Waves Flashcards
(27 cards)
What is a wave
A wave is a disturbance that travels in a medium transferring energy and momentum from one place to another.
Transverse Waves
We call a wave transverse if the displacement is at right angles to the direction of energy transfer.
Longitudinal Waves
In a longitudinal wave the displacement is parallel to the direction of energy transfer.
All longitudinal waves require a medium in which the wave travels. Sound waves are longitudinal waves.
Compression: Coils crowd together - higher than normal density
Rarefaction: Coils move apart - lower than normal wavelength
Longitudinal Waves: Displacement - Distance Graph
Positive: Right
Negative: Left
Electromagnetic waves
All EM waves have in common the fact that they move at the speed of light in a vaccuum.
EM waves are transverse.
Wavefront
A wavefront is a surface through crests and normal to the direction of energy transfer of the wave.
Rays
Lines in the direction of energy transfer of the wave are called rays.
Wavefront and rays
Rays are perpendicular to wavefronts.
Law of reflection
The angle of incidence is equal to the angle of reflection r.
The reflected and incident rays and the normal to the surface lie one the same plane, called the plane of incidence.
Refraction
Refraction is the travel of light from one medium into another where is has a different speed.
Dispersion
Rays with the same angle of incidence but of different wavelength are refracted by different angles.
Total internal reflection
- Total internal reflection is the phenomenon in which waves arriving at the interface from one medium to another are not refracted into the second medium, but completely reflected back into the first medium.
- A wave moving in an optically dense medium arrives at the interface with a less dense medium.
Critical angle
The angle of incidence for which the angle of refraction is 90 is called the critical angle.
Diffraction
The spreading of a wave as it goes past an obstacle or through an aperture is called diffraction.
- The width of the diffracted wavefronts is proportional to the intensity of the wave.
The principle of superposition
When two or more waves of the same type arrive at a given point in space at the same time, the displacement of the medium at that point is the algebraic sum of the individual displacements.
Constructive interference
Constructive interference occurs when the path difference = nλ with n=0, 1, 2,3 (The path difference is an integral multiple of the wavelength).
Destructive interference
Destructive interference occurs when the path difference = (0.5+n)λ with n=0, 1, 2,3 (The path difference is a half-integral of the wavelength).
Standing waves
When two waves of the same speed, wavelength and amplitude travelling in opposite directions meet and interfere, a standing wave is formed.
- Standing waves do not transfer energy, it consists of 2 waves travelling energy in different directions so the standing wave itself transfers no energy.
Nodes
The points on the string where, as a result of destructive interference between the two waves, the displacement is always 0.
Antinodes
Half-way between nodes, are points where, as a result of constructive interference, the displacement gets as large as possible.
Standing waves: Wavelength
The distance between two consecutive nodes is half a wavelength.
Standing waves: Both ends fixed
- First harmonic: Longest wavelength and lowest frequency.
- λn=2L/n, n=1,2,3,4…
- All harmonics have frequencies that are integral multiplies of the fundamental frequency.
- Fn= Vn/2L
Standing waves: Both ends opened
- λn=2L/n, n=1,2,3,4…
- Fn= Vn/2L
Standing waves: One end opened and one end closed
- The distance between a node and an antinode is a quarter of a wavelength.
- λn = 4L/n, n=1,3,5,7…
