Refraction Flashcards
When does refraction occur?
- when light passes a boundary between two different transparent media
- At the boundary, the rays of light undergo a change in direction
Why does a ray of light move towards/ away from a normal
due to the change in speed when travelling in different substances
When light passes into a denser substance the rays will slow down
What wave properties change during refraction
speed and wavelength – the frequency of waves does not change
What do the components of the refractive index equation mean
What is a material with a high refractive index called
optically dense
Why is refractive index (n) always bigger than 1
Because the speed of light in a substance will always be less than the speed of light in a vacuum
What do the components of snell’s law equation mean
When does total internal reflection occur and how does this relate to the angle of incidence
- When the angle of refraction is exactly 90° and the light is refracted along the boundary
- At this point, the angle of incidence is known as the critical angle θc
What do the components of the critical angle equation mean
Definition for when total internal reflection occurs
The angle of incidence is greater than the critical angle and the incident refractive index n1 is greater than the refractive index of the material at the boundary n2
What are the two conditions required for total internal reflection to occur
The angle of incidence > the critical angle
The refractive index n1 is greater than the refractive index n2
What are fibre optics (which use total internal reflection) good for
sending high speed light signals over large distances
Examples of fibre optics uses
- Communications, such as telephone and internet transmission
- Medical imaging, such as endoscopes
What are the three components that make up a fibre optic
- An optically dense core, such as plastic or glass
- A lower optical density cladding surrounding the core
- An outer sheath
What is the purpose of the fibre optic outer sheath?
- Prevents physical damage to the fibre
- Strengthens the fibre
- Protects the fibre from the outside from scratches
What is the purpose of the fibre optic cladding?
- It protects the core from damage
- It prevents signal degradation through light escaping the core
- It prevents scratching of the core
- It 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 and why
when white light is used instead of monochromatic light
because different wavelengths of light travel at different speeds:
- Blue light travels slower than red light due to the greater refractive index
- Therefore, the red light reaches the end before the blue light
When does modal dispersion occur and why
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 total internal reflection at the core-cladding boundaries
To prevent modal dispersion, the core needs to be very narrow
What are the advantages of using a narrower optical fibre core
- Less light is lost by refraction out of the core
- 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 pulse absorption occur AND what are its effects
- Part of the signal’s energy is absorbed by the fibre
- The signal is attenuated by the core
This reduces the amplitude of the signal, which can lead to a loss of information
When does pulse broadening occur AND what are its effects
- is caused by modal and material dispersion
- The consequence of pulse broadening is that different pulses could merge, resulting in a completely distorted final pulse
How can pulse broadening and absorption be reduced
- To reduce absorption:
Use a core which is extremely transparent
Use of optical fibre repeaters so that the pulse is regenerated before significant absorption has taken place - To reduce pulse broadening:
Make the core as narrow as possible to reduce the possible differences in path length of the signal
Use of a monochromatic source so the speed of the pulse is constant
Use of optical fibre repeaters so that the pulse is regenerated before significant pulse broadening has taken place
How are the wavelength, frequency and speed of light affected when a ray of light travels from air to glass
- wavelength decreases
- frequency stays the same
- speed decreases
