3.5 Refraction Flashcards
When does refraction occur?
when light passes a boundary between 2 different transparent media
-the rays if light undergo a change in direction
what is the normal?
- the direction of the refraction is taken as the angle from a hypothetical line
- this line is perpendicular to the surface of the boundaries and is represented by a straight dotted line
What is the change in direction of refraction dependent on?
which media the light rays pass between
What direction does light refract when it passes from less dense to a more dense material?
light bends towards the normal
- e.g. air to glass
What direction does light refract when it passes from a more dense to a less dense media?
light bends away from the normal
-e.g. glass to air
What direction does light refract when it passes along the normal
the light does not bend at all
Why does the direction of light change when refraction occurs?
- due to the change in speed when travelling in different substances
- when light passes into a denser substance the rays will slow down, hence bend towards the normal
What properties change during refraction?
speed and wavelength
What is the refractive index?
a property of a material which measures how much light slows down when passing through it
What is the refractive index used for?
to determine the speed at which light travels within different substances
What property does an optically dense material have?
high refractive index
- causes light to travel slower
What is critical angle?
- when the angle of refraction is exactly 90˚ the light is reflected across the boundry
- the angle of incidence here is known as the critical angle
When does total internal reflection occur?
- when the angle of incidence is greater than the critical angle
- the incident refractive index is greater than the refractive index of the material at the boundary
What are the 2 conditions for TIR to occur?
- angle of incidence > the critical angle
- the refractive index of material 1 is greater than the refractive index of material 2
What uses to fibre optics have?
- communications, such as telephone and internet transmission
- medical imaging, such as endoscopes
What are the 3 main components that make up optical fibres?
- optically dense core, such as glass or plastic
- a lower optical density cladding surrounding the core
- an outersheeth
How does TIR occur in an optical fibre?
since the refractive index of the core is > the refraction index of the cladding TIR can occur
What is the outer sheath of an optical fibre?
- prevents physical damage to the fibre
- strengthens the fibre
- protects the fibre from the outside from scratches
What is the role of cladding in the fibre optics?
- protects the core from damage
- prevents signal degradation through light escaping the core, causing information from signal to be lost
- maintains the quality of the signal whilst keeping it secure
- keeps the core away from adjacent fibre cores hence preventing crossover of information to other fibres
- it provides the fibre with strength and prevents breakage given that the core needs to be very thin
When does material dispersion occur?
when white light is used instead off monochromatic light
why does material dispersion occur?
- different wavelengths of light travel at different speeds
When does modal dispersion occur?
- when the light pulses in the optical fibre spread out due to the different angles of incidence in the original pulse
- this is more prominent in wider cores as the light travelling along the axis of the core travels a shorter distance than light undergoing TIR at the core cladding boundaries
How can modal dispersion be prevented?
- the core needs to be very narrow
What are the advantages of using a narrow core?
- less light is lost by refraction out of the core
- there is a smaller change in angle between each reflection, so the angle of incidence is less likely to fall below the critical angle
- less overlapping pulses hence reduction of modal dispersion
- the quality of the signal will be better and less distorted
- the signal will be transferred quicker leading to improved data and information transfer
When does absorption occur?
- when part of the signals energy is absorbed by the fibre
- the signal is attenuated by the core
What is the effect of absorption?
- reduction of the amplitude of the signal, which can lead to a loss of information
What is pulse broadening caused by?
modal and material dispersion
What are the consequences of pulse broadening?
different pulses could merge, resulting in a completely distorted final pulse
How can you reduce absorption?
- use a core which is extremely transparents
- use of optional fibre repeaters so that the pulse is regenerated before significant absorption has taken place
How can you reduce pulse broadening?
- make the core as narrow as possible to reduce the possible differences in path length of the signal
- use of monochromatic source so the speed of the pulse is constant
- use of optical fibre repeaters so the pulse is regenerated before significant pulse broadening has taken place
- use of single mode fibre to reduce multi path modal dispersion
