Chapter 11- Waves Flashcards
What is a progressive wave?
An oscillation which transfers energy through a medium without transferring matter.
The energy is passed through vibrations of a particle in a medium or fields
What do particles do when they vibrate? Energy transfers
They move from equilibrium to positive(peak) and negative amplitude(trough). The particle has 0 kinetic energy at peaks and troughs because it is converted to potential energy
What is a transverse wave?
A wave where the direction of transfer of energy is perpendicular to the direction of oscillations. Peaks and troughs are maximum displacement from equilibrium
What’s are the 2 types of waves and what is the difference?
Mechanical: particles physically move(requires a medium)
EM: are changes in the electric and magnetic field created by the motion of a charged particle
What is a longitudinal wave? How do they travel?
A wave where the direction of oscillation is parallel to the the direction of energy transfer.
When travelling through a medium they create compressions and rarefactions. Particles displace and collide with neighbours which transfers energy and also provides a restoring force
What is restoring force?
Force felt by a particle to return back to equilibrium
Why does the speed of mechanical waves differ with density?
The particles are closer together in a denser medium
So particles don’t need to be displaced as much before interacting with other particles
So energy is transferred faster
Define displacement
The distance and direction from the equilibrium position of a particle. It can be positive or negative
Define amplitude
Maximum displacement from equilibrium
Define wavelength
The distance between 2 adjacent particles/points in a wave vibrating in phase with the same velocity
What is time period? What is units
Seconds
The time taken for a particle/point to perform one complete oscillation of its motion
What is frequency and unit?
Hz
Number of complete oscillations performed by a particle/point per second
What is wave speed?
The speed at which a wave travels through a medium per unit time
How does speed change when there is or isn’t a medium change?
In the same medium, the wave travels at a constant velocity so a higher frequency means shorter wavelength
Speed changes when a wave crosses the boundary between 2 media which causes wavelength to change, NOT FREQ
What is the wave equation and the other one?
C=f * (lamda) Velocity= freq x wavelength
Freq= 1/time period
What is 1 complete wave cycle?
360* = 2pi
What is phase of a particle?
The fraction of a complete oscillation that a particle is at at a given point in time
What is phase difference?
The difference in phase between 2 points along the same wave or 2 different waves with the same frequency at a given point in time
How is phase calculated?
Fraction of completed wave x 2pi(360*)
How is phase difference and distance calculated?
Distance between 2 points as fraction of wave/wavelength x 2pi (or wavelength)
Phase difference/ 2pi X wavelength = distance
What is anti phase?
When there is a difference in phase of an odd integer of pi radians
How is phase difference and frequency linked?
When 2 points with the same frequency meet at a point in time, phase difference is CONSTANT
When they have different frequencies, phase difference is always changing
What are the 4 types of wave phenomena?
Reflection, refraction, diffraction, superposition
What is optical density? How is it linked to em waves
A measure of the absorbance of a material. EM a waves travel slower through higher optical density while mechanical waves travel faster in higher physical density
What is a wave train?
An infinite number of waves moving together as a unit
What is a wave front?
A line of constant phase of a wave trai ( a line that can link the peak of each wave)
Wave direction is 90* to wave fronts
Changing wavefronts(or wavelengths) means a wave phenomena is occurring.
Define reflection and it’s law
When a wave reverses direction upon meeting the boundary of 2 different media
Angle of incidence=angle of reflection (to the normal)
Wavelength and media DO NOT change
Define refraction
Refraction is the change in a waves direction as it crosses the boundary between 2 media.
Speed changes which means wavelength changes. FREQ IS CONSTANT
Explain how a wave refracts
If the incident ray approaches the boundary at an angle from the normal, it’s wavefronts change speed which causes a change in direction
What happens when a wave speeds up/slows down and why?
When mechanical waves enter a denser medium they bend AWAY from the normal. Wavelength gets LONGER. This is because part of the wavefront meet the boundary and speed up first so the whole wave train bends away
When mechanical waves enter a less dense medium, they bend towards the normal and SLOW DOWN.
Why do waves refract at the boundary of shallow and deep water?
Deep water is more dense than shallow water. Because there is a change in density hence waves will speed up in higher physical and optical density.
Define diffraction
The physical phenomena of waves spreading out when passing through a gap or around an obstacle.
There is no change in media
Relationship between gap size and wavelength
The smaller the gap size relative to the wavelength, the greater the diffraction
Gap size has to be a similar order of magnitude as the wavelength for significant diffraction to occur
Why does sound refract through a door but not light?
The wavelength of sound is the same order of magnitude as the door so sound waves refract. Light has a smaller order of magnitude
What is polarisation?
The process of confining the oscillations of a wave to a specific plane
Light from a laser is polarised but light from a lamp isn’t as oscillation occur on many planes
Why can’t longitudinal waves be polarised by transverse can?
Longitudinal waves have oscillations parallel to the direction of energy transfer so the oscillations are already confined to one plane
Transverse waves can be polarised because the plane of oscillation are perpendicular to the direction of energy transfer so they can be confined to 1 plane
Why does the amount of light vary when 2 polarising filters are rotated?
When the transmission axis of 2 filters are aligned, the same amount of light passes through both. As you rotate the filter, the amount of light that passes through varies.
The larger the vertical component, the more vertically polarised light passes through.
2 perpendicular filters don’t let any light through
If the 2nd filter is 45 degrees rotated from the first one, half the light will go through (intensity halved). Because the vertical component of the second filter decreases
What happens to light reflected off a puddle?
It gets partially polarised on the horizontal plane because oscillations can only happen on that plane
How does a Polaroid filter work?
They only transmit light with e field oscillations and components on a specific plane which decreases the wave intensity.
Used in sunglasses to only let light with the electrostatic field vector oscillating in the vertical plane to pass
What happens to light and mechanical waves the further it travels from a source?
It spreads out more so the energy(power) transferred becomes less concentrated so it is dimmer.
Mechanical waves decrease in amplitude
Explain how a filament lamp works
Heat from the lamp is infrared, bulb has argon gas
Electrons oscillating at different rates and directions form all colours to form white light.
Define intensity and power
Intensity is the radiant power passing through a surface per unit area
Power is energy transferred per unit time
What is the formula for intensity
Intensity(w/m2) = power(w)/area(m2)
I = P/ 4pi r^2
From a point source power is spread in 3D so surface area of sphere
What is the relationship with intensity and distance?
Intensity has an inverse square relationship to distance from source
If distance doubles, intensity decreases by a factor of d2
I= 1/r^2
What is the relationship between energy intensity and amplitude?
Intensity is proportional to amplitude squared
Reason: the further waves travel, amplitude decreases because energy spreads up which means reduced speed. E=1/2 mv^2 so energy is quartered
Double amplitude= quadruple intensity
How are EM waves produced?
They are produced by the movement of a charged particle which causes the magnetic field vector to oscillate in the Z plane and electrostatic field vector to oscillate in the y plane
Why is sunlight unpolarised?
Because the electrostatic field vectors oscillate in all planes because charge particles accelerate in all directions
What is an em wave?
A transverse wave consisting of oscillating electric and magnetic fields which are in PHASE and perpendicular to each other and to the direction of travel
They don not need a medium to travel in and travel in a vacuum at 3x10^8 m/s
Velocity varies with freq
They can be polarised and obey v=f x lamda
What is the em spectrum and what are the order of magnitudes of the categories?
The EM spectrum is the continuum of all em waves
(Wavelength in metres)
Radio: 10^6 to 10 Micro: 10 to 10^-3 Infrared: 10^-3 to 7x10^-7 Visible: 7x10^-7 to 4x10^-7 UV: 4x10^-7 to 10^-8 X Rays: 10^-8 to 10^-10 to 10^-13 Gamma: 10^-10 to 10^-13 to 10^-16
What is absolute refractive index?
N: the ratio between the speed of light in a vacuum and the speed of light in a medium
What is the formula for refractive index?
C/V
Or wavelengthC/wavelengthV
FREQ IS CONSTANT NO MATTER WHAT MEDIUM
What is the effect on the speed of light if there is a change in medium?
Light is fastest in vacuum and slows down in other materials because it interacts with more particles(more optically dense)
What is the relationship between refractive index and the angle of incidence/refraction
N= sin i / sin r
What is snells law?
N1 sin 1 = n2 sin 2
What is total internal reflection?
When light completely reflects back at the boundary between 2 media when the incident angle is larger than the critical angle
What conditions are necessary for total internal reflection?
The media in which the ray is incident in must have a lower refractive index. (More to less optically dense)
The incident angle must be greater than the critical angle
Formula for critical angle
Sin-1(n2/n1)
Or just use snells law
Why are diamonds so shiny?
They have a high refractive index so there is a small critical angle, so light is totally internally reflected many times. It also dispersed light to form a rainbow effect
How do optical fibres work? 3 points
Light is incident to the fibre boundary at an angle bigger than the critical angle to light is constantly totally internally reflected. The fibres are transparent so light is absorbed less.
Core is very thin to stop multipath dispersion(where there is a difference in the path length of rays reflecting off sides and going straight through.
