G482 - Waves Flashcards
Progressive Wave
Definition
A transfer of energy as a result of oscillations
Longitudinal Wave
Definition
A wave in which the oscillations are parallel to the direction of wave travel
Transverse Wave
Definition
A wave in which the oscillations are perpendicular to the direction of wave travel
Longitudinal Wave
Examples
Sound
Transverse Wave
Examples
Light
Water
String
Displacement
Definition
The distance of a particle from the equilibrium position
Measured in metres, m
Amplitude
Definition
Maximum displacement of a particle from equilibrium
Symbol A
Measured in metres, m
Wavelength
Definition
The shortest distance between a point on one wave and the same point on the next wave
Symbol λ
Measured in metres, m
Time Period
Definition
The time taken for one complete pattern of oscillation
Symbol T
Measured in seconds, s
Phase Difference
Definition
Relates to the oscillation at two points
How far out of step one oscillation is from another
Measured in degrees or radians
Frequency
Definition
The number of oscillations per unit time
Symbol f
Measured in hertz, Hz
Wave Speed
Definiton
Distance travelled by a wave per unit time
Measured in m/s
Time Period
Formula
T = 1/f
T = time period, s f = frequency, Hz
Wave Speed
Formula
v = fλ
v = wave speed, m/s f = frequency, Hz λ = wavelength, m
Reflection
Definition
When waves rebound from a barrier changing direction but remaining in the same medium
Diffraction
Definition
When a wave spreads out after passing around an obstacle or through a gap
When the gap is closer in size to the wavelength of the wave there is more diffraction
Diffraction around an obstacle increases with wavelength
Refraction
Definition
When waves change direction when they travel from one medium to another due to a difference in the wave speed in each medium
Typical Wavelengths
Radio Waves
10^-1 -> 10^4 m
Typical Wavelengths
Microwaves
10^-4 -> 10^-1 m
Typical Wavelengths
Infrared
7.4x10^-7 -> 10^-3 m
Typical Wavelengths
Visible Light
3.7x10^-7 -> 7.4x10^-7 m
Typical Wavelengths
Ultraviolet
10^-9 -> 3.7x10^-7 m
Typical Wavelengths
X Rays
10^-12 -> 10^-7 m
Typical Wavelengths
Gamma
10^-16 -> 10^-9 m
Electromagnetic Spectrum Order
Radio Waves Microwaves Infrared Visible Light Ultraviolet X Rays Gamma
Electromagnetic Spectrum
Similarities
Travel at the same speed in a vacuum
All transverse waves
All possess an electric wave and a magnetic wave interlocked at right angles
Electromagnetic Waves
Speed in a Vacuum
3x10^8 m/s
Speed of Sound In Air
330m/s
Radio Waves
Method of Production
Electrons oscillated by an electric field
Radio Waves
Method of Detection
Resonance in electric circuits
Radio Waves
Uses
Television
Radio
Telecommunications
Microwaves
Method of Production
Magnetron
Klystron oscillators
Microwaves
Method of Detection
Heating effect
Electronic circuits
Microwaves
Uses
Radar
SATNAV
Mobile phones
Microwave ovens
Infrared
Method of Production
Oscillation of molecules from all objects above absolute 0
Infrared
Method of Detection
Photographic film
Heating of skin
Infrared
Uses
Heaters
Night vision equipment
Remote controls
Visible Light
Method of Production
High temperature solids and gases
Lasers
Visible Light
Method of Detection
Photographic film
Retina of eye
Visible Light
Uses
Sight
Communication
Ultraviolet
Method of Production
From high temperature solids and gases
Ultraviolet
Method of Detection
Photographic film
Phosphors
Sunburn
Ultraviolet
Uses
Disco lights
Tanning studios
Counterfeit detection
X Rays
Method of Production
Bombarding metals with high energy electrons
X Rays
Method of Detection
Photographic film
Fluorescence
X Rays
Uses
CT scans
X Ray photography
Crystal structure analysis
Gamma Rays
Method of Production
Nuclear decay
In a nuclear accelerator
Gamma Rays
Method of Detection
Photographic film
Geiger tube
Gamma Rays
Uses
Diagnosis and cancer treatment
UV-A
λ = 315 - 400 nm
Causes tanning when skin is exposed
UV-B
280-315nm
Causes damage to skin such as sun burn and skin cancer
UV-C
100-280nm
Is filtered out by the atmosphere so doesn’t reach the earth’s surface
Industrial UV-C is highly damaging
Sunscreen
Contains chemicals designed to filter out UV-B pre eating sun burn and skin cancer
Skin is protected inside as glass absorbs UV-B
Plane Polarised Waves
Definition
Waves that oscillate in only one plane
Only transverse waves can be pale polarised
Malus’s Law
I = Imax x cos²θ
I = intensity transmitted at angle θ Imax = intensity before the filter θ = the angle of the filter in relation to the previous filter that the light has travelled through
Cross Polaroids
If filters are at right angles, θ=90, then no light will pass through
The Principle of Superposition
When two or more waves meet at a point the resultant displacement is equal to the vector sum of the displacements of each wave
Interference
Definition
The addition of two or more waves that results in a new wave pattern
Sources must be coherent
Amplitude must be approximately equal
Coherence
Definition
Two waves are coherent if they have a constant phase difference and the same frequency
Path Difference
Definition
Distance between two identical points on the two waves
Measured in λ
Constructive Interference
Path Difference
Whole number of wavelengths
Constructive Interference
Phase Difference
0°, 360°, 720°… multiples of 360°
0, 2π, 4π radians … multiples of 2π
Destructive Interference
Path Difference
Odd number of half wavelengths
Destructive Interference
Phase Difference
180°, 540°, 900°… multiples of 2n(180)+180
π, 3π, 5π radians… multiples 2nπ +1
Intensity
Formula
Intensity(W/m²) = Power W / cross sectional area m²
Intensity∝Amplitude²
Intensity
Definition
Energy incident per unit area per unit time
Young’s Double Slit Experiment
Description
A monochromatic light source is positioned between two slits
When the light passes through the slits it is diffracted
The two light sources, the two slits, interferes with each other to produce an interference pattern
The pattern consists of a series of maxima and minima
At minima the path difference is an odd number of half wavelengths so there is destructive interference
At maxima the path difference is a whole number of wavelengths so there is constructive interference
Young’s Double Slit Experiment
Formula
λ = ax/D
λ = wavelength m a = distance between slits m x = distance between bright fringes m D = distance between slits and screen
Monochromatic Light
Light waves with a single frequency and wavelength
Finding the Wavelength if Monochromatic Light
Use the laser as the source Carry out young's double slit experiment Measure a and D Take an average measurement for x Use λ=ax/D
Diffraction Grating
Formula
sinθ = nλ/d
θ = angle of the beam from horizontal n = order of the beam λ = wavelength of source d = spacing between slits
Advantages of Multiple Slits
Many sharp maxima can be observed so measurements are easier and more accurate
Double slit images can sometimes be blurry increasing error when measuring fringe spacing
Stationary Wave
Definition
A wave formed by the interference of two waves travelling in opposite directions
Energy is stored in the wave which has nodes and antinodes
Nodes
Definition
Points in a stationary wave at which the displacement is 0
Antinodes
Definition
Points in the stationary wave where the displacement is maximum
Fundamental Frequency
Definition
Frequency that gives a standing wave of half a wavelength
Harmonics
Definition
Whole number multiples of the fundamental frequency of a stationary wave
What is the distance between two nodes or two antinodes?
Half a wavelength
Wave Patterns
Stretched Strings, Open Pipes, Closed Pipes
nth harmonic
Wavelength = Fundamental Wavelength / n
Frequency = Fundamental Frequency x n
Wave Patterns
Stretched Strings, Open Pipes, Closed Pipes
fundamental
Fundamental Wavelength = 2l
Fundamental Frequency = v/2l
l = length of string or pipe
Wave Patterns
Pipes Closed at One End
fundamental
Fundamental Wavelength = 4l
Fundamental Frequency = v/4l
l = length of pipe
Wave Patterns
Pipes Closed at One End
nth harmonic
Wavelength = Fundamental Wavelength / (2n+1)
Frequency = Fundamental Frequency x (2n+1)
Stationary Waves
Microwaves
Set up a transmitter opposite a reflector with a detector in the middle
The detector will register strong signals every half wavelength i.e. nodes
Stationary Waves
Guitars
When the string is plucked a transverse wave is sent down the string and reflected
The reflected wave interferes with the incident wave to produce a stationary wave of the correct pitch
Experiment to Find the Speed of Sound in Air
Put an open ended tube into water to ‘close’ the tube at one end
Sound a tuning fork and hold it close to the tube
The tube is open at one end so there is an antinode, maxima, at the opening
Move the tube up and down in the water to find where the resonance is maximum, where there is λ/4 inside the tube
Measure the distance from the top of the water level to the top of the tube at this point
Multiply this distance by 4 to find the λ
The tuning fork has its frequency written on
Use v=f λ to find v
