Waves Flashcards
Mechanical waves
Waves that move through a medium.
Electromagnetic waves
Oscillating electric and magnetic fields that progress through space without the need for a substance.
Longitudinal waves
Waves in which the direction of vibration of the particles is parallel to the direction in which the wave travels.
Transverse waves
Waves in which the direction of vibration is perpendicular to the direction in which the wave travels.
Plane-polarised
The vibrations stay in one plane only.
Displacement of a vibrating particle
Its distance and direction from its equilibrium position.
Amplitude of a wave
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.
Wavelength
The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time (distance between adjacent crests).
Cycle of a wave
From one wave peak to the next.
Period of a wave
The time for one complete wave to pass a fixed point.
Frequency of a wave
The number of cycles of vibrations of a particle per second. Measured in Hertz (Hz). f = 1 / T.
Wave speed, c
c = f λ
Phase difference
The fraction of a cycle between the vibrations of two particles vibrating at the same frequency, measured in degrees or radians.
Phase difference in radians
2πd / λ
Wavefronts
Lines of constant phase (e.g. crests).
Refraction
The effect that occurs when waves pass across a boundary at which the wave speed changes, the wavelength also changes.
Diffraction
Occurs when waves spread out after passing through a gap or round an obstacle. The narrower the gap, the more the waves spread out. The longer the wavelength, the more the waves spread out.
Principle of superposition
When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point.
Nodes
Points of zero displacement on a wave.
Standing waves
Set up as a result of the superposition of two waves with the same amplitude and frequency, travelling at the same speed but in opposite directions.
Coherent
Waves with a constant phase difference.
First harmonic
Simplest stationary wave pattern consisiting of two nodes at either end.
Phase difference between two particles on a stationary wave
Zero if the two particles are between adjacent nodes or separated by an even number of nodes. 180 degrees or π radians if the two particles are seperated by an odd number of nodes.
The normal
An imaginary line perpendicular to a boundary between two materials or surfaces.
If the incident ray is along the normal
No refraction occurs.
If a light ray passes into a more dense substance
Bends towards the normal.
If a light ray passes into a less dense substance
Bends away from the normal.
Refractive index of a substance, n
n = sin i / sin r = c / cs
Law of refraction
n1 sin i = n2 sin r
Refractive index of air
1
Refractive index of a vacuum
1
Refractive index of water
1.333
Critical angle
The minimum angle of incidence beyond which total internal reflection occurs for light travelling from a medium of higher to one of lower refractive index.
Requirements for total internal reflection
The incident substance has a larger refractive index than the other substance and the angle of incidence exceeds the critical angle.
Total internal reflection
Complete reflection of a ray of light within a medium.
Critical angle equation
sin 𝜃c = n2 / n1 for n1 > n2
Fringe separation
The distance from the centre of a bright fringe to the centre of the next bright fringe.
Fringe separation, w equation
w = λD / s where D is the distance from the slits to the screen and s is the distance between the centres of the slits.
Diffraction grating equation
d sin 𝜃 = n λ
