4.4 waves Flashcards
progressive wave
transfers energy without transferring matter
define: displacement + unit
distance and direction of a point on the wave from the rest position + m
define: amplitude + unit
the max displacement of a point on the wave from the rest position + m
define: wavelength + unit
distance from any point to an adjacent point in the same phase + m
define: period + unit
time taken for one complete wave to pass a fixed point + s
define: frequency + unit
number of waves passing a fixed point per second + Hz
define: wave speed + unit
the speed that a peak travels in the direction of the wave + ms-1
define: intensity + unit
power per unit area + Wm-2
I = P
A
define: path difference
fraction of wavelength by which the oscillations of two waves/points are seperated
define: phase difference
fraction of a cycle by whuch their oscillations are seperated measured in radians
phase difference equation
= path difference x 2π
what type of waves are EM
transverse
properties of EM waves
- dont need a medium
- all travel at c
order of EM spectrum
radio, micro, infared, visible, uv, x-rays, gamma
longitudinal waves + examples
parallel + sound
transverse waves + examples
transverse + EM, waves on a string
polarisation
if oscillations are restricted to a single plane, the wave is polarised
why cant longitudinal waves be polarised
they are oscillations along a line
refraction
change of direction when a wave passes at an angle across a boundry between two media
why does refraction occur?
waves travel at different speeds in different medias
refractive index, n
ratio of the speed of light in a vacuum, to the speed of light in that substance, c
refractive index equation:
n = c
v
snells law:
n₁ sinθ₁ = n₂ sinθ₂
what happens to rays of light travelling from a material with lower n to one with a higher n
they refract towards the normal
what happens to rays of light travelling from a material with higher n to one with a lower n
they refract away from the normal
if the angle of refaction is 90° what is the incidence angle called
critical angle
what happens if the angle of incidence is greater than the critical angle
the light will reflect at the boundry - total internal reflection
define: black-body
a surface that absorbs all EM radiation
define: coherant
waves that have a constant phase difference
define: interferance
the superposition of coherent waves
antinode
point of max amplitude. formed when progressive waves are in phase
node
point of displacement at all times, completely cancel out
distance between 2 nodes
1/2 λ
what is constructive interference
when 2 waves meet, if their displacements are in the same direction, they combine to give a bigger displacement
the points are known as maxima
what is destructive interference
when 2 waves meet exactly out of phase and cancel out
how to observe interference of sound waves
- playing the same sound out of two speakers at the same time
- walk across the room parallel to the line between the speakers
- quietest point = minima
- loudest point = maxima
how to observe interference of radio waves
- emitting radiowaves of same frequency from two stationary microwave emitters
- move a microwave detector along a line parallel
- low readings = minima
- high readings = maxima
diffraction
spreading of waves when they pass through a gap
when does maximum diffraction occur
when the gap is same size as wavelength
laser safety
- turn off when not in use
- never shine at a person
- avoid shining at reflective surfaces
- display warning on door
how are bright fringes formed
path difference between light from each slit is nλ, interfere constructively
how are dark fringes formed
path is (n+1/2)λ, interfere destructively
fringe seperation
distance between the centres of two adjacent minimas or maximas
wavelength of light equation using seperation:
λ = ax
D
number of orders of fringes seen for a given wavelength equation:
n = d
y
