why is there refraction:
two materials of different optical densities, light will travel at different speeds in each
from less optically dense to more optically dense, light bends toward the normal
from more optically dense to less optically dense, light bends away from the normal
refractive index: a measure of optical density
nx = c/cx
where
nx is the refractive index of material x
c is the speed of light in a vacuum (ms^-1)
cx is the speed of light in material x (ms^-1)
n>1, as if n<1 the speed of light in x is greater than the speed of light in a vacuum, which isn't the case
the greater the value of n, the more optically dense, as light is being slowed more
refractive index of air is approximately 1
snell's law of refraction:
n1sin𝛳1 = n2sin𝛳2
where
n1 is the refractive index of material 1
sin𝛳1 is angle 1
n2 is the refractive index of material 2
sin𝛳2 is angle 2
critical angle:
sin𝛳c = n2/n1
note: this is only for more dense to less dense i.e. n1 > n2
fibre optics:
optical fibre: a thin flexible transparent fibre used to carry light pulses from one end to the other
uses: communication, endoscopes
cladding:
prevents crosstalk
provides protection
stops signals escaping
needs a lower refractive index than the core
ideally:
the core is thin to reduce the effects of MODAL DISPERSION
use monochromatic light to reduce effects of MATERIAL DISPERSION
modal dispersion: the lengthening of a light pulse as it travels along an optical fibre, due to rays that repeatedly undergo total internal reflection having to travel a longer distance than rays that undergo fewer total internal reflections
material dispersion: caused by using white light. all wavelengths of white light travel at slightly different speeds and so leads to pulse broadening
path difference: the difference in distances from two coherent sources to an interference fringe
coherence: two waves sources are coherent if they emit waves with a constant phase difference
superposition: the process by which two waves combine into a single waveform when they overlap
stationary waves: wave pattern with nodes and antinodes formed when two or more progressive waves of the sae frequency and amplitude pass through each other
nodes: positions on a stationary wave which do not vibrate, where there is zero displacement
antinodes: positions on a stationary wave where there is maximum displacement
frequency for first harmonic only:
f = (1/2L)√(T/μ)
where:
f is the frequency (Hz)
T is the string tension (N)
L is the length of the string (m)
μ is the mass per unit length of the string (m/l)
longitudinal waves:
direction of travel/propagation is parallel to direction of vibration
e.g. sound
transverse waves:
direction of travel/propagation is perpendicular to direction of vibration
e.g. EM waves, waves on a string
all EM waves travel at 3 x 10^8 ms^-1 in a vacuum
polarisation: restricts the oscillations of a transverse wave to one plane
this is proof that the waves of the EM spectrum are transverse. if they were longitudinal waves the oscillations would not be restricted by the polarising filter
applications of polarisers:
TV aerials get the best reception when they point to the transmission source so they absorb the maximum amount of the radio waves
polaroid glasses reduce unwanted reflected light
polaroid material
oscillation of the particles of the medium
progressive waves: waves which travel through a substance or through space if electromagnetic
amplitude: distance from the equilibrium to maximum displacement or minimum displacement, measured in metres (m)
frequency: the number of cycles of a wave that pass a point in 1 second, measured in Hertz (Hz) (F=1/T)
wavelength: the least distance between two adjacent vibrating particles with the same displacement and velocity at the same time (e.g. distance between two adjacent peaks), measured in metres (m)
displacement: distance from the equilibrium position, measured in metres (m)
time period: time for one complete wave cycle, measured in seconds (s)
wave speed = frequency x wavelength (c=f𝜆)
measured in metres per second (ms^-1)
phase: the position of a point in time on a wave cycle
phase difference: the fraction of a cycle between the vibrations of two vibrating particles, measured either in radians or degrees, or as fractions of a cycle
two sources are IN PHASE if both waves are at the same point in their cycles at the same time
two sources are in ANTIPHASE if both waves are at exactly opposite points in their cycles at the same time
two sources are OUT OF PHASE if the waves are at different points in their cycles at the same time
if the phase difference is an odd number of 𝜋 radians, the waves are in antiphase
if the phase difference is an even number of 𝜋 radians, the waves are in phase
