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PHYSICS PAPER ONE > Define// Waves > Flashcards

Define// Waves Flashcards

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1
Q

Define progressive waves and state what they do to the particles of the medium through which they travel

A

A wave that transfers energy from one point to another without transferring the medium itself

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2
Q

State common properties of waves

A

Properties of a Progressive Wave
-Displacement (x) of a wave is the distance of a point on the wave from its equilibrium position
-Amplitude (A) is the maximum displacement of a particle in the wave from its equilibrium position
-Wavelength (λ) is the distance between points on successive oscillations of the wave that are in phase
-Period (T) or time period, is the time taken for one complete oscillation or cycle of the wave
-Frequency (f) is the number of complete oscillations per unit time.
-Speed (v) is the distance travelled by the wave per unit time

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3
Q

Define longitudinal waves and give an example

A

A wave in which the particles oscillate parallel to the direction of the wave travel (and energy transfer)

Examples of longitudinal waves are:
-Sound waves
-Ultrasound waves

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4
Q

Define transverse waves and give an example

A

A wave in which the particles oscillate perpendicular to the direction of the wave travel (and energy transfer)

Examples of transverse waves are:
-Electromagnetic waves e.g. radio, visible light, UV
-Vibrations on a guitar string
-Seismic waves

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5
Q

Define the term amplitude

A

The maximum magnitude of displacement from its equilibrium position

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6
Q

Define the term wavelength

A

The distance between points on successive oscillations that are in phase/ the distance between two consecutive crests-trough.

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7
Q

Define the term phase

A

A measurement of the position of a particular point in a wave cycle.

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8
Q

Define the term phase difference

A

The phase difference between two waves is a measure of how much a point or a wave is in front or behind another

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9
Q

Define the term polarisation

A

Property of transverse waves in which particle oscillations occur in only one of the directions perpendicular to the direction of wave propagation.

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10
Q

Explain the effect of a polarising filter on longitudinal and transverse waves

A

When transverse waves are polarised, this means:
-Vibrations are restricted to one direction
-These vibrations are still perpendicular to the direction of propagation / energy transfer

Longitudinal waves (e.g. sound waves) cannot be polarised, this is because they oscillate parallel to the direction of travel.

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11
Q

Explain how polarisation is evidence for the nature of transverse waves

A

In 1808, Malus discovered that light was polarised by reflection. At that time light was though of as a longitudinal wave.
In 1817, Young suggested light was a transverse wave consisting of vibrating electric and magnetic fields at right angles to the transfer of energy. This explained why light can be polarised.

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12
Q

Give applications of polarisation

A

-Polaroid sunglasses are glasses containing lens with polarising filters with transmission axes that are vertically oriented. This means the glasses do not allow any horizontally polarised light to pass through.
-Glare reduction as polarising filters can reduce it by filtering out the partially plane polarised light when incident light is reflected
-Radio and television services are broadcast either horizontally-polarised or vertically-polarised

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13
Q

Define frequency

A

The number of cycles per second passing a given point.

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14
Q

Define period

A

the time taken for a whole cycle to complete

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15
Q

State the property that is the same for all EM waves in a vacuum

A

All electromagnetic waves: are transverse waves; can travel through a vacuum ; travel at exactly the same speed in a vacuum, the speed of light, 3.0 x 10^8

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16
Q

State the principle of superposition of waves and explain how it causes the different types of interference

A

When two or more waves overlap, the resultant displacement at a point is equal to the sum of the individual displacements at that point.
At the point of overlap:
-if two waves have the same sign displacement CONSTRUCTIVE INTERFERENCE (reinforcement) will occur
-if the have opposite sign displacement DESTRUCTIVE INTERFERENCE (cancellation) will occur.

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17
Q

Define stationary waves and give a graphical explaination of how they are formed

A

Standing waves are produced by the superposition of two coherent progressive waves of the same frequency and amplitude travelling in opposite directions

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18
Q

Explain what is meant by the harmonics of stationary waves

A

Stationary waves can have different wave patterns, known as harmonics
These depend on the frequency of the vibration and the situation in which they are created

1st Harmonic: also called fundamental 1/2 λ, 2N and 1A
2nd Harmonic: 2f of first harmonic, λ, 3N and 2A
3rd Harmonic: 3f of first harmonic, 3/2λ, 4N and 3A

19
Q

Define the terms nodes

A

Nodes are regions where there is no vibration, y=0

20
Q

Define the terms antinodes

A

Antinodes are regions where the vibrations are at their maximum amplitude

21
Q

Distinguish between phase difference and path difference and state the units for path difference

A

The phase difference is the difference in the phase angle of the two waves. Path difference (in metres) is the difference in the path traversed by the two waves. The relation between phase difference and path difference is direct. They are directly proportional to each other.

22
Q

Explain how path difference from between two coherent waves produces constructive and destructive interference

A

Constructive interference at n lambda
Destructive at 1/2+n lambda

23
Q

Define and describe diffraction

A

Diffraction is the spreading out of waves when they pass an obstruction
This obstruction is typically a narrow slit known as an aperture
The only property of a wave that changes when it diffracts is its amplitude
This is because some energy is dissipated when a wave is diffracted through a gap

24
Q

Explain why a single source with double slits can be used instead of two separate coherent sources

A

Using a single source with double slits is simpler and more practical than using two separate coherent sources to produce interference patterns. It eliminates the need to align two sources, is more economical, and allows for the study of interference patterns with different sources. The interference pattern produced by a single source with double slits is similar to that produced by two separate coherent sources as long as the distance between the slits is small compared to the distance from the slits to the screen.

25
Q

Explain why laser light is often used in the double slit experiment to produce clear interference patterns

A

Laser light is ideal for producing clear interference patterns in the double-slit experiment due to:

Coherence: All the light waves emitted from the laser have the same frequency, phase, and polarization, resulting in a well-defined and stable interference pattern.
Monochromaticity: Laser light consists of a single wavelength of light, ensuring a sharp and well-defined interference pattern.
Directionality: Laser light produces a narrow beam, reducing diffraction and ensuring the light passes through the double-slit setup in a well-defined manner.
High intensity: Laser light has high intensity, ensuring a bright enough interference pattern to be easily observed.

26
Q

Use Young’s double slit formula to explain the interference pattern produced by white light

A

Young’s double-slit formula is given by:

d * sinθ = m * λ

where d is the distance between the two slits, θ is the angle between the central maximum and the mth bright fringe, λ is the wavelength of the light, and m is the order of the bright fringe.

When white light is used in the double-slit experiment, the interference pattern produced consists of a series of bright fringes separated by dark fringes. This pattern arises because the different colors of white light have different wavelengths and, therefore, different values of θ for each order of the bright fringe.

As a result, the bright fringes of each color are displaced from each other, producing a multicolored pattern. The central maximum is white because all the colors overlap at this point, while the other bright fringes are different colors, with red being the least deviated and violet being the most deviated.

27
Q

Derive the diffraction grating equation

A
28
Q

Briefly outline why an interference pattern is formed when EM waves pass through a single slit

A

When an electromagnetic (EM) wave passes through a single slit, it diffracts, bending and spreading out into the region behind the slit. The diffracted waves interfere with each other and produce an interference pattern on a screen placed some distance away from the slit.

The interference pattern arises because the waves emerging from different points on the slit have different path lengths to the screen, resulting in phase differences between them. At some points, the waves add up constructively, producing a bright fringe, while at other points, they cancel out destructively, producing a dark fringe.

The width of the slit determines the extent of the diffraction, with narrower slits producing more diffraction and broader interference patterns. The distance between the screen and the slit also affects the pattern, with greater distances resulting in narrower fringes.

29
Q

State the factors that affect the degree of diffraction

A
  1. Wavelength: Shorter wavelengths diffract less than longer wavelengths, meaning they have a more direct path and show less bending.
  2. Size of the aperture or obstacle: A smaller aperture or obstacle causes greater diffraction because it limits the extent to which the wave can spread out.
  3. Distance between the wave source and the aperture or obstacle: The closer the wave source is to the aperture or obstacle, the greater the degree of diffraction.
  4. Shape of the aperture or obstacle: The shape of the aperture or obstacle affects the degree of diffraction. An object with sharp edges diffracts more than an object with smooth edges.
  5. Polarization of the wave: The degree of diffraction also depends on the polarization of the wave. Waves polarized perpendicular to the aperture or obstacle will diffract more than waves polarized parallel to it.
30
Q

Compare the interference pattern caused by the diffraction of white light through a single slit with the interference pattern caused by the diffraction of monochromatic light and explain the differences

A

The interference pattern produced by the diffraction of monochromatic light through a single slit consists of a central bright fringe surrounded by a series of alternating bright and dark fringes, with the intensity of the fringes decreasing as the distance from the central maximum increases.

In contrast, the interference pattern produced by the diffraction of white light through a single slit consists of a broad central maximum surrounded by a series of rainbow-colored fringes, with the colors changing from one fringe to the next. The central maximum is white because it contains all the colors of the spectrum, while the colored fringes arise because different colors of light have different wavelengths and therefore diffract by different amounts.

The colored fringes are narrower than the fringes produced by monochromatic light, and they are separated by a distance that depends on the wavelength of the light. This is because different wavelengths of light diffract by different amounts, causing the colors to spread out and producing the rainbow effect.

31
Q

Define the refraction index of a substance and state the refractive index of air

A

The refractive index, n, is a property of a material which measures how much light slows down when passing through it.
n of air= 1

32
Q

Define the term critical angle

A

When the angle of refraction is exactly 90° the light is refracted along the boundary

33
Q

Define the term total interal reflection

A

Total internal reflection occurs when a wave, such as light or sound, reaches an interface between two media and is completely reflected back into the original medium instead of being transmitted into the second medium. The angle of incidence is greater than the critical angle and the incident refractive index n1 is greater than the refractive index of the material at the boundary n2

34
Q

Show how the critical angle formula comes from Snell’s law

A

n1 sin θ1 = n2 sin θ2
the critical angle θc is the angle of incidence at which the angle of refraction is 90 degrees
n1 sin θc = n2 sin 90°
sin 90° = 1
n1 sin θc = n2
sin θc = n2 / n1

35
Q

Describe the purpose of step-index optical fibres

A

Fibre optics utilise the phenomenon of total internal reflection to send high speed light signals over large distances
These have many important uses, including:
-Communications, such as telephone and internet transmission
-Medical imaging, such as endoscopes

36
Q

Describe the function of the core and the cladding of a step-index optical fibres

A

CORE= transmission path for the light/infrared
CLADDING= used to TIR the rays so that they can travel over long distances and around curves. its refractive index has to be lower than the core

37
Q

Describe and explain material dispersion

A

Material dispersion, or spectral dispersion, occurs when white light is used instead of monochromatic light
This is because different wavelengths of light travel at different speeds
Blue light travels slower than red light due to the greater refractive index
Therefore, the red light reaches the end before the blue light
This results in pulse broadening

38
Q

Describe and explain modal dispersion

A

Modal dispersion occurs when the light pulses in the optical fibre spread out due to the different angles of incidence in the original pulse
This is more prominent in wider cores as the light travelling along the axis of the core travels a shorter distance than light undergoing total internal reflection at the core-cladding boundaries
This causes pulse broadening as the pulses emerging are longer than they should be

39
Q

Explain the principles and consequences of pulse broadening and absorption

A

Absorption occurs when part of the signal’s energy is absorbed by the fibre or the signal is attenuated by the core
This reduces the amplitude of the signal, which can lead to a loss of information

Pulse broadening is caused by modal and material dispersion
The consequence of pulse broadening is that different pulses could merge, resulting in a completely distorted final pulse

40
Q

Explain how pulse broadening and absorption are minimised

A

To reduce absorption:
-Use a core which is extremely transparent
-Use of optical fibre repeaters so that the pulse is regenerated before significant absorption has taken place

To reduce pulse broadening:
-Make the core as narrow as possible to reduce the possible differences in path length of the signal
-Use of a monochromatic source so the speed of the pulse is constant
-Use of optical fibre repeaters so that the pulse is regenerated before significant pulse broadening has taken place
-Use of single-mode fibre to reduce multipath modal dispersion

41
Q

How is the change in direction determined in refraction

A

The change in direction depends on which media the light rays pass between:
From air to glass (less dense to more dense): light bends towards the normal
From glass to air (more dense to less dense): light bends away from the normal
When passing along the normal (perpendicular) the light does not bend at all

42
Q

advantages of copper wires

A

-easier to join together

43
Q

advantages of optical fibres

A

-greater bandwidth
-less signal losses between repeaters
-not susceptible to EM interference
-high resistance

PHYSICS PAPER ONE (6 decks)

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