3.5 - Refraction Flashcards
When does refraction occur?
When light passes a boundary between 2 different transparent media at an angle.
What happens at the boundary between two mediums when light refracts?
The rays of light undergo a change in direction.
How is the angle of direction taken in refraction?
From the normal (perpendicular to the surface of the boundaries).
What happens to the direction of light when going from a less dense to more dense medium?
The light bends towards the normal.
(air to glass)
What happens to the direction of light when going from a more dense to less dense medium?
The light bends away from the normal.
(glass to air)
What happens to the direction of light when passing along the normal?
Light does not bend at all.
Why does light refract when going to a more or less dense medium?
The change in direction occurs due to the change in speed when travelling in different substances (light slows down when going into a denser medium so it bends towards the normal).
What is the refractive index? (n)
A property of a material which measures how much light slows down when passing through it.
How do you calculate the refractive index? (equation)
n = c / cₛ
c = the speed of light in a vacuum (3 x 10⁸) (ms⁻¹)
cₛ = the speed of light in a substance (ms⁻¹)
What is a material with a high refractive index called?
An optically dense material.
Why must n always be above 1 when calculating the refractive index?
Because the speed of light in a substance can never be higher than the speed of light in a vacuum.
What is Snell’s Law?
n₁sinθ₁ =n₂sinθ₂
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
Where are the angles θ1 and θ2 are always taken from?
The normal.
What is the angle of incidence?
The angle between the normal and the ray of light.
What is the critical angle? θ𝒸
When the angle of incidence goes over 90 degrees and the ray changes from just refracting to total internal reflection.
How can the critical angle be found?
sin θ𝒸 = n₁ / n₂
What is total internal reflection (TIR)?
When 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 2 conditions for TIR?
- The angle of incidence > the critical angle
- The refractive index n₁ is greater than the refractive index n₂
How do you calculate the angle of reflection?
critical angle - angle of incidence
What are 2 things fibre optics are used for?
Communications - (telephone/internet)
Medical imaging - (endoscopes)
What do fibre optics do?
Utilise the phenomenon of total internal reflection to send high speed light signals over large distances.
What is the cladding of a fibre optic?
The material with a lower refractive index that surrounds the core.
What is the core of a fibre optic?
An optically dense material (like plastic/glass).
What are the 3 components of optical fibres?
- core
- cladding
- outer sheath
Why is the core more optically dense?
So that total internal refraction occurs.
n cladding < n core
What are the 3 purposes of the outer sheath?
- Prevents physical damage to the fibre
- Strengthens the fibre
- Protects the fibre from the outside from scratches
What are 3 purposes of the cladding?
- Keeps signals secure and maintains quality
- Keeps the core away from adjacent fibre cores so there is no crossover signals
- Prevents core from damage
What is material dispersion?
When white light is used instead of monochromatic light.
What is pulse broadening?
Wen material dispersion occurs and the difference frequencies of light travel at different rates.
The pulses that emerge are longer than they should be.
What is modal dispersion?
When the light pulses in the optical fibre spread out due to the different angles of incidence in the original pulse.
Why is the core so narrow?
To prevent modal dispersion.
