Waves - Kerboodle Flashcards
1
Q
4.1 Waves and vibrations
Types of waves
A
- Waves that pass through a substance are vibrations which pass through a substance
- Sound waves, seismic waves and waves on a string are examples of waves that pass through a substance
- These types of waves are referred to as mechanical waves
- When waves pass through a substance, particles of the substance vibrate in a certain way which makes nearby particles vibrate in the same way
- Electromagnetic waves are an oscillating and magnetic fields that progress through space without the need for a substance
- The vibrating electric field generates a vibrating magnetic field, which generates a vibrating electric field further away, and so on
- examples of electromagnetic waves are radio waves, microwaves, infrared, radiation, light, ultraviolet radiation, X-rays and gamma radiation.
2
Q
Longitudinal and transverse Waves
A
- Longitudinal waves are waves in which the direction of vibration of the particles is parallel to the direction in which the wave travels
- Sound waves, Primary seismic waves and compression waves on a slinky toy are all longitudinal waves
- Transverse waves are waves in which the direction of vibration is perpendicular to the direction in which the wave travels
- Electromagnetic waves, secondary seismic waves and waves on a string or a wire are all transverse waves.
3
Q
Polarisation
A
- Transverse waves are plane-polarized if the vibrations stay in one plane only. If the vibrations change from one plane to another, the waves are unpolarised
- Longitudinal waves cannot be polarised
- If unpolarised light is passed through a polaroid filter, the transmitted light is polarised as the filter only allows through the light which vibrated in a certain direction
- If unpolarised light is passed through two polaroid filters, the transmitted light intensity changes if one polaroid is turned relative to the other one
- The filters are said to be crossed when the intensity is at a minimum, at this position the polarised light from the first filter cannot pass through the second filter
- Light is a part of the spectrum of electromagnetic waves. The plane of polarisation of an electromagnetic wave is defined as the pane in which the electric field oscillates.
4
Q
4.2 Measuring waves
Key terms
A
- The displacement of a vibrating particle is its distance and direction from its equilibrium position
- The amplitude of a wave is the maximum displacement 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
- The wavelength of a wave is the least distance between two adjacent vibrating particles with the same displacement and velocity at the same time
- One complete cycle of a wave is from maximum displacement to the next maximum displacement
- The period of a wave is the time for one complete wave to pass a fixed point
- 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
5
Q
Wave speed
A
- The higher the frequency of a wave, the shorter its wavelength
- The speed of a wave can be calculated using c = Frequency * wavelength
6
Q
Phase difference
A
- The phase of a vibrating particle at a certain time is the duration of a cycle it has completed since the start of the cycle
- The phase difference between two particles vibrating at the same frequency is the fraction of a cycle between the vibrations of the two particles
- Phase difference is measured in degrees or radians, were 1 cycle = 360° = 2π radians.
7
Q
4.3 Wave properties 1
Reflection
A
- Straight waves are directed at a certain angle so a hard flat surface reflects off at the same angle
- The angle between the reflected wave front 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.
8
Q
Refraction
A
- When waves pass across a boundary at which the wave speed changes, the wavelength also changes
- If the wavefronts approach at an angle to the boundary, they change direction as well as speed
- This effect is known as Refraction
- Refraction of light is observed when a light ray is directed into a glass block at an angle. The light ray changes direction when it crosses the glass boundary. This happens because light waves travel more slowly in glass than in air.
9
Q
Diffraction
A
- Diffraction occurs when waves spread out through a gap or around an obstacle
- The narrower the gap, the more the waves spread out
- The longer the wavelength, the more the waves spread out
- Diffraction is waves spreading out past a gap to fill a room, you can think of it as how you can hear the sound even if the emitter isn’t pointed at you.
10
Q
4.4 Wave properties 2
The principle of Superposition
A
- When waves meet, they pass through each other. At this point where they meet, they combine for an instant before they move apart. This combining effect is known as superposition
- There are two types of superposition, Constructive interference, where a crest meets a crest
- Constructive interference forms a super crest, the combination of two crests, or a super tough, the combination of two troughs
- The second type of superposition is destructive interference
- Destructive interference is where two waves of equal amplitude, frequency and wavelength meet, but have a phase difference of 90°
- Destructive interference causes the two colliding waves to cancel each out.
11
Q
Further examples of superposition
1 Stationary waves on a rope
A
- Stationary waves are formed on a rope if two people send waves continuously along the rope from either end.
- The two sets of waves which cancel each other out are known as progressive waves
- Progressive waves combine at fixed points along the rope to form nodes (points of no displacement) and antinodes (points of maximum displacement)
- At each node, the two waves are always 180° out of phase so they cancel each other out
12
Q
Further examples of superposition
2 Water waves in a ripple tank
A
- A vibrating dipper on a water surface sends out circular waves, with the points where it makes contact with the water being the epicentre
- Points of cancellation are created where a crest from one dipper meets a trough from the other dipper. These points of cancellation are seen as gaps in the wavefronts
- Points of reinforcement are created where a crest from one dipper meets a crest from the other dipper, or where a trough from one dipper meets a trough from the other dipper
- As the waves formed by the two dippers are continuously passing through each other at a constant frequency and phase difference, cancellation and reinforcement occur at fixed points
- The effect that causes this is known as interference
- Interference is where coherent sources of waves produce an interference pattern where they overlap because they vibrate at the same frequency with a constant phase difference
- If the phase difference changed at random, the points of cancellation and reinforcement would move at random, and no interference pattern would be seen
13
Q
4.5 Stationary and progressive waves
Formation of stationary waves
A
- A stationary wave is formed when two progressive waves pass through each other
- This can be achieved on a string in tension by fixing both ends and making the middle part vibrate, so progressive waves travel towards each end, reflect at the end and pass through each other
- Stationary waves can have different harmonics, which show how many nodes/antinodes there are, waves of the first harmonic have 2 nodes, while waves of the second have 3, etc.
- To change a wave from the first harmonic to the second, you need to gradually increase the frequency
- Stationary waves that vibrate freely do not transfer energy to there surroundings
14
Q
Explanation of stationary waves
A
- some differences between stationary waves and progressive waves can be seen in their frequency, amplitude and the phase difference
- For stationary waves, the frequency stays constant at all particles except those at a node, whereas for progressive waves, all the particles vibrate at the same frequency
- For stationary waves, the amplitude varies from zero at the nodes to a maximum at the antinodes, whereas for progressive waves the amplitude is the same for all particles
- For stationary waves the phase difference between two particles is equal to mπ, where m is the number of nodes between the two particles, whereas, for progressive waves, the phase difference between two particles is equal to 2πd/λ, where d = distance apart and λ is the wavelength.
15
Q
More examples of stationary waves
Sound in a pipe
A
The sound resonates at certain frequencies in an air-filled tube or pipe. In a pipe closed at one end, these resonant frequencies occur when there is an antinode at the open end and a node at the other end.
