Waves Flashcards
Define displacement as applied to wave motion
The displacement of any particle of the wave is its distance in a given direction from the rest or
equilibrium position. It is a vector quantity.
State the meaning of the term amplitude
The amplitude is the magnitude of the maximum displacement of any particle on the wave from its
equilibrium or rest position. It is a scalar quantity.
Define Frequency, f
The frequency of a wave is the number of cycles of a wave that pass a given point in one second.
State the meaning of the term phase difference
Phase difference is the difference in position of 2 points within a cycle of oscillation. It is given as a
fraction of the cycle or as an angle (where one whole cycle is 2π or 360°) together with a statement
of which point is ahead in the cycle.
Define Wavelength, λ
The wavelength of a progressive wave is the minimum distance, measured along the direction of
propagation, between two points on the wave oscillating in phase.
Explain what is meant by wave speed, v
The speed of a wave is the distance that the wave profile moves per unit time. SI unit: ms-1
Describe how a transverse wave differs from a longitudinal wave.
A transverse wave is one where the particle oscillations are at right angles to the direction of travel
of the wave energy.
A longitudinal wave is one where the particle oscillations are parallel to the direction of travel of
the wave energy.
Explain what is meant by a progressive wave.
A progressive wave is a pattern of disturbances travelling through a medium and carrying energy
with it, involving the particles of the medium oscillating about their equilibrium positions.
Explain briefly what is meant by the term progressive when used to describe wave motion.
In a progressive wave, the wave profile can be seen to propagate (move forward) and so transfer
energy in direction of travel of the wave.
Describe what is meant by a plane polarised wave
A polarised wave is a transverse wave in which particle oscillations occur in only one of the
directions at right angles to the direction of wave propagation.
State two properties which distinguish electromagnetic waves from other transverse waves.
- electromagnetic waves can travel through a vacuum.
- electromagnetic waves are oscillating electric and magnetic fields.
State one similarity and one difference between X-rays and radio waves.
- Both are transverse waves
- Both travel at the same speed in a vacuum
- Both be can be polarised.
- They have different wavelengths,
- They have different frequencies and
- They have different (photon) energies.
Explain the term Intensity
The intensity is of a wave is the radiant power per unit area perpendicular to the direction of energy
transfer.
Explain what is meant by infrared radiation
Infrared is part of the electromagnetic spectrum. It has longer wavelength (and lower frequency)
than light.
Electromagnetic waves and sound waves can be reflected. State two other wave phenomena
that apply to both electromagnetic waves and sound waves.
- Both can be refracted
- Both can be diffracted
- Both can experience constructive and destructive interference.
Explain why electromagnetic waves can be polarised but sound waves cannot be polarised.
Electromagnetic waves are transverse; sound waves are longitudinal waves. Only transverse waves
can be polarised.
State Snell’s law
At the boundary between any two given materials, the ratio of the sine of the angle of incidence to
the sine of the angle of refraction is a constant
Define refractive index
For light, Snell’s law may be written: n1 sin θ1 = n2 sin θ 2 in which θ1 and θ2 are angles to the
normal for light passing between medium 1 and medium 2; n1 and n2 are called the refractive
indices of medium 1 and medium 2 respectively.
Define critical angle
When light approaches the boundary between two media from the ‘slower’ medium, the critical
angle is the largest angle of incidence for which refraction can occur. The refracted wave is then
travelling at 90° to the normal.
Explain what is meant by total internal reflection
When light travels through one transparent material towards the boundary with a second transparent
material with a lower refractive index, all the light is reflected at the boundary if the angle of
incidence is greater than the critical angle. This is total internal reflection.
A glass fibre used for the transmission of data consists of a central glass core with a cladding
of glass of lower refractive index. Suggest one advantage of having a glass cladding rather
than simply an air surround.
- The beam is confined to small angle to axis and so there will be fewer reflections and so less
spreading of the pulses. - The cladding prevents damage (scratches) to the surface of the core which result in light being
lost into the cladding.
What is a monomode fibre?
A monomode fibre is an optical fibre whose diameter is equal to a few wavelengths of the
transmitted light. This ensures that the light only propagates along one path parallel to axis without
being reflected.
Explain the advantage of a monomode fibre over a fibre with a much thicker core, for the
transmission of a rapid stream of data.
In a monomode fibre there is only one route or path for the light to follow whereas in a multimode
fibre there are many paths. Therefore, in a monomode fibre each pulse arrives without any
spreading and so there is less chance of pulses overlapping.
Give two advantages of using optical fibres rather than copper wires for transmitting
information.
- The bandwidth is greater and so there is a larger information-carrying capacity.
- There is less attenuation of the signal in optical fibres and so data can be transmitted over
greater distances before boosting is required. - Optical fibres are more secure than copper cables and so less susceptible to unauthorised access
to transmitted data (i.e. tapping).
In a multimode fibre, light travels at a range of angles to the fibre axis. Explain why, for clear
communication of rapid streams of data, the range of angles should be restricted so that even
the greatest value of the angle is very small.
- By reducing the maximum angle to the fibre axis the range of path lengths is decreased.
- This reduces multimode dispersion and so produces less time broadening of the pulses.
- This means that the pulses are less likely to overlap, which allows a higher rate of data transfer.
Explain why multimode fibres are used for transmitting data over short distances only.
- In a multimode fibre there are many possible paths for the light to follow.
- This causes the light pulses time broaden (dispersion), the pulses may then overlap.
- The shorter the fibre the less the broadening or the less chance of pulses overlapping.
Modern communications systems require very high data transmission rates, for which mono-
mode optical fibres are needed. Explain why optical fibres with thick cores (multi-mode
fibres) are not suitable.
- Paths at different angles to the axis are of different lengths.
- Data travelling on different paths arrive at different times.
- This may cause the data pulses to overlap.
State the Principle of Superposition:
The principle of superposition states that if waves from two sources [or waves travelling by
different routes from the same source] occupy the same region then the total displacement at any
one point is the vector sum of their individual displacements at that point.
Explain the meaning of the term Interference
Interference occurs when two or more waves travelling in the same region superpose (overlap and
interact).
Explain the meaning of the term Coherence
Waves or wave sources, which have a constant phase difference between them (and therefore must
have the same frequency) are said to be coherent.
Explain what is meant by Coherent sources.
Two wave sources are said to be coherent if they have the same frequency and a constant phase
difference (e.g. in phase or in antiphase).
Use the idea of phase difference to explain how an interference pattern is formed
For waves from two coherent sources:
- At those points where the waves arrive in phase they constructively interference to produce
a wave of maximum amplitude (or intensity), sometimes called a maximum.
- At those points where the waves arrive in antiphase they destructively interference to
produce a wave of minimum amplitude (or intensity), sometimes called a minimum.
Use the idea of path difference to explain how an interference pattern is formed
For waves from two coherent sources oscillating in phase:
- At those points where the path difference for the two waves is n λ constructive interference
occurs to produce a wave of maximum amplitude (or intensity), sometimes called a
maximum.
- At those points where the path difference for the two waves is (n + 1⁄2) λ destructive
interference occurs to produce a wave of minimum amplitude (or intensity), sometimes
called a minimum.
Use the concepts of phase difference and path difference to explain how destructive
interference is produced.
When waves from two sources arrive at a point in antiphase (180° phase difference) they
destructively interfere. At a point of destructive interference, the waves travel different distances to
that point from the sources. The two waves will arrive at that point in antiphase if the path
difference path difference from sources to point of destructive interference is given by: Path
difference = (n + 1⁄2) λ
Diffraction
Diffraction is the spreading out of waves (or wavefronts) as they pass the edges of an obstacle such
as the edges of a slit or gap. Some of the wave’s energy travels into the geometrical shadows of the
obstacles.
Explain the historical importance of Young’s double slit experiment
- It established the wave nature of light
- Where previously light was thought to be particles (corpuscles)
Describe a diffraction grating.
- A diffraction grating is a flat opaque screen made from a material with slits (or gaps)
- There are multiple equal spaced parallel slits
To obtain an accurate value of wavelength, it is better to use a diffraction grating than a
double slit. Give two reasons for this.
- The beams produced by a diffraction grating are further apart and as such it is easier to
measure the angle between them accurately, compared to the fringe separation produced by
a double slit that is very small - The beams produced are very bright and as such sharper and more defined, making it
easier to identify their centres to measure the angle from, compared to the fringes of a
double slit that are dim, not well defined and very hard to identify their centres - The slit separation in a diffraction grating may be calculated with a smaller uncertainty,
since a longer measurement is made and then divided by the large number of slits. The slit
separation in a double slit set up might be larger, but it is a very small length to measure
accurately.
Explain what is meant by the term stationary wave.
A stationary wave is a pattern of disturbances in a medium, in which energy is not propagated. The
amplitude of particle oscillations is zero at equally-spaced nodes, rising to maxima at antinodes,
midway between the nodes.
State two features of a stationary wave
- Energy is trapped or stored in pockets so that there is no net transfer of energy.
- There are positions, called nodes, where the amplitude is zero (or minimum).
- There are positions, called antinodes, where the amplitude is maximum.
- Adjacent (neighbouring) points on the wave have different amplitudes.
- All points between successive nodes oscillate in phase with each other and in antiphase with
all points between the next two successive nodes.
State what is meant by a node in a stationary wave, and state how far apart the nodes will be
in this stationary wave.
- It is a point that does not move or oscillate and as such has zero (minimum) amplitude
- It is a point of destructive interference
State how, if at all, the phase of the oscillations of the particles of the string varies with
distance along the string, for a stationary wave.
- All points between successive (consecutive or adjacent) nodes are in phase
- All points on opposite sides of a node are in antiphase
Explain, in terms of interference , how the stationary wave is produced for a stationary wave
on the string.
- The incident progressive wave reflects at the fixed end
- Now there are two waves, the incident and reflected, that move in opposite directions on the
string - Where the two waves meet, they overlap and superpose
- Producing a stationary wave with nodes where the two waves interfere destructively and
antinodes where they interfere constructively
Explain the effect of increasing the frequency will have on a stationary wave on a string
Because
c = f λ as the frequency increases, the wavelength will decrease. This means that the stationary waves observed at higher frequencies will have more nodes,
closer together
State the two differences between progressive and stationary waves
- a progressive wave transfers energy whereas a stationary wave traps energy in pockets (as
such there is no transfer of energy). - every point on a progressive wave has the same amplitude whereas on a stationary wave
the amplitude is maximum at the antinodes and decreases to zero (or minimum) at the nodes. - On a progressive wave, all points (over a distance of one wavelength) oscillate out of phase
with each other whereas on a stationary wave all points between nodes oscillate in phase
with each other.
When used to describe stationary (standing) waves, explain the terms node and antinode
- A node occurs where the amplitude (or displacement) of the standing wave is always zero
- An antinode occurs where the amplitude of the standing wave takes the maximum possible
value.
Explain how a stationary wave can be regarded as being formed from progressive waves.
- Two waves travelling in opposite directions (or a wave and its reflection)
- With similar amplitude and the same frequency
- Interfere or superpose to produce a stationary wave
Explain how a stationary wave can be formed inside a tube closed on one end
- A sound wave is produced on the open end of the tube
- The sound wave travels towards the closed end where it is reflected
- The incident and reflected wave superpose and interfere
- To produce a standing wave with a node in the closed end and an antinode a ( actually
slightly above) the open end - All stationary waves in this tube will have a node at the closed end and an antinode at
(actually slightly above) the open end
