refractive index

Define refractive index (n) of a material.

A property of material which measures how much it slows down light passing through it

What happens when a material is more optically dense (higher refractive index).

Light travels slower in a material and bends more sharply

What’s the difference between refractive index and optical density?

Refractive index is a quantitative description (numbers, measuring speed reduction and bending of light)

Optical density is a qualitative description (resistance to light propagation)

(They both describe the same thing one with numbers the other with words- e.g. ‘more dense’ (therefore higher refractive index,usually))

What’s the difference between ‘refractive index’, ‘absolute refractive index’ and relative refractive index

Refractive index= absolute refractive index which is a measure optically density- comparing light speed in a medium (cs) to its speed in a vacuum (c)

Relative refractive index compares light speed between two non-vacuum media (e.g. water and oil)

What does equation 1n2=n2/n1 mean

What is the angle of incidnece?

The angle the incoming light makes to the normal

What is the angle of refraction?

The angle the refracted ray makes with the normal

What happens when light enters an optically denser medium?

The refracted ray bends towards the normal

(Because the light slows down in the denser material, causing a smaller angle of refraction compared to angle of incidence)

What is meant by a coherent fibre bundle?

Maintains the spatial arrangement of its constituent fibres, preserving the image

Constituent= the individual optical fibres that, when bundled together, form the complete fiber (basically refers to the individual components that make up a larger whole)

What is critical angle (0c)?

The angle of incidence, in an optically denser medium, at which the angle of refraction is exactly 90 degrees from the normal, traveling along the boundary between the two media .

What is Total Internal Reflection (TIR)?

When the angle of incidence is greater than the critical angle (below/equal to 90 degrees)

When you reach critical angle, the angle of refraction is exactly 90 degrees.

Why does TIR happen?

Because light speeds up when moving from a denser material to a less dense one - this speed change forced the light to bend away from the normal line

What is a useful application of total internal reflection?

Optical fibres

Describe how step index optical fibre work relative to optical density.

They have an optically dense core (high refractive index) surrounded by cladding (lower optical density) which allows TIR to occur

TIR can only occur when a wave passes from a high refractive index into a lower one

What other uses are there for the cladding?

1. Cladding protects the core from damage

2. Prevents signal degradation through light escaping the core

3. Provides tensile strength to the optical fibre- so it doesn’t break

What are the two reasons why the core must be protected from scratches?

1. Water can get into the scratch- which increases the refractive index to a level that may be higher than the core, prevent TIR to occur, and signal will exit the core Provides tensile

2. Scratch may alter angle at which the signal interacts with the cores boundary such that the angle is lowered below critical angle - prevents TIR and causes signal loss

What is signal degradation?

The loss of signal quality as it travels through a medium making it weaker, distorted, or harder to interpret.

What is signal degradation caused by?

Absorption- where part of the signals energy is absorbed by the materials fibre, resulting in loss of amplitude of the signal, leads to loss of information (doesn’t affect frequency)

Dispersion- causes pulse broadening- received signal is broader than the original transmitted signal. Broadening signals can overlap causing loss of information.

How can the effect of absorption be reduced?

Use an optical fibre repeater- boost signal at periods along the fibre

What are the two types of dispersion?

Modal dispersion

Material dispersion

what is modal dispersion?

When light rays enter the fibre at different angles, therefore taking different paths along the fibre, leading to rays taking a different amount of time to travel along the fibre, causing pulse broadening.

causes different path lengths

How can modal dispersion be reduced?

Make the core very narrow, making the possible difference in path lengths smaller

What is material dispersion?

caused by light consisting of different wavelengths, therefore light rays will travel at different speeds along the fibre, leading to pulse broadening

Caused by different speeds for different wavelengths

How can material dispersion be reduced?

Use monochromatic light

What is the consequence of modal and material dispersion?

Pulse broadening- the pulse (flash of light) last longer when it gets to a certain point due to the wave stretching out

How does pulse broadening link to modal and material dispersion?

Modal and material dispersion are the reason pulse broadening happens (So pulse broadening is the result of these dispersions)

How can both absorption and dispersion be reduced?

Use an optical fibre repeater which regenerates (restored a weak/distorted etc to its original state) the signal during its travel to its destination

Define signal.

a stream of pulses of light

What happens to a light ray that passes through a medium with parler entry and exit surfaces?

The emergent ray is parallel to the incident ray

The angle of emergence (in the final medium) equals the angle of incidence (in the initial medium)

A light ray passes through a glass block with parallel sides. If it enters the glass from the air at 45 degrees, what angle will it make with the normal when it exits back into the air?

45 degrees

What does snells describe?

Snells law describes how light bends (refracts) when it passes from one medium to another