Refraction is the change in direction of a wave when it passes through a boundary between mediums of different density, caused by a change in the speed of the wave.

Define refractive index .

The refractive index of a material is a measure of how much light slows down when passing through it, calculated as the ratio of the speed of light in a vacuum to the speed of light in the substance.

How does the optical density of a medium affect the speed and direction of light entering it?

The more optically dense the medium, the slower light travels and the light bends towards the normal The less optically dense the medium, the faster light travels and the light bends away from the normal

Since the speed of light in a substance is always less than the speed of light in a vacuum, the value of n is always ..........

Since the speed of light in a substance is always less than the speed of light in a vacuum, the value of n is always greater than one .

True or False? The refractive index of air is taken as one because light does not travel through air.

False. Light does travel through air. The refractive index of air is taken as approximately one because light does not slow down significantly when travelling through air, compared with a vacuum.

In Snell's law, what do n 1 and n 2 represent?

n 1 is the refractive index of material 1, and n 2 is the refractive index of material 2.

From where are the angles θ 1 and θ 2 always measured in Snell's law?

θ 1 and θ 2 are always measured from the normal .

In Snell's law, θ 1 is the angle of .......... of the ray in material 1, and θ 2 is the angle of .......... of the ray in material 2.

In Snell's law, θ 1 is the angle of incidence of the ray in material 1, and θ 2 is the angle of refraction of the ray in material 2.

True or False? If n 2 is greater than n 1 , then θ 2 is greater than θ 1 .

False. If n 2 > n 1 , then by Snell's law, θ 2 must be smaller than θ 1 — light bends towards the normal when entering a more optically dense material.

Define a step-index optical fibre .

A step-index optical fibre is one in which the refractive index of each component (core, cladding, sheath) increases moving from the outside to the centre, so that total internal reflection occurs at the core-cladding boundary.

What are the three main components of an optical fibre?

An optically dense core tube A lower optically dense cladding surrounding the core An outer sheath

What condition must the refractive indices of the core and cladding satisfy for total internal reflection to occur?

Total internal reflection only occurs when n cladding < n core .

State two roles of the cladding in an optical fibre.

Protects the core from damage and scratching Prevents signal degradation by stopping light escaping the core Keeps signals secure and maintains signal quality Keeps the core separate from other fibres, preventing information crossover

Why does material dispersion cause pulse broadening in an optical fibre carrying white light?

Each wavelength travels at a different speed in the fibre. Violet light travels slowest and undergoes more reflections , so it takes longer to reach the end of the fibre, causing the pulse to broaden.

What causes modal dispersion, and how can it be reduced?

Each part of the wavefront has a different angle of incidence , so it undergoes total internal reflection a different number of times and arrives at a different time. It is more prominent in wider cores, so it can be reduced by using a narrower core.

Absorption in an optical fibre reduces the .......... of the signal, which can lead to a loss in the information transmitted.

Absorption in an optical fibre reduces the amplitude of the signal, which can lead to a loss in the information transmitted.

State three ways to reduce pulse broadening in an optical fibre.

Use a core that is as narrow as possible Use a monochromatic source Use optical fibre repeaters Use a single-mode fibre

True or False? Increasing the width of an optical fibre's core reduces modal dispersion.

False. Modal dispersion is more prominent in wider cores, since total internal reflection occurs more times. Using a narrower core reduces modal dispersion.

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Refraction (AQA A Level Physics): Flashcards

Exam code: 7408

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FrontRefraction at a Plane Surface
Define refraction.
FrontRefraction at a Plane Surface
State the equation for refractive index and define each term.
BackRefraction at a Plane Surface
$n = \frac{c}{c_{s}}$
n = refractive index of the material
c = speed of light in a vacuum (m s^-1)
c_s = speed of light in the substance (m s^-1)

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Cards in this collection (21)

Define refraction.
Refraction is the change in direction of a wave when it passes through a boundary between mediums of different density, caused by a change in the speed of the wave.
Define refractive index.
The refractive index of a material is a measure of how much light slows down when passing through it, calculated as the ratio of the speed of light in a vacuum to the speed of light in the substance.
State the equation for refractive index and define each term.
$n = \frac{c}{c_{s}}$
- n = refractive index of the material
- c = speed of light in a vacuum (m s^-1)
- c_s = speed of light in the substance (m s^-1)
How does the optical density of a medium affect the speed and direction of light entering it?
- The more optically dense the medium, the slower light travels and the light bends towards the normal
- The less optically dense the medium, the faster light travels and the light bends away from the normal
Since the speed of light in a substance is always less than the speed of light in a vacuum, the value of n is always ..........
Since the speed of light in a substance is always less than the speed of light in a vacuum, the value of n is always greater than one.
True or False?
The refractive index of air is taken as one because light does not travel through air.
False.
Light does travel through air. The refractive index of air is taken as approximately one because light does not slow down significantly when travelling through air, compared with a vacuum.
Define Snell's law.
Snell's law relates the angle of incidence to the angle of refraction at a boundary between two media:
$n_{1} \sin θ_{1} = n_{2} \sin θ_{2}$
In Snell's law, what do n₁ and n₂ represent?
n₁ is the refractive index of material 1, and n₂ is the refractive index of material 2.
From where are the angles θ₁ and θ₂ always measured in Snell's law?
θ₁ and θ₂ are always measured from the normal.
In Snell's law, θ₁ is the angle of .......... of the ray in material 1, and θ₂ is the angle of .......... of the ray in material 2.
In Snell's law, θ₁ is the angle of incidence of the ray in material 1, and θ₂ is the angle of refraction of the ray in material 2.
A light ray hits a glass surface with an angle of incidence of 39° in air and refracts at 25°. Calculate the refractive index of the glass.
$n_{2} = \frac{\sin 39 °}{\sin 25 °} = 1.5 (2 s . f .)$
True or False?
If n₂ is greater than n₁, then θ₂ is greater than θ₁.
False.
If n₂ > n₁, then by Snell's law, θ₂ must be smaller than θ₁ — light bends towards the normal when entering a more optically dense material.
Define a step-index optical fibre.
A step-index optical fibre is one in which the refractive index of each component (core, cladding, sheath) increases moving from the outside to the centre, so that total internal reflection occurs at the core-cladding boundary.
What are the three main components of an optical fibre?
- An optically dense core tube
- A lower optically dense cladding surrounding the core
- An outer sheath
What condition must the refractive indices of the core and cladding satisfy for total internal reflection to occur?
Total internal reflection only occurs when n_cladding < n_core.
State two roles of the cladding in an optical fibre.
- Protects the core from damage and scratching
- Prevents signal degradation by stopping light escaping the core
- Keeps signals secure and maintains signal quality
- Keeps the core separate from other fibres, preventing information crossover
Why does material dispersion cause pulse broadening in an optical fibre carrying white light?
Each wavelength travels at a different speed in the fibre. Violet light travels slowest and undergoes more reflections, so it takes longer to reach the end of the fibre, causing the pulse to broaden.
What causes modal dispersion, and how can it be reduced?
Each part of the wavefront has a different angle of incidence, so it undergoes total internal reflection a different number of times and arrives at a different time. It is more prominent in wider cores, so it can be reduced by using a narrower core.
Absorption in an optical fibre reduces the .......... of the signal, which can lead to a loss in the information transmitted.
Absorption in an optical fibre reduces the amplitude of the signal, which can lead to a loss in the information transmitted.
State three ways to reduce pulse broadening in an optical fibre.
- Use a core that is as narrow as possible
- Use a monochromatic source
- Use optical fibre repeaters
- Use a single-mode fibre
True or False?
Increasing the width of an optical fibre's core reduces modal dispersion.
False.
Modal dispersion is more prominent in wider cores, since total internal reflection occurs more times. Using a narrower core reduces modal dispersion.

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