3.3.2 - Refraction, Diffraction & Interference

Refractive Index (n)

A measure of the relative speed of light in a material compared to in a vacuum (3 x 10^8 ms^-1)

Refraction

Light changing velocity when it travels across the boundary between two materials.

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A more optically dense material (higher refractive index) causes it to slow down and bend towards the normal and vice versa.

What changes when a wave refracts?

Wave speed and wavelength change.

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Frequency stays the same

Calculating Refractive Index

n = c (vacuum) / c (material) = λ (vacuum) / λ (material

Refractive index of air

Air is considered a vacuum as it doesn’t slow light down significantly so has a ^^refractive index of one^^

Snell’s Law

n1 sinθ1 = n2 sinθ2

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* n1 is the refractive index of material 1


* n2 is the refractive index of material 2


* θ1 is the angle of incidence of the ray in material 1
* θ2 is the angle of refraction of the ray in material 2

Total Internal Reflection

All light gets reflected off of a surface instead of passing through and being refracted.

TIR - Conditions

* Light has to be more from a more optically dense medium (higher n value) into a less optically dense medium (lower n value).

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* Angle of Incidence > Critical angle

Critical Angle

The angle of incidence which causes light to travel alongside the boundary due to an angle of refraction = 90\*

Critical Angle Formula

sinθ c = n2 / n1

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where n1 > n2

Uses of TIR

TIR is used in optical fibres which carry information in the form of light signals.

How do optical fibres work?

A light pulse is sent down an optical fibre and is detected at the other end generating a signal.

Optical Fibre Structure

Flexible thin tube of plastic or glass.

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Inner core is more optically dense and surrounded by cladding which is less optically dense

Function of cladding

* protects the core from damage

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* prevents signal degradation through light escaping the core, which can cause information to be lost

Signal Degradation causes -

* Absorption
* Dispersion

Absorption

Parts of the signal’s energy is absorbed by the fibre reducing the overall amplitude of the signal - could cause a loss of information

Dispersion

Causes Pulse Broadening which is when the received signal is broader then the original transmitted signal - broadened signals can overlap which causes loss of information.

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Types of dispersion

* Modal
* Material

Modal Dispersion - what is it?

Light rays enter fibre at different angles so take different paths hence they could take different amounts of time to travel along the fibre

Modal Dispersion - How to reduce it?

Make the core very narrow to reduce the possible path differences the light could have.Mater

Material Dispersion - what is it?

Light consisting of different wavelengths will travel at different speeds in a material

Material Dispersion - How to reduce it?

Use monochromatic light

Path difference

The difference in the distance travelled by two waves

Coherent Light Source

A light source where all light waves emitted have the ^^same frequency and wavelength^^ with a ^^constant phase difference.^^

Monochromatic light

Light of a single wavelength

Young’s Double Slit Experiment purpose

To demonstrate the wave properties of light.

How to make a light source cohesive

* Place single slit before a double slit to make the light have a constant phase difference

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* Use a filter to make the light monochromatic

What did young’s double slit experiment show?

A series of bright (maxima) and dark (minima) fringes - the central fringe is always a bright fringe.

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The path difference for light from one slit is different from the other slight so light meets on the screen in different phases.

Bright Fringes - causes

Light meets in phase and interferes constructively which occures when the path difference is a whole number of wavelengths (^^nλ^^).

Dark Fringes - Causes

Light meets out of phase and interferes destructively which occurs when the path difference is a whole number and a half wavelengths (^^(n+½)λ^^).

Young’s double slit equation

sw = λD

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where -

* s = slit separation (m)
* w = fringe separation (m)
* λ = wavelength (m)
* D = distance from screen (m)

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slit separation is measured from the centre of one slit to the centre of the next

Diffraction

The spreading out of waves as they pass through or around a gap.

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The greatest diffraction occurs when the gap is the same size as the wavelength - when the gap is smaller most waves get reflected.

Single Slit Diffraction Pattern

A bright(est) central fringe double the width of all the other fringes with alternating dark and bright fringes on either side.

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Intensity of fringes decreases from central fringe,

Singe slit - changing slit width

Increasing slit width = less diffraction so central maxima is narrower and more intense

Single Slit - changing wavelength

Increasing the wavelet will increase how much light diffracts causing a thicker less intense central maxima

White light single slit

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White light double slit

Diffraction Grating Pattern

More distinct dark and bright fringes.

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The central fringe is called the zero order line and then the first order lines and then the second order lines etc.

What is a diffraction grating?

A slide containing many equally spaced slits.

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d = 1/N

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where -

* d = distance between the slits
* N = number of slits ^^per metre^^

Diffraction Grating equation

d sinθ = nλ

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where -

* d = distance between the slits
* θ = the angle between the zero order line and next order line
* n = the order
* λ = the wavelength

Diffraction Grating - changing wavelength