Waves Flashcards
(137 cards)
1
Q
What is a progressive wave?
A
A wave that transfers energy from one point to another without transferring the medium itself.
2
Q
What is displacement in a wave?
A
The distance of a point on the wave from its equilibrium position; it is a vector quantity (can be positive or negative).
3
Q
What is amplitude?
A
The maximum displacement of a particle in the wave from its equilibrium position.
4
Q
What is wavelength (λ)?
A
The distance between points on successive oscillations of the wave that are in phase.
5
Q
What is the period (T) of a wave?
A
The time taken for one complete oscillation or cycle, measured in seconds (s).
6
Q
What is frequency (f)?
A
The number of complete oscillations per unit time, measured in Hertz (Hz).
7
Q
How is frequency related to period?
A
𝑓=1/𝑇
8
Q
What is wave speed (v)?
A
The distance travelled by the wave per unit time, measured in m/s.
9
Q
What is the wave equation linking speed, frequency, and wavelength?
A
c=fλ
10
Q
What happens to frequency when wavelength increases (assuming constant speed)?
A
Frequency decreases.
11
Q
What is phase difference?
A
A measure of how much one wave is ahead or behind another, in terms of fraction of wavelength, degrees, or radians.
12
Q
When are two waves said to be in phase?
A
When their crests or troughs are aligned (phase difference = 0 or 360° or 2π radians).
13
Q
When are two waves said to be in antiphase?
A
When the crest of one aligns with the trough of another (phase difference = 180° or π radians).
14
Q
What are the two main types of mechanical waves?
A
Transverse waves and longitudinal waves.
15
Q
How do particles oscillate in a transverse wave relative to the direction of wave travel?
A
Particles oscillate perpendicular to the direction of wave travel.
16
Q
Give three examples of transverse waves.
A
Electromagnetic waves (radio, visible light, UV), vibrations on a guitar string, waves on a rope.
17
Q
Can transverse waves be polarised?
A
Yes, transverse waves can be polarised.
18
Q
How do particles oscillate in a longitudinal wave relative to the direction of wave travel?
A
Particles oscillate parallel to the direction of wave travel.
19
Q
What features do longitudinal waves show?
A
Compressions and rarefactions.
20
Q
Define compressions and rarefactions.
A
Compressions are regions of increased pressure; rarefactions are regions of decreased pressure.
21
Q
Give three examples of longitudinal waves.
A
Sound waves, ultrasound waves, waves in a slinky spring.
22
Q
Can longitudinal waves be polarised?
A
No, longitudinal waves cannot be polarised.
23
Q
What is a wavelength in a longitudinal wave?
A
The distance between two compressions or two rarefactions.
24
Q
How does energy transfer happen in mechanical waves?
A
Energy is transferred through moving oscillations or vibrations of particles in the medium.
25
What types of seismic waves correspond to transverse and longitudinal waves?
S-waves are transverse; P-waves are longitudinal.
26
What is polarisation?
Polarisation is when particle oscillations occur in only one direction perpendicular to the direction of wave propagation.
27
Can longitudinal waves be polarised? Why or why not?
No, because longitudinal waves oscillate parallel to the direction of travel, not perpendicular.
28
What happens when transverse waves are polarised?
Their vibrations are restricted to one direction, still perpendicular to the wave propagation.
29
How does a polariser work?
It only allows oscillations in a certain plane to be transmitted, blocking other directions.
30
What happens to unpolarised light passing through two polarising filters with their transmission axes parallel?
Maximum intensity of light is transmitted.
31
What happens when two polarisers are perpendicular to each other?
No light passes through, minimum intensity is transmitted.
32
How does light intensity vary as the angle between two polarisers changes?
It varies periodically, reaching maximum at 0° and minimum at 90° between their transmission axes.
33
Why do polaroid sunglasses reduce glare from water surfaces?
Because reflected light is partially horizontally polarised, and the sunglasses have vertical polarising filters that block horizontally polarised glare.
34
How do polaroid filters help in photography?
They reduce glare and enhance colour, especially useful for shooting scenes with reflective surfaces or underwater objects.
35
How does polarisation affect radio and TV aerials?
Aerials must be aligned with the polarisation (horizontal or vertical) of the transmitted signals for proper reception.
35
What is a stationary (standing) wave?
A stationary wave is formed by the superposition of two waves of the same frequency and amplitude travelling in opposite directions, typically a wave and its reflection.
36
How do stationary waves differ from progressive waves in terms of energy?
Stationary waves store energy; progressive waves transfer energy.
37
What are nodes in a stationary wave?
Nodes are points where there is no vibration (zero displacement).
38
What are antinodes in a stationary wave?
Antinodes are points where the vibration amplitude is at a maximum.
39
Do nodes and antinodes move along the medium?
No, nodes are fixed points and antinodes only oscillate vertically but do not move along the medium.
40
What is the phase relationship between points on a stationary wave?
Points between nodes are in phase; points separated by an odd number of nodes are out of phase; points separated by an even number of nodes are in phase.
41
How is a stationary wave usually formed practically?
By a travelling wave reflecting back and interfering with the incoming wave.
42
What does the stationary wave pattern look like?
Peaks and troughs appear fixed in position, oscillating in place without moving along the medium.
43
Why don’t stationary waves transfer energy along the medium?
Because energy oscillates between kinetic and potential forms at fixed points rather than moving along the medium.
44
What does the principle of superposition state?
When two or more waves with the same frequency arrive at a point, the resultant displacement is the sum of the displacements of each wave.
45
What happens when two waves are in phase during superposition?
They interfere constructively, meaning their peaks and troughs line up and the resultant wave has double the amplitude.
46
What happens when two waves are in anti-phase during superposition?
They interfere destructively, meaning the peaks of one align with the troughs of the other and the resultant wave has zero amplitude.
47
What conditions are required for the formation of a stationary wave?
Two waves must travel in opposite directions along the same line with the same frequency, wavelength, and similar amplitude.
48
What causes nodes in a stationary wave?
Nodes form at points where the two waves are in anti-phase and destructive interference causes them to cancel out.
49
What causes antinodes in a stationary wave?
Antinodes form where the two waves are in phase, causing constructive interference and maximum amplitude.
50
How do stationary waves produce sound on a stretched string?
Vibrations at resonant frequencies cause stationary waves with nodes and antinodes along the string, producing sound
51
How can microwaves be used to demonstrate stationary waves?
A microwave source is placed opposite a reflecting plate, and a detector is moved between them to detect nodes (minima) and antinodes (maxima) of the stationary wave.
52
How are stationary sound waves produced in air columns?
A loudspeaker at one end creates sound waves that reflect and interfere in the column, forming nodes and antinodes; powder inside the column collects at nodes showing zero displacement.
53
What is required at the ends of an air column to form stationary sound waves?
There must be a node (minimum displacement) at the closed end and an antinode (maximum displacement) at the open end (where the loudspeaker is).
54
What happens to the resultant displacement when waves are neither completely in phase nor completely out of phase?
The resultant amplitude is between the maximum (constructive) and minimum (destructive) values, depending on their relative phase.
55
Why do wave displacements combine like vector quantities during superposition?
Because displacement can be positive or negative, and they add algebraically considering their direction (phase).
56
What physical condition must be met for two waves to produce a stable interference pattern?
The waves must be coherent — having the same frequency and a constant phase difference.
57
How can two speakers produce an interference pattern?
By emitting sound waves of the same frequency and fixed phase relationship, creating regions of loud (constructive) and quiet (destructive) sound.
58
What effect does changing the frequency of waves have on the interference pattern?
It changes the wavelength, thus altering the spacing of nodes and antinodes in the pattern.
59
What is a fringe in terms of wave interference?
The phase difference between waves does not change over time, allowing stable interference patterns.
60
Why are lasers ideal for studying diffraction and interference?
Because lasers produce coherent (constant phase difference) and monochromatic (single wavelength) light.
61
What do the bright fringes in a laser diffraction pattern represent?
Areas of constructive interference where waves reinforce each other.
62
What causes the dark fringes in a laser diffraction pattern?
Areas of destructive interference where waves cancel each other out.
63
Why do filament bulbs or sodium lamps not produce clear interference patterns like lasers?
Because they emit non-coherent, polychromatic (multiple wavelengths) light.
64
List three key safety precautions when using lasers.
1) Never look directly at the laser beam or its reflection
2) Wear laser safety goggles
3) Place warning signs and stand behind the laser
65
How is two-source interference demonstrated with sound waves?
Using two speakers emitting coherent sound waves of the same frequency and phase difference.
66
When does constructive interference occur with sound waves?
When compressions and rarefactions from both waves line up, increasing the amplitude and loudness.
67
How does destructive interference occur with sound waves?
When a compression from one wave aligns with a rarefaction from another, cancelling each other out and producing silence.
68
What practical application uses destructive interference of sound waves?
Noise-cancelling headphones.
69
How can microwave interference be detected?
Using a movable microwave detector to measure intensity variations due to constructive and destructive interference.
70
What does the microwave detector measure in regions of constructive interference?
Maximum amplitude or intensity (a strong signal)
71
What does the microwave detector measure in regions of destructive interference?
Minimum or zero amplitude (no signal).
72
How is the intensity of a wave related to its amplitude?
Intensity is proportional to the square of the amplitude,
𝐼∝𝐴^2
73
Why does intensity relate to the square of amplitude?
Because energy transferred by the wave is proportional to the square of the amplitude.
74
What does Young’s double-slit experiment demonstrate?
Diffraction and interference patterns from two coherent light sources or a single source passing through two slits.
75
In Young’s experiment, why is a single slit placed before the double slit?
To diffract the light, producing two coherent sources at the double slits
75
What forms the bright fringes (maxima) in the double-slit diffraction pattern?
Constructive interference of the two coherent light waves.
75
What forms the dark fringes (minima) in the double-slit diffraction pattern?
Destructive interference of the two coherent light waves.
76
What determines the position of maxima and minima in the interference pattern?
he path difference between the two waves arriving at a point on the screen.
77
State the condition for constructive interference in Young's double slit experiment.
Path difference = n × λ (where n = 0, 1, 2, ...)
78
State the condition for destructive interference in Young’s double slit experiment.
Path difference = (n + ½) × λ
79
What is meant by the order of maxima (n) in the interference pattern?
It indicates the position of the maxima relative to the central maximum, with n = 0 at the center.
80
How does the fringe spacing (w) change if the wavelength (λ) increases?
Fringe spacing increases.
81
How does fringe spacing change if the slit separation (s) decreases?
Fringe spacing increases.
82
How does fringe spacing change if the distance (D) between slits and screen increases?
Fringe spacing increases.
83
Write the fringe spacing equation for Young’s double slit experiment.
𝑤=𝜆𝐷/𝑠
where
w = fringe spacing,
λ = wavelength,
D = distance to screen,
s = slit separation.
84
What happens to the interference pattern when white light passes through a double slit?
A central white maximum is produced, with spectra (rainbow colors) forming on either side.
85
In a white light interference pattern, which color appears closest to the central maximum?
Violet/blue (shortest wavelength).
86
Which color appears furthest from the central maximum in a white light interference pattern?
Red (longest wavelength).
87
Why do the spectral fringes in a white light interference pattern become blurry further from the center?
Because fringe spacing decreases and different wavelengths overlap, merging the colors.
88
What is diffraction?
The spreading out of waves after they pass through a narrow gap or around an obstacle.
89
How does the size of the gap relative to the wavelength affect diffraction?
Diffraction is most significant when the gap size is about the same as the wavelength. Much larger or smaller gaps cause little to no diffraction.
90
What happens to wavefronts after they pass through a narrow gap?
They spread out and curve, showing diffraction.
91
Which property of a wave changes during diffraction?
The amplitude decreases; wavelength stays the same.
92
Describe the diffraction pattern formed by a single slit using monochromatic light.
A central bright fringe (maximum) much wider and brighter than others, with alternating dark and less bright fringes on either side that get dimmer with increasing order.
93
Why is the central maximum in single slit diffraction wider than other maxima?
Because constructive interference from all parts of the wavefront happens here.
94
What causes the dark fringes in a single slit diffraction pattern?
Destructive interference between parts of the wavefront.
95
How does the intensity of fringes change as you move away from the central maximum in single slit diffraction?
Intensity decreases with distance from the central maximum.
96
What difference does white light make to the single slit diffraction pattern?
The central maximum is bright white; other maxima show spectra of colors, with blue/violet fringes closest to center and red fringes furthest away.
97
Why do fringes in white light diffraction appear blurry further from the central maximum?
Because fringe spacing decreases, and spectra from different wavelengths overlap and merge.
98
How does increasing the wavelength of light affect diffraction through a single slit?
Increases the angle of diffraction and widens the bright fringes.
99
Compare diffraction patterns of red and blue lasers through a single slit.
Red light produces wider fringes due to longer wavelength; blue light produces narrower fringes.
100
What happens to the diffraction pattern if the slit width is narrowed?
The angle of diffraction increases, the central maximum widens, fringe spacing increases, and the intensity of maxima decreases.
101
What effect does making the slit wider have on the diffraction fringes?
Fringes become narrower and closer together.
102
What is a diffraction grating?
An optical device with many thin, equally spaced parallel slits that diffract light into sharp bright and dark fringes.
103
How does the diffraction pattern from a diffraction grating compare to that of a double slit?
The grating produces narrower, sharper bright fringes and wider, darker dark regions.
104
Write the diffraction grating equation.
dsinθ=nλ
Where:
d = slit spacing
θ = angle of diffraction
n = order of maxima (integer)
λ = wavelength of light
105
What does the order n represent in diffraction gratings?
The number of maxima away from the central maximum
106
What causes the maxima in the diffraction grating pattern?
Constructive interference where the path difference between adjacent slits equals nλ.
107
What are some common applications of diffraction gratings?
- Spectrometers for analyzing star light and chemical composition
- Measuring red shift and atomic spacing (X-ray crystallography)
- Monochromators to select specific wavelengths for medical or optical fiber use
108
Why are diffraction gratings preferred over double slits in spectrometers?
Because they produce sharper, more intense maxima and allow better wavelength separation.
109
How does increasing the number of lines per mm on a grating affect slit spacing d?
Increasing lines/mm decreases slit spacing d
110
What happens to the diffraction angle θ as the order n increases?
θ increases, meaning higher order maxima appear at larger angles from the central maximum.
111
What causes refraction?
Change in direction of a wave due to a change in speed when passing between media of different optical density.
112
How does the angle of refraction compare to the angle of incidence when light goes from air to glass?
The angle of refraction is smaller; light bends towards the normal.
113
What property of a material is described by the refractive index?
How much the speed of light is reduced inside the material compared to in a vacuum.
114
What happens to the frequency of light when it refracts?
Frequency remains the same.
115
What happens to wavelength when light passes into a more optically dense medium?
Wavelength decreases.
116
What is the approximate refractive index of air?
Approximately 1.
117
If light hits a boundary at 90°, does it refract?
No, it passes straight through without changing direction.
118
When light moves from glass (more dense) to air (less dense), what happens to the speed and wavelength?
Both speed and wavelength increase.
119
State Snell’s Law formula.
n1sinθ 1=n2sinθ 2
120
Under what conditions does total internal reflection occur?
Angle of incidence = angle of reflection.
121
Why do optical fibers rely on total internal reflection?
to trap light inside the core by keeping the angle of incidence greater than the critical angle, allowing light to travel long distances with minimal loss.
122
What principle allows light to travel along an optical fibre?
Total internal reflection, where light reflects repeatedly inside the fibre because the angle of incidence at the core-cladding boundary is greater than the critical angle.
123
Name the three main components of an optical fibre.
- Core (optically dense, plastic or glass)
- Cladding (lower refractive index than core)
- Outer sheath (protective layer)
124
Why must the cladding have a lower refractive index than the core?
To enable total internal reflection at the core-cladding boundary, keeping light trapped inside the core.
125
What is a step-index fibre?
An optical fibre where the refractive index changes sharply (steps) from the core to the cladding.
126
List two functions of the cladding in an optical fibre.
- Protects the core from damage and scratches
- Prevents signal loss by stopping light from escaping the core
- Prevents cross-talk between adjacent fibres
127
What is material dispersion in optical fibres?
When white light spreads into its different wavelengths, each traveling at different speeds, causing the pulse to broaden.
128
Which colour travels slowest in an optical fibre and why?
Violet light travels slowest because it has the shortest wavelength and undergoes more reflections due to a smaller angle of incidence.
129
What causes modal dispersion in optical fibres?
Different parts of the wavefront travel at different angles, undergoing varying numbers of total internal reflections, causing pulses to spread out over time.
130
How can modal dispersion be reduced?
By using a very narrow core so that light paths are more similar and fewer reflections occur.
131
What is pulse broadening, and why is it a problem?
Pulse broadening is the lengthening of light pulses due to dispersion, which can cause pulses to merge and distort the signal, leading to information loss.
132
How does absorption affect signals in optical fibres?
The fibre absorbs some signal energy, reducing amplitude and potentially causing loss of information.
133
Name three ways to reduce absorption and pulse broadening in optical fibres.
- Use very transparent core materials
- Use optical fibre repeaters to regenerate signals
- Use monochromatic light and single-mode fibres to reduce dispersion
133
Why are single-mode fibres used in optical communication?
They allow only one wavelength and mode to propagate, minimizing modal dispersion and improving signal quality over long distances.
