Waves Flashcards
Progressive waves
There is a transfer of energy as a result of oscillations, hence they transfer information
Stationary waves
No net transfer of energy, traps energy in pockets between nodes
Mechanical wave
Waves which need a medium through which to travel
Longitudinal waves
The particles of the medium oscillate parallel to the direction of energy transfer
Transverse waves
The particles of the medium oscillate perpendicularly to the direction of energy transfer
Phase difference
The fraction of an oscillation between two particles or points on the a wave
What is polarisation?
A wave is plane polarised if the oscillations are in one direction only and perpendicular to the direction of energy transfer
What is a polarising filter?
An object that will absorb the energy from the oscillating electric field of an electromagnetic wave. It consists of long chain molecules perpendicular to the polarising axis.
Principle of superposition
When two or more waves meet the resultant displacement is equal to the vector sum of the displacements of each wave
How are stationary waves formed?
When a progressive wave is reflected between two fixed points, so two waves with the same frequency and amplitude travelling in opposite directions, meet and superpose/interfere creating regions of nodes and antinodes
Phase difference in stationary wave
Oscillating particles between nodes are in phased. Oscillating particles in adjacent segments are in antiphase. Oscillating particles in alternate segments are in phase
What is the fundamental mode of vibration?
The pattern of a stationary wave, which appears at the lowest possible resonant frequency for a particular system
Define coherence
Two waves are coherent if they have a constant phase difference, and same frequency
Define interference
when two or more waves meet and superpose at the point there is a change int the overall intensity
What is diffraction?
waves spread due to an obstacle or gap, most notable when the gap spacing is about the same size as the wavelength
Describe the intensity profile of single slit diffraction
Wide central maximum with lower intensity maxima half as wide on either side
Describe the intensity profile of double slit diffraction?
Same overall shape, with more maxima in a “single slit envelope”
What is refraction?
Part of the wave energy is transmitted into another medium, exhibiting refraction i.e. change of direction of wave propagation
What is Snells Law
The ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of the velocities, or reciprocal of the ratio of the indices of refraction
Define dispersion
Different wavelengths of light propogating through the glass slow down by different amounts since the refractive index varies with wavelength
Describe material dispersion
Different wavelengths have different speeds due to different refractive indices within the core, a sharp pulse will spread out into a broader signal - reduce by using a monochromatic beam or shorter repeaters to reform the pulse
Describe modal dispersion
Different paths have different lengths so effective time along fibre differs. This is causes pulse broadening and limited bandwidth - reduce by using a monomode fibre
What is a monomode fibre?
Fibre with a narrow core and the change in n between core and cladding is small, hence light ray is nearly confined to one single path along the cable
