3.3.2.3 - Refraction At A Plane Surface Flashcards
Refractive index of a material =
C / Cs (speed of light/speed of light in that material)
What is the refractive index of air?
1.
What is snell’s law?
n1 sin 01 = n2 sin 02
n = refractive index.
What causes light to be refracted along the boundary?
When light is reflected at the critical angle.
What causes light to be totally internally reflected?
When the angle is greater than the critical angle.
What values can sin take?
Smaller than 1.
Sin c =
1/ refractive index
n2 / n1
What are optical fibres used for?
To carry signals over long distances and round corners.
Why is cladding needed for optical fibres?
Optical fibres themselves have a high refractive index but are surrounded by cladding with a lower refractive index to allow total internal reflection. Moreover cladding protects the fibre from scratches which could let light escape.
Why is the fibre narrow?
So that light always hits the boundary between the fibre and cladding at an angle bigger than the critical angle.
What two things cause signal degradation?
Absorption and dispersion.
What does absorption cause?
Loss in amplitude.
How is absorption caused?
As signal travels, some of its energy is lost through absorption by the material the fibre is made from.
What does dispersion cause?
Pulse broadening.
What are the two types of dispersion?
Modal and material.
How does Modal dispersion occur?
Light rays enter the fibre at different angles, and so take different paths. The rays which take a longer path take longer to reach the other end than those that travel down the middle of the fibre.
How and why can modal dispersion be stopped?
A single-mode fibre only lets light take one path, so it stops modal dispersion.
What causes material dispersion?
As light consists of different wavelengths that travel at different speeds in the fibre, this causes some light wavelengths to reach the nod of the fibre faster than others.
What can stop material dispersion?
Using monochromatic light.
Why is pulse broadening a problem?
Broadened signals can overlap each other and confuse the signal.
What can be used to reduce both signal absorption and dispersion?
An optical fibre repeater can be used to boost and regenerate the signal every so often.
