11,12. Waves (4.4) Flashcards
What is a progressive wave? (2)
- a transfer of energy
- as a result of oscillations
(A wave which travels continuously in a medium in the same direction without a change in its amplitude)
Longitudinal vs transverse
- L has oscillations parallel to direction of energy transfer
- T has oscillations perpendicular to direction of energy transfer
What is the displacement of a wave?
the relative change in position of a wave as it traveled through a medium.
amplitude
The amplitude is the perpendicular distance from the centre line to the peak of the wave
wavelength
The length travelled in one full wave in the direction of energy transfer
wave period
The time it takes for two successive peaks (one wavelength) to pass a specified point
Phase difference
The phase difference is the difference in the phase angle of the two waves.
(How many degrees apart they points if the wave are apart should the wave be turned into a sin/cos function)
Frequency
How many peaks of a wave pass one point in one second
Wave speed (and equation)
Wave speed is the distance a wave travels in a given amount of time
v = f λ
Wavefront
Lines of constant phase (which are perpendicular to the direction of energy transfer)
What does a change in wavefronts signify?
Some wave phenomenon
Reflection
When a wave reverses direction upon meeting the boundary between two different media.
Angle of incidence (i) vs angle of refraction (r) of a reflected wave
i = r
What does the distance between wavefronts represent?
Wavelength
Refraction
When a wave changes speed (and usually direction) upon crossing the boundary between two different media.
Does f or λ change during refraction?
λ
What is the relationship between change in speed and angle of refraction relative to the normal?
- if speed decreases, bends towards normal
- if speed increases, bends away from normal
When do sound waves speed up during refraction
If they go into physically denser media
When do EM waves speed up in refraction?
When they go into less optically dense media
Is physical density proportional to optical density
Not always
Diffraction
Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture.
What is polarisation?
A property of only transverse waves which defines the plane of oscillation of the wave
What is the plane of alignment of a polarising filter?
The plane in which it will allow oscillations.
When travelling through a filter, what happens to the intensity of an unpolarised wave?
It halves
Relationship between intensity and amplitude
I is directly proportional to A^2
Relationship between intensity, power, area:
I =P/A
Order of EM wavelength from longest to shortest:
- radio
- micro
- infrared
- visible
- UV
- X-ray
- gamma
Radiowave λ (m)
10^-1 - 10^6
Microwave λ (m)
10^-3 - 10^-1
Infrared λ (m)
7x10^-7 - 10^-3
Visible light λ (m)
4x10^-7 - 7x10^-7
UV λ (m)
10^-8 - 4x10^-7
X-ray λ (m)
10^-13 - 10^-8
Gamma λ (m)
10^-16 - 10^-10
Which 2 EM waves have an overlap of λ?
EM and X-ray
refractive index formula
n = c/v
Refractive index definition
The ratio between the speed of light in a vacuum to the speed of light in the medium
Snell’s law equation
n1sinθ1=n2sinθ2
What is the critical angle
The angle where total internal reflection occurs
Total internal reflection
When light completely reflects back at a boundary between two media as opposed to transmitting across the boundary and refracting.
Critical angle formula
sinC = 1/n (when the other medium is air)
General formula:
sinC = n2/n1
Conditions for total internal refraction:
- n1>n2
- i>=iC
What is the definition of superposition?
When two waves meet at a point, the resultant displacement is equal to the sum of the displacements of the individual waves.
PAG 5
Interference
When two coherent waves superimpose?
Constructive interference
Larger amplitude
Destructive interference
Smaller amplitude
Coherence
Two waves have a constant phase difference
(so same frequency and wavelength)
Path difference
The difference in length in the paths travelled by each of the waves that interfere
Phase difference (in terms of two dif waves)
The cycle difference between two coherent waves at the same point
Path difference and phase difference of constructive interference
- PLD = multiple of wavelength
- Phase difference = multiple of 2pi
Path difference and phase difference of destructive interference:
- PLD = odd multiple of (wavelength/2)
- Phase dif = odd multiple of pi
What did Young’s double slit experiment show about the nature of light?
It showed that light has wave nature.
What is Young’s double slit equation?
λ = (ax)/D
where a = distance between slits, x = distance between consecutive fringes/bright spots, D = horizontal distance between slits and wall
Remember how a stationary wave looks.
Similarities between stationary and progressive waves:
- both are comprised of oscillations
- both have wavelength, frequency, amplitude
Differences between stationary and progressive waves:
- progressive has energy transfer in direction of wave, stationary has no net energy transfer
- wavelength of stationary =2 x distance between two adjacent nodes but wavelength of oregressive is distance between two points in phase
- all points between two nodes are in phase for stationary but the phase changes across one complete cycle of the wave for progressive
- in prigressive all points have same amplitude, in stationary max amplitude accurs at antipode and drops to 0 at node
Node
A point where there is 0 displacement in a stationary wave
Antinode
A point in a stationary wave where there is maximum displacement
Draw out first 5 harmonics, with expressions for λ, of a stationary wave on a string fixed at both ends
Textbook 235
Draw out first 4 harmonics (and wavelength) of a stationary wave in an open tube
Textbook 239
Draw out first 5 harmonics (and expressions of wavelength) for a stationary wave in a tube with only one side open
Textbook 237
