Topic 4: Waves Flashcards
Define displacement (x).
The distance the medium has moved from the equilibrium position in a specific direction (m).
Define amplitude (x₀).
The maximum displacement from the equilibrium position (m).
Define frequency (f).
The number of oscillations per second (Hz).
Define period (T).
The time for one oscillation (s).
Define wavelength (λ).
The shortest distance between two points that are in phase along a wave (m). Eg. crest to crest.
Define wavespeed (v).
The distance travelled per unit time by the energy of the wave (or by a wavefront) (ms⁻¹).
Define phase (φ).
A measure of how “in step” different particles are.
When does simple harmonic motion (SHM) occur?
SHM is a type of oscillation that takes place when the acceleration of (and the force on) an object is;- proportional to its displacement from the equilibrium position.- in the opposite direction to the displacement (i.e. towards the equilibrium position).*The motion is due to a restoring force.
If a graph of acceleration against displacement is plotted what would a straight line and negative gradient show?
- A straight line shows that acceleration and displacement are proportional.- A negative gradient shows that acceleration and displacement are in opposite directions.
Define wave.
A movement of energy through a medium.
Describe the two types of wave motion.
Longitudinal waves - the particles of the medium vibrate parallel to the direction of the energy transfer. Eg. sound, earthquake P waves. Rarefaction and compression.Transverse waves - the particles of the medium vibrate at right angles to the direction of the energy transfer. Eg. light, earthquake S waves. Crest and trough.
Define intensity.
The power per unit area received by an observer. Measured in Wm⁻².
What is the intensity of a wave related to?
It is proportional to the square of the amplitude of the wave. I∝A²It is inversely proportional to the square of the distance from the source. I∝x⁻²
Explain the two types of waves.
Mechanical waves. These require a medium to travel through. Eg. sound and earthquake waves.Electromagnetic waves. These do not require a medium to travel through i.e. they can pass through a vacuum. All electromagnetic waves are transverse and travel at the speed of light in a vacuum.
Give the order of magnitude for the electromagnetic spectrum.
(Roman Men Invented Very Useful Xray Guns)In order of decreasing wavelength: Radiowaves 10²Microwaves 10⁻³Infrared 10⁻⁵Visible 10⁻⁷Ultraviolet 10⁻⁹Xray 10⁻¹²Gamma rays 10⁻¹⁴
Define the refractive index (n).
A measure of how fast a wave travels in the medium. The higher the value of n, the slower the wave travels.
Give Snell’s Law.
n = sini/sinr*Only applies if the first medium is air. If the first medium is not air, the law given in the data booklet applies.
When does total internal reflection occur?
When light is travelling from a more to a less optically dense medium and the angle of incidence at the boundary is greater than the critical angle.
Give the formula to work out the critical angle or refractive index.
sinθc = 1/n₁
Define wavefront.
A line connecting points on a wave with the same phase/displacement.
How is a ray drawn on a wavefront?
A ray is a line drawn to represent the direction a wave is travelling when viewed from above. It is drawn at 90° to the wavefronts.
Define diffraction.
When waves move past an object or through a gap the waves tend to spread out. This is called diffraction.
What is affected in refraction?
Speed and wavelength are affected.Frequency stays the same.
What is affected in diffraction?
Amplitude decreases as the energy is spread out over a longer wavefront.Frequency, speed and wavelength stay the same.
Describe constructive interference.
When waves meet in phase, they will form a resultant wave with an amplitude equal to the sum of the two individual waves.
Describe destructive interference.
When waves meet out of phase, they will cancel each other out to give a wave of zero amplitude (assuming both waves are of equal amplitude).
Define the principle of superposition.
If two or more waves meet, the resultant displacement at any point is found by adding the displacements produced by each individual wave.
What are antinodal and nodal lines?
Antinodal lines are drawn when waves meet in phase (constructive interference). Path difference = nλ. Antinodes are the points at max displacement.Nodal lines are drawn when waves meet out of phase (destructive interference). Path difference = (n + 1/2)λ Nodes are the points at 0 displacement.
What are the units the formula s = λD/d?
Metres.s = distance between the bright fringesλ = wavelength of the lightD = distance from the double slits to the interference pattern on a screend = distance between the double slits
How do standing waves form?
When two waves of the same type meet. They must be;- of the same amplitude- of the same frequency- travelling in opposite directions*Note: Energy is not transferred in standing waves.Eg. when a wave reflects back on itself from a boundary. The inverted reflected wave interferes with the forward moving wave to create a standing wave.
What is polarisation?
If a wave is polarised, then all the oscillation of the medium is only in one plane. When light is polarised, the electric field vector is oscillating in one plane only.*Note: only transverse waves can be polarised.
How can polarised light be produced from unpolarised light?
- Reflection of a surface (Brewster’s Law)- Using polarising filters (Malus’ Law)
Discuss Brewster’s Law.
When light reflects off a surface, it will be polarised in the plane of the surface. n = tanθ
When is polarisation 100% when light reflects of a surface?
If the angle between the reflected and refracted ray is 90°.
Discuss Malus’ Law.
Light is passed through a polarising filter and then an analyser.
What happens when unpolarised light is passed through a polariser?
The light intensity is reduced by half.
nm -> m
divide by 10^9
Define an optically active substance.
One that rotates the plane of polarisation of light that passes through it.
What is the graph for SHM?
Upside down parabola.
What is the graph for intensity against an angle?
Parabola with a minimum at 90° and a maximum at 180°.
How do sunglasses work?
Sunglasses work by blocking the horizontally polarised light in order to stop light glare from reflected rays off of water. So they have vertical slits only.
Describe interference with polarised light.
Two waves must have the same plane of polarisation and phase in order to produce an interference pattern. If waves of two different polarised light meet, a wave of a different plane of polarisation is produced.
What type of wave are standing waves in pipes?
Sound waves which are longitudinal so the particles are vibrating horizontally.
Do waves travel faster in deep water or shallow water?
Deep water.
