Refraction

What is the refractive index of a material?

#The ratio of how fast light travles through that particular substance compared to the speed of ligth in a vacuum

What is refraction?

#The change in direction of a wave as it passes trhough a boundary between mediums of different density

What happens when light passes into a more optically dense medium?

#the slower the waves travel and the smaller the angle of refraction

• The light bends towards the normal

What happens when light passes into a less optically dense medium?

#the faster the waves travel and the larger the angle of refraction

• The light bends away from the normal

What happens to the waves properties as it passes through a different medium?

#its speed and wavelength change, but its frequency remains the same

What happens when a light ray is incident n the bundary at 90?

#The wave passes straight through without direction

• This is because the whole wavefront enters the boundary at the same time

How can the refractive index of a material be calculated

#N = c/c_s

• Where:

  • n = refractive index of material 

  • c = the speed of light in a vacuum (m s^–1)

  • c_s = the speed of light in a substance (m s^–1)

What is the equation for Snell’s law?

#n₁sin0₁ = n₂sin0₂

• Where:

  • n₁ = the refractive index of material 1

  • n₂ = the refractive index of material 2

  • θ₁ = the angle of incidence of the ray in material 1

  • θ₂ = the angle of refraction of the ray in material 2

What happens to the critical angle as refractive index of a material gets bigger?

#For a larger n then θ_C is smaller

What happens when the angle of incidence = critical angle?

#--Angle of refraction = 90°

• The refracted ray is refracted along the boundary between the two materials

What happens when the angle of incidence <critical angle?

• #When the angle of incidence < critical angle then:

  • the ray is refracted and exits the material

What happens when the angle of incidence >critical angle?

• When the angle of incidence > critical angle then:

  • the ray undergoes total internal reflection

How do you work out the critical angle of material?

#sin0_c = n₂/n₁= sin0₂/sin0₁

• Where:

  • n₁ = refractive index of material 1

  • n₂ = refractive index of material 2

  • θ_c = critical angle of material 1

What are the requirements for total internal reflection?

• #The angle of incidence within the denser medium is greater than the critical angle (I > θ_c)

• The incident refractive index n₁ is greater than the refractive index of the material at the boundary n₂ (n₁ > n₂)

What law does total internal reflection follow?

#angle of incidence = angle of reflection

What doe fibre optic use?

#Total internal reflection

How do fibre optics use TIR?

#

1- Light (normally monochromatic), refracts when it enters the optical fibre at one end

2- It undergoes total internal reflection against the sides of the fibre unitl it reaches the other end where it is refracted out

What are the three components of optical fibres?

#

• An optically dense core tube, made of plastic or glass

• A lower optically dense cladding surrounding the core

• An outer sheath

How are fibre optics used?

• #Communications, such as telephone and internet transmission

• Medical imaging, such as endoscopes

When can total internal reflection only occur in fibre optic ?

#TIR only occurs when n_cladding < n_core

What is the role of cladding?

• #Protect the thin core from damage and scratching

• Prevent signal degradation through light escaping the core, which can cause information from the signal to be lost

• It keeps the signals secure and maintains the original signal quality

• It keeps the core separate from other fibres preventing information crossover

What are the two types of dispersion?

#Pulse broadening 

What is pulse broadening?

• #Where the pulses emerging from the fibre are longer than those entering

(White light in an optical fibre)

(White light splits into all colours of the spectrum, causing dispersion.)
(Beam widening in an optical fibre)

White light in an optical fibre

White light splits into all colours of the spectrum, causing dispersion.

Beam widening in an optical fibre

Different wavelengths travel at different speeds, so they spread apart and the beam widens.

Material dispersion

White light is separated into its spectral components because each wavelength travels at a different speed in the fibre material.

Wavelength speeds in a medium

All wavelengths travel at the same speed in a vacuum, but at different speeds in a medium like glass.

Violet light speed

Violet light travels slowest because it has the shortest wavelength and is refracted the most.

Violet light angle of incidence

Violet light has the smallest angle of incidence on the fibre boundary compared to other colours.

Angle of incidence vs angle of reflection

They are equal; a smaller angle of incidence means a smaller angle of reflection.

Violet light travel time in fibre

Violet light takes longer because it undergoes more reflections, increasing the total path length.

When does modal dispersion occur?

#Modal dispersion occurs when the monochromatic light pulses in the optical fibre spread out

—> This is because each part of the wave front has a different angle of incidence and consequently a different angle of reflection. Therefore the wavefront undergoes TIR a different number of times and each wavefront teaches he end of the fibre at different times

When does the modal dispersion occur more?

#When the core of the fibre is wider

How can modal dispersion be prevented?

#the core needs to be very narrow

When does absoprtion of a signal in an optical fibre occur?

#When the fibre absorbs part of the signal’s energy

• This reduces the amplitude of the signal, which can lead to a loss in the information transmitted

What can pulse broadening lead to?

#This can result in the merging of pulses, which distorts the information in the final pulse and decreases the amplitude of the signal

How can absorption be reduced?

#Use an extremely transparent core

• Use optical fibre repeaters so the pulse is regenerated before significant absorption has taken place

How can pulse broadening be reduced?

• #Use a core that is as narrow as possible to reduce the possible differences in the path length of the signal

  • Use of a monochromatic source so the speed of the pulse is constant

  • Use optical fibre repeaters so the pulse is regenerated before significant pulse broadening has taken place

  • Use a single-mode fibre, where only a single wavelength of light passes through the core, to reduce multipath modal dispersion