Waves SAEED Flashcards
Define what a wave is in physics
A wave is the transfer of energy without the transfer of matter
What is the differences between longitudinal waves and transverse waves?
The direction of the oscillations
Longitudinal - parallel
Transverse - perpendicular
Which direction do the longitudinal waves have?
The vibrations are parallel to the direction of wave travel
Which direction do the transverse waves have?
The vibrations are at right angles to the direction of wave travel.
Information about mechanical waves….
Mechanical waves cause oscillations of particles in a solid, liquid or gas and must have a medium to travel through
Information about electromagnetic waves….
Electromagnetic waves cause oscillations in electrical and magnetic fields
What does the “rest position” in a wave show?
The undisturbed position of particles or fields when they are not vibrating
What does the “displacement” in a wave show?
The distance that a certain point in the medium has moved from its rest position
What does the “peak” in a wave show?
The highest point above the rest position
What does the “trough” in a wave show?
The lowest point below the rest position
What does “amplitude” mean in terms of waves?
The maximum displacement of a point of a wave from its rest position
What does “wavelength” mean in terms of waves?
Distance covered by a full cycle of the wave, usually measured from peak to peak, or trough to trough
What does “time period” mean in terms of waves?
The time taken for a full cycle of the wave, usually measured from peak to peak, or trough to trough
What does “frequency” mean in terms of waves?
The number of waves passing a point each second
What are electromagnetic waves?
Are oscillating electric and magnetic fields that progress through space without the need for a substance
The vibrating electric field generates a vibrating magnetic field further away and so on
Examples of electromagnetic waves
Radio waves Microwaves Infrared radiation Light Ultraviolet radiation X - rays Gamma radiation
Examples of longitudinal waves
Sound waves
Primary seismic waves
Compression waves on a slinky toy
How do you send longituinal waves along a slinky?
When one end of the slinky is moved to and fro repeatedly, each “forward” movement causes a compression wave to pass along the slinky as the coils push into each other. Each “reverse” movement causes the coils to move apart so rarefaction (expansion) wave passes along the slinky
Examples of transverse waves
Electromagnetic waves
Secondary seismic waves
Waves on a string or a wire
How do transverse waves travel along a rope?
When one end of the rope is moved from side to side repeatedly, these sideways movements travel along the rope, as each unaffected part of the rope is pulled sideways when the part next to it moves sideways
What is polarisation?
Transverse waves are plane-polarised if the vibrations stay in one plane only
Longitudinal waves cannot be polarised
What is meant by unpolarised waves?
This is if the vibrations change from one plane to another
Light from a filament lamp or a candle is unpolarised. If unpolarised light is passed through a polaroid filter what happens?
The light transmitted is polarised as the filter only allows through the light which vibrates in a certain direction, according to the alignment of its molecules
What happens if unpolarised light is passed through two Polaroid filters, what would happen with the transmitted light?
If unpolarised light is passed through two Polaroid filters the transmitter light intensity changes if one Polaroid is turned relative to the other one
The filters are said to be crossed when the transmitted intensity is minimum
What is said to happen when the light intensity is a minimum?
Unpolarised light
The filters are said to be crossed when the transmitted light intensity is a minimum. At this position the polarised light from the first filter cannot pass through the second filter as the alignment of the molecules in the second filter is at 90* to the alignment in the first filter
What is a plane?
Polarisation
Light is part of the spectrum of electromagnetic waves
The plane of polarisation of an electromagnetic wave is defined as the plane in which the electric field oscillates
How do polarised sunglasses work?
They reduce the glare of the light reflected by water or glass
The reflected light is polarised and the intensity is polarised and the intensity of reduced when it passes through the Polaroid sunglasses
Displacement
The displacement of a vibrating particle is its distance and direction from it’s equilibrium position
Amplitude
The amplitude of a wave is the maximum placement of a vibrating particle
For a transverse wave this is the height of a wave crest or the depth of a wave trough from its equilibrium position
Wavelength
The wavelength of a wave is the least distance between two adjacent vibrating particles with the same displacement and velocity at the same time
Cycle
One complete cycle of a wave is from maximum displacement to the next maximum displacement
Eg/from one peak to the next
Period
The period of a wave is the time for one complete wave to pass a complete point
Frequency
The frequency of a wave is the number of cycles of vibration of a particle per second or the number of complete waves passing a point per second
Unit of frequency is hertz
For waves of frequency f the period of a wave = 1/f
The higher the frequency of a wave…..
The shorter it’s wavelength
How is amplitude measured?
From the equilibrium position to maximum positive or maximum negative (not from maximum positive to maximum negative)
Phase difference is measured as an angle (in radians or degrees) not in terms of wavelength
Phase
The phase of a vibrating particle at a certain time is the fraction of a cycle it has completed since the start of the cycle
What is phase difference?
The phase difference between two particles vibrating at the same frequency is the fraction of a cycle between the vibrations of the two particles, measured either in degrees or radians where 1 cycle = 360* = 2(pie) radians. For two points at distance d apart along a wave of wavelength
The phase difference in radians =
2(pie)d / wavelength
What is the calculation for the speed of waves?
Speed of waves = distance travelled in one cycle / time taken for one cycle
What are some key properties of waves?
Reflectection
Refraction
Diffraction
Me
Occur with many different types of waves
A ripple tank can be used to study these properties
How does a ripple tank work?
The tank is a shallow transparent tray of water with sloping sides
The slopes prevent waves reflecting off the sides of the tank
If they did reflect it would be difficult to see the waves
The waves observed in a ripple tank see referred to as wavefronts which are lines of constant phase
The direction in which a wave travels is at right angles to the wave front
How does reflection work?
WAVES
Straight waves directed at a certain angle to a hard flat surface (the reflector) reflect off at the same angle
The angle between the the reflected wavefront and the surface is the same as the angle between the incident wave front and the surface
Therefore the direction of the reflected wave is at the same angle to the reflector as the direction of the incident wave
This effect is observed when a light ray is directed at a plane mirror
The angle between the incident ray and the mirror is equal to the angle between the reflected ray and the mirror
What is refraction?
When waves pass across a boundary at which the wave speed and wavelength changes
What would happen if the wavefronts approach at an angle to the boundary?
REFRACTION
The waves change direction as well as speed
How does refraction of water in a ripple tank occur?
From deep to shallow water at an angle to the boundary
Because they move more slow long in the shallow water, the wavelength is smaller and therefore they change direction
How do we observe the refraction of light?
Observed when a light ray is directed into a glass block at an angle (eg/not along the normal)
The light ray changes direction when it crosses the glass boundary
This happens because light waves travel more slowly in glass than in air
What is diffraction?
Diffraction occurs when waves spread out after passing through a gap or round an obstacle
How can the effect of diffraction be seen? What does a narrower gap and a longer wavelength mean?
In a ripple tank when straight waves are directed at a gap
The narrower the gap the more the waves were spread out
The longer the wavelength the more the waves spread out
What are wavelets and what do they do?
The wavelets from the points along a wavefront travel only in the direction in which the wave is travelling nlt in the reverse direction and they combine to form a new wavefront spreading beyond the gap
Why do satellite tv dishes in Europe need to point south?
DISH DESIGN
The satellites orbit the earth directly above the equator
The bigger the dish….
DISH DESIGN
The stronger the signal it can receive because more radio waves are reflected by the dish onto the aerial
But a bigger dish reflects the radio waves to a smaller focus because it diffracts the waves less
The dish therefore needs to be aligned more carefully than a smaller dish otherwise it will not focus the radio waves onto the aerial
What is superposition?
When waves meet they pass through each other
At the point where they meet they combine for an instant before they move apart
This combining effect is know as superstition
What is the principle of superstition?
The principle of superstition states that when two waves meet the total displacement at a point is equal to the sum of the individual displacements at that point
What happens when a crest meets a crest?
A super crest is created - the two waves reinforce each other
What happens where a trough meets a trough?
A super trough is created
The two waves reinforce each other
What happens where a crest meets a trough at the SAME amplitude?
The resultant displacement is zero the two waves cancel each other out
What happens where a crest meets a trough at a DIFFERENT amplitude?
The resultant is called a minimum
How are stationary waves formed on a rope?
If two people send waves continuously along a rope from either end
What do the two waves along a rope get referred to as?
Progressive waves to distinguish them from stationary waves
They combine at fixed points along the rope to form two points of no displacement or nodes along the rope
What happens at each node during the stationary waves in a rope experiment?
At each node the two sets of waves are always 180* out of phase so they cancel each other out
What happens during the water waves in a ripple tank experiment?
SUPER POSITION
A vibrating dipper on a water surface sends out circular waves
The waves pass through each other continuously
As the waves are continuously passing through each other at constant frequency at and at a constant phase difference cancellation and reinforcement occurs at fixed positions - aka interference
What are created in the water waves ripple tank experiment?
Points of cancellation
Points of reinforcement
How are points of cancellation created?
Are created when a crest from one dipper meets a trough from the other dipper
These points of cancellation are seen as gaps in the wavefronts
How are the points of reinforcement created?
A crest from one dipper meets a crest from the other dipper or where a trough from one dipper meets a trough from another dipper
After interference what happens during the water waves in a ripple tank experiment?
Coherent sources of waves produce an interference pattern where they overlap because they vibrate at the same frequency with a constant phase difference
What happens if the phase difference is changed at random during the water waves in a ripple tank experiment?
If the phase difference if changed at random the points of cancellation and reinforcement would love about at random and no interference pattern would be seen
Stationary waves that vibrate freely….
Do not transfer energy to their surroundings
What happens when two progressive waves are in phase?
They reinforce each other to produce a large wave
Consider a snapshot of two progressive waves passing through each other, why would you see/happen?
When they’re in phase they reinforce each other to produce a large wave
A quarter of a cycle later the two waves have each moved one quarter of a wavelength in opposite directions - now in antiphase so they cancel each other
After a further quarter cycle the two waves are back in phase - the resultant is again a large wave except reversed
What are light rays?
Represent the direction of travel of wavefronts
What is the normal?
An imaginary line perpendicular to a boundary between two materials or a surface
What is refraction?
The change of direction that occurs when light passes at an angle across a boundary between two transparent substances
If the incident light ray is along the normal what would happen?
No refraction would take place
At a boundary between two transparent substances, the light ray bends….
Towards the normal if it passes into a more dense substance
Away from the normal if it passes into a less dense substance
How would you investigate the refraction of light by glass?
Use a ray box to direct a light ray into a rectangular glass box at different angles of incidence at the midpoint P of one of the longer sides
Note that the angle of incidence is the angle between the incident light ray and the normal at the point of incidence
For each angle of incidence at P mark the point Q where the light ray leaves the block
Angle of refraction is the angle between the normal at point P and the line PQ
What is white light composed of?
Composed of light with a continuous range of wavelengths from red to violet
What did youngs double slit experiment demonstrate?
The interference of light
How can we observe the interference of light?
We can illuminate two closely spaced parallel slots (double slits) using a suitable light source
The two slits act as coherent sources of waves which means that they emit light waves with a constant phase difference and the same frequency
What can be seen on the screen in Young’s experiment?
Alternate bright and dark fringes can be seen where the diffracted light from the double slits overlaps
The fringes are evenly spaced where the diffracted light and parallel to the double slits
What would happen if the single slit is too wide?
Young’s
Each part of it produces a fringe pattern which is displaced slightly from the pattern due to adjacent parts of the single slit
As a result the dark fringes of the double slit pattern become narrower than the bright fringes and contrast is lost between the dark and the bright fringes
Why should the fringes be displayed on a screen when used a low power laser?
A beam of laser light will damage the retina if it enters the eye
Why are the fringes formed?
They’re formed due to interference of light from the two slits
