waves- refraction Flashcards
what happens when light is absorbed in e.g glass
as light travels through glass some of its energy is transferred to the molecular structure of the material through interaction between the light and the molecules of the glass. the amplitude and/or intensity of the light is reduced by the absorption of the light by the glass
angle of incidence
angle between the normal to the interface and a light ray incident on the interface
angle of refraction
angle between the normal to an interface and a light ray refracting from that interface.
critical angle
the angle of incidence within a medium at which the angle of refraction is 90 degrees. thus TIR will occur if the angle of incidence is greater than the critical angle, and the ray is within the more optically dense material.
dispersion
light energy being lost to the material through which the light is moving.
one cause of pulse broadening signals transmitted along optical fibres
electromagnetic spectrum
a continuous spectrum comprising the whole wavelength (/ frequency) rage of electromagnetic radiation
frequency
the frequency of a wave is the number of whole wave cycles passing a point in unit time
intensity
of light (or other radiation) is the power per unit area in Wm-2
material dispersion
different wavelengths of light have slightly different speeds in the core (the refractive index of the core is slightly different for different wavelengths which is why glass can be used to separate white light into colours). no signal is perfectly monochromatic , it always contains a range of wavelengths . thus some wavelengths will take different times to travel along a length of optical fibre, even if they follow the same path. one cause of pulse broadening.
modal dispersion
of light when travelling along an optical fibre, is caused by light taking different length paths a long the optical fibre ( by reflecting from the is for the optical fibre at different angles) and so taking different amounts of time to travel a given distance.
“non axil rays take longer to travel along the fibre then axil rays”
one cause of pulse broadening
monochromatic light
light of a single frequency or wavelength
optical density
light moves slower through a medium or greater optical density.
TIR only occurs in the more optically dense material of the two materials (or vacuum) at an interface
optical fibre
a thin flexible strand of transparent material ( typically glass) that carries light along its length by repeated total internal reflection from its sides . an optical fibre consisting of a core whatever material the fibre is immersed in. it also provides strength ( the core has to be very thin to reduce modal dispersion) to the fibre. where fibres are bundles together it keeps the core of the fibre away from the adjacent fibres. if adjacent fibre cores touched the light could cross from one fibre to another.
pulse broadening
when a pulse of radiation, typically visible light of infra red travels along an optical fibre , different parts of the pulse take different amounts of time to travel a given distance due to modal and material dispersion . this means the received pulse is spread over a longer time than it was when is was transmitted . if sufficient time were left between pulses, pulse broadening might cause them to overlap and make it impossible to distinguish between one pulse and another. pulse broadening therefore limits the number of pulse and another. pulse broadening therefore limits the number of pulses that can be transmitted per unit time without loss of information, so is one limit on the rate of data transmission that can be achieved in digital systems
total internal reflection
whenever a wave crosses an interface between two media in which the wave has different velocities, a fraction of the wave energy is reflected from that interface. if a wave is incident on the interface within the more optically dense medium and the angle of incidence within that medium is equal to , or greater than , the critical angle , All of the wave is reflected and total internal reflection occurs.
refraction
as a wave crosses an interface between two media in which it has differing velocities, if it is not normal to the interface , it changes direction as it crosses the interface. the wave is closer to the normal in the more optically dense medium.
the wave frequency its he same in both media , but its wavelength changes , being longer in the medium in which its velocity is higher.
refractive index
the ratio of speed of light in a vacuum to the speed of light in the material
signal degradation
where a signal loses amplitude/ intensity , or is subject to pulse broadening e.g of light pulses along an optical fibre
transverse wave
light and all electromagnetic wave are transverse waves, that is waves in which oscillations are perpendicular to the direction of energy propagation
wave speed
the speed thatva wave travels at
