Waves Flashcards
types of wave, polarisation, wave phases, double slit, diffraction grating, stationary waves, refraction, optical fibres
what is a transverse wave?
including examples
particles oscillate perpendicular to direction of propagation
-EM waves
-water waves
what is a longitudinal wave?
including examples
each particle oscillates parallel to direction of propagation
-sound waves
what is a mechanical wave?
oscillation of particles in a physical medium (cannot be in a vacuum) and can be transverse or longitudinal
what is an EM wave?
produced by acceleration of charged particles and can transmit energy through a vacuum.
- they are always transverse
definition of time period and frequency
time period - time taken for one oscillation
frequency - number of waves passing any one point in one second
what happens when unpolarised light is shone through a polarising filter?
what happens when a second filter is placed perpendicular to the first?
-all the waves move in the same plane absorbing any that aren’t in the same plane (very few oscillations are let through)
- when perpendicular, no light is let through because the plane of polarised light is also at right angles to the second filter.
wave phases
1) what happens when waves are in phase? how do they interfere?
2) what happens when waves are 180 degrees out of phase (in anti-phase)? how do they interfere?
1) waves are oscillating together and causes constructive interference
2)waves are oscillating so that when wave 1 peaks, wave 2 troughs and causes destructive interference (nothing is seen)
What is the principle of superposition
When two or more waves overlap, the resultant displacement at a point is equal to the sum of the individual displacements at that point
What does coherent mean
Waves that have a fixed phase
What are the conditions for constructive interference
The path difference between coherent sources is a a whole number of wavelengths (waves are in phase)
What are the conditions for destructive interference
The path difference is half a wavelength (in antiphase) so waves are cancelled out
What is a monochromatic light source
The waves have a single wavelength
What does the results of Youngs double slit experiment look like on the screen
A bright maxima with spots either side getting dimmer and dimmer. There is a path difference of one wavelength between each spot
Which colours of light diffracts the most and the least
Red diffracts the most (had a wider maximum)
Blue diffracts the least
Diffraction
what happens when a light wave passes through a gap that is
1) smaller than its wavelength?
2) larger than its wavelength
3) equal to its wavelength?
1) no light passes through so there is no diffraction
2)diffracts a little bit but a lot of light passes through
3) diffracts a lot but not as much light passes through
Conditions for a stationary wave
Two progressive waves with the same frequency travelling in opposite directions
How many nodes/antinodes in the
First harmonic
Second harmonic
First harmonic - two nodes, one antinode
Second - three nodes two antinodes
What happens to the energy in a stationary wave
It’s stored
What is the phase difference in stationary waves
All particles between two adjacent nodes are in phase
What is refraction
Change of direction a light ray undergoes when it enters a medium with a different optical density
When does total internal reflection occur
When the angle of incidence exceeds the critical angle
What happens if the angle of incidence is
Smaller than the critical angle
Equal to the critical angle
Greater than the critical angle
Smaller - light ray is refracted out of the material
Equal- refracted along the boundary of the material
Greater- total internal reflection
what is absorption in optical fibres and what is the effect of it?
some of the signal’s energy is absorbed by the fibre which results in the amplitude of the signal being reduced, making it harder to detect
what is modal dispersion and why does it happen?
when the received signal is broader than the signal sent due to some of the light taking a longer path. this results in information loss
what is material dispersion and how can it be reduced?
different wavelength of light travel at different speeds which is shown more as it refracts off the cladding, so the signals will arrive at different times.
use monochromatic light to reduce this
