Zusammenfassung der Ressource
Waves - Chapter 11
- Basics
- Waves are produced by oscillation
- Oscillation is the constant
to and throe motion
- Waves transmit through
different substances (or
mediums)
- Primary waves are
longitudinal and fastest
- Secondary waves are transverse and slower
- A transverse wave oscillates 90
degrees to the direction of travel
- A longitudinal wave
oscillates in the
same direction as
the wave
- This wave is made up of
lots of compressions and
rarefactions
- Compressions = high
density and high
pressure
- Rarefactions = low
density and low pressure
- Waves seem to be progressing but actually
the particles say in the same place and
oscillate to transfer their energy
- Superposition
- This is when you are at a point listening to music and if the waves are anti
phase then they are destructive and there are no waves (so you can't hear
anything), but if they are in phase they are constructive and there are waves
and you can hear sound
- For calculations
- Electromagnetic
waves travel at
3x10^8
- Sound travels at 340ms^-1
- Frequency = 1/time period (s)
- The time period is the
time to complete an
oscillation
- Speed = frequency
x wavelength
- C = Sin^-1*1/n
- Intensity
- Intensity is directly
proportional to 1/r^2
- It is also directly
proportional to
A^2
- Therefore, if you half the amplitude
you half the speed and hense waves
have 1/4 of energy and intensity
- Intensity = Power/Area
- Inverse square law
- I = P/4πr^2
- Power = watts/time
- Unit is Wm^-2
- The radiant power passing
through a surface per unit
area
- Superposition
- This is a combination of wave
displacements when two or more
waves meet and combine
- This can be observed (e.g.) when coherent
waves from 2 sources meet and combine to
form an interference pattern
- Refractive index
- n = speed of light in
vacuum/speed of light in
medium
- n2 = sine(angle in air)/sine(angle in
medium 2)
- Total Internal
Reflection
- This can be used in optic fibres to
pass large amounts of information as
light (in binary)
- The two conditions for this are
- The light must be travelling through a medium
of a higher refractive index as it strikes a
boundary with a lower refractive index
- The angle at which the light strikes the
boundary must be above the critical
angle
- nSinC = n(air)Sin90
- SinC = 1/n
- Wave phase
- Phase is a particular point in the cycle
of a wave form
- This is usually
measured as an
angle
- Two waves are said to be in phase if
displacement of points on each wave is the
same (phase difference of 0 degrees)
- Waves are anti-phase if the
displacement points are opposite (i.e.
phase difference of 180 degrees
- A wave is can be seen as a circle as its
draw out, 2π radians is a full circle
- Refraction
- Waves can slow down when changing medium
- The wavelength decreases (just like the speed) and therefore light bends
- Red light is slowest and blue light is fastest
- Red light will therefore bend the
most and blue light the least
- If waves slow down they refract to
the normal
- If waves speed up they refract
away from the normal
- Water waves
- Speed of water is affected by
the change in the depth
- When a water wave enters shallower water, it
slows down and the wavelength gets shorter
- Waves
- Progressive waves
- This is an oscillation that travels through matter
- It transfers energy but no matter
- Particles vibrate in plane
parallel to the direction
of energy transfer
- A displaced particle
experiences a restoring force
from its neighbour
- e.g. sound
- Transverse waves
- The oscillations are
perpendicular to the
direction of energy transfer
- Polarisation
- Malus' Law
- Intensity is
proportional to cos^2
of the angle
- Polarisation means the
particles oscillate along in
one direction only
- Light from an unpolarised source
(like a filament lamp) is made of
oscillations in many possible planes
- Partial polarisation
- When transverse waves reflect
off a surface they become
partially polarised
- More waves
oscillating in one
particular plane
- Polarisation of EM waves
- Most natural EM waves
are unpolarised
- Polarisers can stop all
vibrations apart from the
direction you want them in
- Phase difference
- This describes the difference of the
displacement of particles on a wave
- If two waves are in phase, the
phase difference is 0
- If two particles are separated by a whole
wavelength they are said to be 360 degrees
out of phase (or 2pi radians)
- If two particles are anti phase then
the phase difference is 180 degrees
- Reflection
- Angle of
incidence = angle
of reflection
- Diffraction
- How much a wave diffracts depends on
the size of the wavelength and gap
- If gap size = wavelength then maximum
diffraction
- When waves pass through a gap or travel around an
obstacle they spread out