplitude of a vibrating particle varies with position from 0 at a node to maximum amplitude at an antinode.
- Phase difference between 2 vibrating particles is 0 if they are between adjacent nodes or separated by an even number of nodes.
- Phase difference is 180 degrees if the 2 particles are separated by an odd number of nodes.
State how the frequency differs between stationary and progressive waves
In stationary waves, all particles except those at the nodes vibrate at the same frequency.
In progressive waves, all particles vibrate at the same frequency.
State how the amplitude differs between stationary and progressive waves
In stationary waves, the amplitude varies from 0 at the nodes to a maximum at the antinodes.
In progressive waves, the amplitude is the same for all particles.
State how the phase difference between 2 particles differs between stationary and progressive waves
In stationary waves, the phase difference is equal to mpi, where m is the number of nodes between the 2 particles.
In progressive waves, the phase difference is equal to 2dpi/wavelength, where d is the distance apart.
Explain what condition must be satisfied at both ends of the vibrating string
A string must be tied to a mechanical vibrator which is connected to a frequency generator. The other end of the string passes over a pulley and supports a weight, which keeps the tension in the string constant.
Describe the simplest possible stationary wave pattern that can be formed
The first harmonic pattern of vibration is seen at the lowest possible frequency that gives a pattern.This has an antinode in the middle and 2 nodes at either end. Since the length L of the vibrating section is between adjacent nodes, and the distance between them is half a wavelength.
Wavelength = 2L
Frequency = c/2L
Explain how a stationary wave pattern forms on a vibrating string
A progressive wave is sent out by a vibrator. The crest reverses its phase when it reflects at the fixed end, and travels back along the string as a trough. When it reaches the vibrator, it reverses and changes phase again. If 2 crests reinforce each other, the amplitude of the wave increases. The time taken for a wave to travel along and back = time taken for a whole number of cycles.
How can you increase the pitch?
- Raising the tension
- Shortening the length of the string
What happens to the light ray at a boundary between 2 transparent substances?
The light ray bends towards the normal if it passes into a more dense substance.
The light ray bends away from the normal if it passes into a less dense substance.
What is Snell's Law?
Snell's Law is a law stating that the ratio of the sines of the angles of incidence to refraction of a wave is constant when it passes between 2 given media.
Compare glass to air refraction with air to glass refraction
The angle of refraction of the light ray emerging from the block is the same as the angle of incidence of the light ray entering the block. This is because the 2 sides of the block where refraction occurs are parallel.
Refractive index of glass = n
Refractive index when leaving block = 1/n
Relate the refractive index to the speed of light waves
n = c/cs where cs is speed of light in a substance
n = w/ws where ws is speed of light in a substance
Explain why a glass prism splits white light into the colours of a spectrum
- White light is composed light with a continuous range of wavelengths, from red with the largest wavelength to violet with the shortest.
- The glass prism refracts light by different amounts dependent on the wavelength. Shorter wavelength = greater refraction.
- Each colour in the white light is refracted by different amounts.
- This dispersive effect occurs because the speed of light in glass depends on wavelength.
- Violet light travels more slowly that red light, so n is greater for violet light that red light.
Define the term "critical angle"
The angle of incidence of a light ray must exceed the critical angle for total internal reflection to occur.
At this angle, the light ray refracts along the boundary.
State the conditions for total internal reflection
1. The incident substance has a larger refractive index than the other substance.
2. The angle of incidence must exceed the critical angle.
Relate the critical angle to the refractive index
At the critical angle, the angle of refraction is 90 degrees since the light ray emerges along the boundary.
Sine of critical angle = n2/n1
where n1 is the incident substance and n2 is the refractive substance.