Waves Flashcards by Ashraf Ali

What is a wave caused by?

Oscillations of particles or FIELDS

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What is a progressive wave?

Transfers energy from one place to another without transferring matter

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What are the features of waves? [5]

-Amplitude (A), metres
-Wavelength (λ), metres
-Frequency (f), hertz
-Time Period (T), seconds
-Wavespeed (c), m/s

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What are radians and what is its symbol?

Its another measurement for angles

Its symbol is pi

1 radian = 180 degrees

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What is phase difference?

How far one wave lags behind the other

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What is path difference?

The distance, measured in metres between two points which are out of phase

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What happens if two points are one wavelength apart?

Then there is no phase difference between them

[this is because they are in phase]

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What is an unpolarised wave?

Waves that oscillate in multiple directions/planes

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What is polarisation?

The process of restricting an unpolarised wave into one plane
[this is done by using a polarising filter aka polaroid]

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What is superposition?

When two waves combine /overlap to form a single wave aka resultant wave

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What happens when 2 waves travelling in opposite directions superpose?

It forms a standing waves which doesn’t transfer energy ; instead it uses its energy to oscillate

[they need to be travelling opposite directions to superpose]

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What is coherence?

When waves have similar wavelength, frequency and a fixed phase difference

[standing waves have to come from coherence]

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Why are they called standing waves?

Because they’re restricted to one medium

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What is interference?

An overall pattern that results when two or more waves interfere with each other

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What is destructive interference?

When two waves are out of phase and their individual displacements cancel each other out producing zero resultant displacement OR it can reduce it

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What is constructive interference?

Occurs when two waves are in phase and their individual displacements sum to produce a larger resultant displacement

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What are harmonics?

All possible standing waves in a medium

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What are nodes?

Position on standing wave that has no displacement

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What are antinodes?

Position on standing wave that has maximum displacement

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What is the fundamental frequency?

The lowest frequency in a periodic waveform

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What is refraction?

The bending of light as it passes from one medium to another

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What are the conditions for refraction to occur?

Wave must be travelling to a different medium
The incident ray must be at an angle to the boundary/normal

How do you know whether a wave will bend towards the normal or away from the normal?

From less dense to more dense (e.g air to glass), light bends towards the normal.

From more dense to less dense (e.g. glass to air), light bends away from the normal

What is the refractive index?

A measure of how easy it is for light to pass through a medium

( has no units )

At what speed does the an electromagnetic wave travel at?

Speed of light = 3x10^8ms^-1

Name the waves in the EM spectrum in order from lowest frequency to highest [7]

Radio , microwave, infrared, visible , ultraviolet, x-ray, gamma

Define critical angle

What happens if the critical angle is equal to the angle of incidence?

Then light will refract at 90 degrees

What happens if the critical angle is less than the angle of incidence?

Then light will refract

What happens if the critical angle is greater than the angle of incidence?

Then TIR occurs

What is Total Internal Reflection?

A phenomenon where critical angle is greater than the angle of incidence so light is TOTALLY reflected into the 2nd medium

What are the conditions for Total Internal Reflection to occur?

1. The light ray must be travelling from a denser medium to a less dense medium 2. Angle of incidence must be greater than the critical angle

What is an optical fibre?

A thin wire of flexible glass or plastic that transmits light signals

What is cladding?

The outer layer of material of an optical fibre that surrounds the core

What is a core?

The central part of the optical fiber where light travels

Out of the core and cladding, which one has a greater refractive index and why?

signal can be lost and to reduce that , you get the light to travel from a denser medium to a less dense one so that TIR occurs which causes the light to reflect totally in the core

Why is cladding used?

Because sometimes light refracts into the cladding so it prevents the light signal escaping the optical fibre by refracting it back into the core

What is absorption?

The loss of light signal as it travels through the fibre due to it being absorbed by the material itself

What is an optical fibre repeater?

A device used in long-distance communications systems to keep signals strong and clear as it travels through the fibre

How do optical fibre repeater work?

It boots the signal at periodic positions along the fibre

What is modal dispersion?

When light beams enter the fibre at different angles resulting in TIR in multiple different paths , causing the beams to reach the end of the fibre at different times

What is material dispersion?

When each light signal has different wavelengths so they travel at different speeds in the core

Why do we only observe the superposition of coherent waves?

Because when the waves overlap, they form a repeating pattern of maxima and minima areas but when the waves are not coherent , the interference pattern would change rapidly and continuously

What is the path difference of waves in constructive interference?

A whole number of wavelengths Eg 2 λ , 6 λ, 9 λ

What is the path difference of waves in destructive interference?

A half number of wavelengths Eg 1/2 λ , 2 and 1/2, 7 and 1/2

What is the significance of Young’s double slit experiment?

It proved that light behaved as a wave

What is the central maxima?

The brightest fringe at the centre of the interference pattern

What is diffraction?

The spreading out of waves as they pass through a gap

What does angle of diffraction depend on?

The wavelength of the light source compared to the size of the gap ; needs to be similar

What is diffraction grating?

A series of narrow, parallel slits

How do diffraction gratings work?

Light shone on grating diffracts Diffracted light waves from different points of the gratings interfere Interference pattern of diffraction minima and maxima occurs resulting in bright and dark fringes

What is zero order maximum?

It is the first bright spot and it lies directly behind where the light shines on the grating n=0 Path difference of 0 ; constructive interference

What is the first order maximum?

The next bright spot after the zero-order maximum that’s at an angle to it They have a path difference of 1 λ from the zero-order maximum and so on

What should you remember when using the equation dsin θ= n λ

That n must be an integer because it’s the order of fringes. Therefore when doing calculations and n is a decimal you ROUND DOWN and ignore normal rounding principles