Waves Flashcards
Define frequency and give its units
The number of waves passing through a point per second. (hertz) Hz
Define wavelength
The distance between two adjacent peaks on a wave
Define amplitude
The maximum displacement of the wave from its equilibrium position
What is phase difference
How much a particle / wave lags behind another particle / wave.
What is a longitudinal wave
A wave in which the oscillation of the particles is parallel to the direction of energy transfer.
There are rarefactions (areas of low pressure) and compressions (areas of high pressure)
What is a transverse wave?
Waves where the particle oscillations are perpendicular to the direction of energy transfer
True or False? The magnetic field and electric field in a electromagnetic wave are parallel to each other
False.
The electric and magnetic field are at right angles to each other
What does a polarising filter do
Only allows oscillations in one plane
How is polarisation used as evidence of the nature of transverse waves
Polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel (as they are in transverse waves)
How is polarisation used in antennas
TV and radio signals are usually plane-polarised by the orientation of the rods on the transmitting aerial, so the receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength
What is a stationary wave
A wave which transfers no energy and whose positions of maximum and minimum amplitude are constant
What is a node
A point on a stationary wave where the displacement is 0
What is an antinode
A point on a stationary wave with maximum displacement
What are the conditions for a stationary wave to be produced
● The waves must be of the same frequency, wavelength and amplitude.
● They must be travelling in opposite directions.
How are stationary waves produced
A stationary wave is formed from the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude.
Define coherence
Coherent waves have a fixed phase difference and the same frequency and wavelength
Why is a laser useful in showing
interference and diffraction
It produces monochromatic (same wavelength / colour) light so diffraction and interference patterns are more defined
What was Young’s double-slit experiment
A single light source is directed towards two slits, which each act as a coherent light source, the light interferes constructively and destructively to create an interference pattern
Describe the interference pattern created
using white light
A bright white central maximum flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red furthest
Why does an interference pattern form when light is passed through a single slit
The light diffracts as it passes through the slit, where the waves are in phase, constructive interference occurs making bright fringes and, where the waves are completely out of phase, destructive interference occurs making a dark fringe
When light enters a more optically dense medium does it bend towards or away from the normal
Towards the normal
When does total internal reflection occur
When light is at a boundary to a less optically dense medium and the angle of incidence is greater than the critical angle
What is the purpose of the cladding
● Protects core from scratches which would allow light to escape and degrade the signal.
● Allows TIR as it has a lower refractive index than the core
How does signal degradation by absorption in an optical fibre affect the received signal
Part of the signal’s energy is absorbed by the fibre so its amplitude is reduced
What is pulse broadening
When the received signal is wider than the original, this can cause overlap of signals leading to information loss
How does modal dispersion cause pulse
broadening
Light rays enter the fibre at different angles so they take different paths along it, some may travel down the middle while others are reflected repeatedly, so the rays take different times to travel along the fibre, causing pulse broadening
What is material dispersion
When light with different wavelengths is used some wavelengths slow down more than others in the fibre so they arrive at different times causing pulse broadening
How can modal dispersion be reduced
Use a single mode fibre (very narrow fibre) so the possible difference in path lengths is smaller
How can material dispersion be reduced
Use monochromatic light
How can both absorption and dispersion
be reduced
Use a optical fibre repeater to regenerate
the signal now and then
State the advantages of optical fibres
over traditional copper wires
● Signal can carry more information as light has a high frequency
● No energy lost as heat
● No electrical interference
● Very fast
What path does a light ray take when the angle of incidence is equal to the critical angle
It goes along the boundary ie. the angle
of refraction is 90°
State 2 applications of diffraction gratings
● Splitting up light from stars to make line absorption spectra- used to identify elements present in the star.
● X-ray crystallography, a crystal sheet acts as the diffraction grating the X-rays pass through, used to find the spacing between atoms
When light passing through a diffraction grating is changed from blue to red, do the orders get closer together
The wavelength of light has increased so it will diffract more, the orders will become further apart
What is diffraction
The spreading out of waves when they
pass through or around a gap
How did Young’s double slit experiment provide evidence for the wave nature of light
Diffraction and interference are wave properties hence the interference pattern of light shows light has wave properties
What are 4 safety precautions that must be followed when using a laser
● Wear laser safety goggles
● Don’t shine the laser at reflective surfaces
● Display a warning sign
● Never shine the laser at a person
What is path difference
The difference in distance travelled by 2
waves
How could you investigate stationary sound waves
- Place a speaker at one end of a closed glass tube
- Lay powder across the bottom of the tube
- It will be shaken from the antinodes and settle at the nodes
- The distance between each node is half a wavelength
What is ‘phase’
The position of a certain point on a wave cycle
True or False: ‘Only light can produce interference patterns’
False: interference patterns can be formed by sound waves and all EM waves too
