3.3 Waves

what is a wave?

a disturbance or variation that transfers energy from point to point

how may a wave take form?

• elastic deformation

• variation of pressure

• electric/ magnetic intensity

• electric potential

how are all waves caused?

oscillations or vibrations

what do all waves do?

transfer energy or infomation without transferring matter

what are the two categories of waves?

- mechanical
- electromagnetic

what is a mechanical wave?

waves that lass through a substance via a vibrations of the substance

how do mechanical waves move?

as waves pass through a substance, particles vibrate in a certain way, making the nearby particles vibrate the same way

what are some examples of mechanical waves?

- sound waves
- seismic wave
- waves on a string

what are electromagnetic waves?

oscillating electric and magnetic fields thag progress, through space without the need for a substance

how do electromagnetic waves travel?

vibrating electric field generates a vibrating magnetic field, which generates a vibrating electric field, etc etc. this is how they move without a medium

what are some examples of electromagnetic waves?

- radio waves
- microwaves
- infrared
- visible light
- ultraviolet
- x-rays
- gamma rays

what is a longitudinal wave?

waves with a direction of vibration parallel to the direction of propagation of the wave (energy transfer)

what are some examples of longitudinal waves?

- sound
- p waves
- compression waves in a medium

do longitudinal waves need a medium to transfer energy?

yes

what is the peak?

maximum positive displacement from equilibrium

what is the trough?

the maximum negative displacement from equilibrium

what is the wavelength?

the distance from one peak of trough to the next

what are plane polarised waves?

transverse waves that vibrate in one plane only.

when are waves unpolarised?

if vibrations can change from one plane to another

can longitudinal waves be polarised?

no

what happens if unpolarised light is passed through a filter?

- light will become polished
- light will now be propagating in only one plane

what is a filter called?

a polaroid

in what plane will now polarised light propagate in now?

only one plane m: the one that matched the polaroid it was passed through

what happens if i polaroid if light is passed through two successive filters?

- light passed through when both filters are aligned in the same place

what happens if when light is passed through two successive filters, the second filter is alighted at a perpendicular angle to the first?

no light will be transmitted

what are some uses of polarisation?

- liquid crystal displays (LCDs)
- aerial
- su

how do liquid crystal displays use polarisation?

- use liquids that align and act as a polaroid when current flows across them
- lowers power consumption

how do aerials use polarisation?

-radio waves polarised into vertical or horizontal
-aerial must be alighted correctly to receive a clear signal

T/F: electromagnetic waves done travel at the same speed in a vacuum?

false, they do travel at the same speed

what is a complete cycle?

measured from a position of maximum displacement to the next ,aximum displacement

what is the equation for frequency, in relation to time period?

frequency = 1/ time period

what is the phase difference?

the fraction of a cycle between the vibrations of two vibrating particles, measured either in radians or in degrees

what is the equation for phase difference in radians?

2πd/λ

what is reflection?

the return of light or sound waves from a surface

what is the normal?

line drawn perpendicular to the surface

what is always true about the angle of incidence for a plane surface?

the angle of incidence = the angle of reflection

what is refraction?

the change of direction of a wave when it crosses a boundary where its speed changes

what happens to the wavelength when waves travel across a boundary at which their speed changes?

the wavelength also changes

what happens if the wave crossing a boundary is incident at a no-zero angle to the boundary?

they change direction as well as speed

what happens if the ray of light is incident at 90°?

there is no change in direction, only in speed?

what happens when a wave enters a denser material?

bends towards the normal

what happens when a wave enters a less dense material?

bends away from the normal

what is diffraction?

the spreading out of waves on passing through a gap or near an edge

what does the amount of diffraction depend on?

the size of the wavelength compared to the size of the gap

what happens in diffraction when there is a narrower gap?

the more the waves are spread out

what happens in diffraction when the wavelength is longer?

the more the waves are spread out

what is phase difference?

the fraction of a cycle between the vibrations of two vibrating particles measured in either radians or degrees

what is the phase of a vibrating particle at a certain time?

The fraction of the cycle it has completed since the start of the cycle

what is phase difference equation in radians?

2πd/ λ

where d is the distance between two points along a wave and one cycle is 360° (2π)

what speed do different electromagnetic waves travel at in a vacuum ?

the same speed

how does polarisation provide evidence to the nature of a transverse wave?

because polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel

how do sun glasses use polarisation?

* reduce glare by blocking partially polarised light reflected from water and tarmac
* only allow oscillations in the plane of the filter, making it easier to see

how do TV and radio signals use polarisation?

* they are usually plane-polarised by orientation of the rods on transmitting aerial
* receiving aerial must be aligned in the same plane

what is super position?

where the displacements of two waves are combined as they pass each other

what is the resultant displacement when two waves super pose?

the vector sum of each wave’s displacement

what is constructive interference?

occurs when two waves have displacements in the same direction

what is destructive interference?

when one wave has positive displacement and the other has negative displacement.

what is total destructive interference?

if the waves have equal but opposite displacements

what is interference?

the formation of points of cancellation and reinforcement where two coherent waves pass through each other

what is a stationary wave?

formed from the superposition of two progressive waves, travelling in opposite directions, in the same plane, with the same frequency, wavelength and amplitude

T/F: stationary waves transfer energy.

FALSE. no energy is transferred by a stationary wave

what is an antinode?

regions of maximum amplitude

where are antipodes formed?

where waves meet in phase, constructive interference occurs

what is a node?

region of no displacement

where are nodes formed?

where waves meet completely out of phase pm destructive interference occurs

what is the first harmonic?

lowest frequency at which a stationary wave forms

what does the first harmonic of a stationary wave look like?

stationary wave with two nodes and a single antinode

what is the distance between adjacent nodes (or anti nodes) in relation to wavelength, for any harmonic?

distance is half a wavelength

how can you calculate the frequency of a stationary wave in the first harmonic?

f = 1/2L\* sqr(T/μ)

where L is the length of the vibrating string

T is the tension

μ is mass per unit length

how would you find the second harmonic frequency using first harmonic frequency?

double it

how are stationary microwaves formed?

reflecting a microwave beam at a metal plate

what is a coherent light source?

has the same frequency and wavelength and a fixed phase difference

what does young’s double slit experiment?

demonstrates interference of light from two sources, showing that light is a wave

why do some people put a a single slit before the double slits in young’s double slit expeirment?

to make the light have a fixed path difference

why are lasers used as sources of light in diffraction experiments?

* form clear interference patterns
* coherent and monochromatic (emit a single (or small range of) wavelength(s) of light)

where are light fringes formed in young’s double slit experiment?

where the light meets in phase and interferes constructively.

* where the path difference between waves is a whole number of wavelengths (nλ, where n is an integer)

where do dark fringes form in young’s double slit experiment?

where th eight meets completely out of phase and interferes destructively

* sheerest the path difference is a whole number and a half wavelengths ((n+1⁄2)λ)

what is the equation for fringe spacing?

w =λD/ s where D is the distance between the screen and slits Where s is slit separation

what happens if you use white light in young’s double slit experiment?

• wider maxima

• less intense diffraction pattern

  • centeral white fringe with alternating bright fringes which are spectra

when using white light in young's double split experiment what order do the colours go from the central white fringe?

violet is the closest to the central maximum and red is the furthest

what laser safety measures should you follow when you are using them?

• wear laser safety goggles

• don’t shine the laser at reflective surfaces

• display a warning sign

• never shine the laser at a person

how can you adapt young’s double slit experiment for sound waves?

• two speakers connected to the same signal generator

• measure intensity using a microphone to find the maxima and minima

how did the double slit experiment prove that light is a wave?

• diffraction and interference are wave properties and so proved that em radiation must act as a wave, at times

how does knowledge and understanding change over time?

accordance to the experimental evidence gathered by the scientific community

what is diffraction?

spreading out of waves when they pass through or around a gap

when does the greatest diffraction happen?

when the gap is the same size as the wavelength

what happens when the gap is smaller than the wavelength?

most waves are reflected

what happens when the gap is bigger than the wavelength?

less noticeable diffraction

what is formed when monochromatic light is diffracted through a single split onto a screen?

forms an interference pattern of light and dark fringes.

• bright central fringe

• alternating dark and bright fringes on either side

what is the size like of the bright central fringe in an interference pattern?

double the width of all the other fringes

what are bright fringes in an interference pattern caused by?

constructive interference

what is constructive interference

where the waves meet in phase

what are the dark fringes caused by in an interference pattern?

destructive interference

what is destructive interference?

where waves arrive completely out of phase

how does the intensity of outer fringes differ from the central fringe in an interference pattern?

intensity decreases

what happens to a diffraction pattern if you use white light instead of monochromatic light?

• you would see all different wavelengths of visible light

• different wavelengths of light are all diffracted by diffferent amounts so you get a spectrum of colour in the diffraction pattern

what does a diffraction pattern if you use white light instead of monochromatic light look like?

• central white maximum with alternating bright fringes which are spectra

• violet is the closest to the central maximum

  red furthest away

how does increasing the slit width of a diffraction pattern vary the centeral maximum?

decreases diffraction so the central maxmimum becomes narrower and its intensity increases

how does increasing the light wave vary the central maximum?

increases amount of diffraction. as the slit is clower in size to the light;s wavelength, so central maximum becomes wider and intensity decreases