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🔱Physics > 5. Waves > Flashcards

5. Waves Flashcards

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1
Q

What are waves?

A

Waves transfer energy from one place to another
But don’t mean they transfer mass

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2
Q

What is a transverse wave?

A

A wave that’s particles oscillate perpendicular to the direction of the wave
Move up and down
The wave has:
- amplitude
- crest (top)
- trough (bottom)
- wavelength

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3
Q

What is a longitudinal wave?

A

A wave where the energy travels parallel to the oscillation of the waves particles
Moves in compressions and rarefactions
They are slower than transverse waves
They need a median to travel through
- the denser median, the faster travel
They move through diffusion
- high to low conentration
Tempuratue affect the speed of them

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4
Q

What is a compression segment in a longitudinal waves?

A

An area of high concentration

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5
Q

What is a rarefaction segment in a longitudinal waves?

A

An area of low concentration

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6
Q

What types of waves are longitudinal?

A

Sound Waves
Ultrasound waves
Primary seismic waves (P-waves)
- waves generated by an earthquake
- these are fast and arrive first

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7
Q

What is the displacement of a wave?

A

The position of a particular point on a wave at a particular instant in time
Measured from the mean/ equilibrium position (x-axis)
measured in (m)

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8
Q

Where are the compressions and rarefactions of longitudinal waves on a transverse form wave?

A

Longitudinal waves can still be drawn as transverse waves

The rarefactions have a low pressure
- So there is rarefactions at the mean/ equilibrium position (x-axis)

Compressions have high pressure
- So they exist at the crests and troughs

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9
Q

What types of waves are transverse?

A

EM waves
Waves on a rope or slinky
Vibrations on a guitar string
Secondary seismic waves (S waves)

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10
Q

What’s the wave equation?

A

This equation tells us how fast a wave moves
The wave will move at the number of wave cycles per second x the wavelength, so
v=f λ

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11
Q

What is amplitude?

A

The magnitude of the maximum displacement reached by an oscillation in the wave

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12
Q

What is frequency, f?

A

The number of wave cycles per second
So 1/time of one complete oscillation (T)
f=1/T

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13
Q

What is a period, T?

A

The time taken for one complete oscillation at one point of a wave
Measured as T

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14
Q

What is the adjacent maxima?

A

The distance between the maxima and minima of a wave
same as λ/2
As ‘adjacent’ connects stuff together

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15
Q

What is interference of waves?

A

When 2 or more waves combine, they produce a resultant wave with a new amplitude

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16
Q

What is superposition?

A

When the waves are positioned on top of each other
- the reason why waves interfere and combine
Waves superposing = waves interfering

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17
Q

What is constructive interference?

A

When waves superpose to have a larger amplitude than any of the individual waves

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18
Q

What is destructive interference?

A

When the resultant wave has a smaller amplitude than the individual waves

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19
Q

What does it mean for waves to be in coherence?

A

Waves must have a constant phase difference
Must have the same frequency

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20
Q

What is a monochromatic source

A

a source of light that emits only one colour of light

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21
Q

What are waves in anti-phase?

A

When waves are 180 / π out of phase
so half way out of phase

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22
Q

What are waves in quadrature?

A

When waves are 90 / π/2 out of phase
so a quater out of phase

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23
Q

What is path difference?

A

The difference in distance travelled by 2 waves from their source to where they meet
If their path difference is a multiple of λ, then they interfere constructively
If their path difference is +λ/2 out, then they are in anti phase, so act destructively

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24
Q

What are stationary/ standing waves?

A

Waves produced by superposition of 2 waves with the same frequency and amplitude, travelling in opposite directions
Creates a wave where the peaks and troughs don’t move along the string
Has a fixed end if being reflected
Makes nodes and anti-nodes

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25
Q

What happens with energy in a stationary wave?

A

The energy is stored in the wave, as no energy is transferred anywhere

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26
Q

What is a node?

A

On a stationary wave, a node is where there is no vibration and no movement

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27
Q

What is an anti-node?

A

On a stationary wave, an anti-node is where there is vibration and max amplitude

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28
Q

How do nodes and anti-nodes move?

A

They do not move along the string
The only movement is from the by anti-nodes, up and down
The waves are in anti phase at the nodes and anti-nodes

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29
Q

What is a fundamental wave?

A

The first standing wave we can produce
The smallest wave in terms of λ we can make
half a wavelength
Called the first harmonic

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30
Q

What is a harmonic?

A

A harmonic is the multiple of a fundamental wave
E.g a full wave is the 2nd harmonic

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31
Q

What is the intensity, I of a wave?

A

The amount of energy passing through a wave per second and unit area is the intensity of the wave
As P=E/t
I=P/A
Where A=area(m^2)
I is in (Wm^-2)

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32
Q

How does reflection work?

A

Reflection occurs when a wave hits a boundary between 2 media and doesn’t go through
As the 2 different media are too different to each other
The surface also has to be very flat, this is why we don’t see refection from a wall

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33
Q

What is the angle proportionality during reflection?

A

Angle of incidence = Angle of reflection
θ(i)=θ(r)

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34
Q

What is the normal?

A

The normal is the line perpendicular to the medium change

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35
Q

What’s the angle of incidence?

A

The angle from the incident ray (the ray going towards new media) to the normal

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36
Q

Where are all the angles measured from

A

All angles used are measured from the normal to their ray
- θ incidence
- θ reflection
- θ refraction

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37
Q

What surfaces reflect light best?

A

Flat surfaces are the best
- they are the smoothest
Opaque surfaces are good
- the light is absorbed by electrons and will re-emit as a reflected wave
Rough surfaces are the worst
- won’t reflect as light is scattered in all different directions

38
Q

What is refraction?

A

When light hits a similar, but different median, light is refracted as it slows down or speeds up
Slows down when:
- faster median to slower median
- angle decreases
- θ(i) > θ(r)
Speeds up when:
- slower median to faster median
- angle increases
- θ(i) < θ(r)

39
Q

What determines the speed light goes through the median?

A

The density of the median
The more dense, to slower light will travel

40
Q

What is the refractive index, n?

A

The refractive index, n measures how much light slows down when passing through the median compared to speed of light in a vacuum, c
n=c/v
where v=speed of light in median

41
Q

What does the number of the refractive index, n tell us?

A

The smaller n, the closer v is to the speed of light
so smaller n, the faster v
The higher n, the slower v
n will always be <1
- as nothing is faster than c

42
Q

What is the speed of light, c?

A

c=3x10^8ms^-1

43
Q

What is Snell’s Law?

A

Snell’s Law relates the angle of incidence to the angle of refraction by using their refractive index’s
n(1)sin θ(1)=n(2)sinθ(2)
where θ(1) = θ(i)
where θ(2) = θ(r)

44
Q

What’s the critical angle?

A

The critical angle is when light goes straight through the median wall
So the critical angle will be the angle of incidence
And the angle of refraction =90 deg
θ(2) = 90
This means sinθ(2)=1
and n(2)=1 as it’s air
so…
nsin(C)=1x1
sin(C)=1/n

45
Q

What is Total Internal Reflection (TIR)

A

TIR is when the ray totally reflects
so Aal of the light is reflected in TIR
As it’s reflection:
- θ(i)=θ(r)

TIR happens on 2 conditions:
- θ(i) > θ(C)
- going from a more dense median to a less dense one, so n(1)>n(2)

46
Q

What’s a converging (convex) lens?

A

A converging lens brings light onto one point (principal focus)
‘converges’ the light rays
The more curved the lens, the smaller the focal length
Creates a real image
Image is created inverted

47
Q

What is a diverging (concave) lens?

A

The rays diverge away
Unlike converging lenses, a virtual is created as the image isn’t actually there, it just a appears like it is
- in reality, it’s further away
Image is created upright

48
Q

What is the principal focus, F?

A

The point where all the rays go together
In a converging lens, it’s on the right
In a diverging lens, it’s on the left, but dotted lines are drawn to it

49
Q

What is the focal length, f?

A

The length between the middle of the lens and the principal focus

50
Q

How do you draw a converging lens diagram?

A

Draw a line down the middle of the converging lens
Draw a line from the object to the centre of the lens on the principle axis and a line horizontal to the lens middle
Then carry on the line from the floor all the way, and cannot the other one to the principal focus, F and follow it down to connect the lines
This will create the image

51
Q

How do you draw a diverging lens diagram?

A

Draw a horizontal and line to centre of lens
Then draw a dotted line from the horizontal line, at the centre of lens to the principle focus, which is on the same side
Where the 2 lines meet is the top of the image

52
Q

What’s the principle axis

A

The line that doesn’t get deviated (converged or diverged)
The middle of the lens horizontally

53
Q

What is the power of a lens?

A

The more powerful a lens, the more the rays will curve
The more curved the lens the more power
The more power means that the image will appear closer
so the more powerful the lens, the shorter the focal length
Power (dioptures, D)=1/f(m)
P=1/fW
The shorter the length, the more powerful the lens

54
Q

What is the power in a diverging lens?

A

In a diverging lens, the focal length is negative, at it’s on the other side
So. the power is also negative

55
Q

How do you find the total power of a lens?

A

You add all the powers together
p=p(1)+p(2)+p(3)…

56
Q

When do images become enlarged with a converging lens?

A

When the object is closer than 2f, the object will enlarge
When it’s exactly f, or less, the image is too big and is infinite

57
Q

When do images stay the same size with a converging lens?

A

When the focal length =2f

58
Q

When do images become smaller with a converging lens?

A

When the object is further than 2f, the image will be smaller

59
Q

What’s the lens equation?

A

We can find the actual position of an image through ray diagrams, or the lens equation
1/f=1/u+1/v
when f= focus length
when u= object distance
when v= image distance
This equation only works for thin converging or diverging lenses
v and f are negative in diverging lenses

60
Q

What 2 medians are used in our core practicals?

A

Air
Perspex block

61
Q

How do you find the magnification of a lens?

A

M=image size/object size
M=h(i)/h(o)
or M=v/u

62
Q

What is polarisation?

A

Occurs when particles are only allowed to oscillate in one of the directions perpendicular to the direction of wave propagation
Waves only pass in one wave angle

63
Q

What do EM waves consist of?

A

EM waves are made out 2 different types of waves:
- electric field
- magnetic field
They are perpendicular to each other
They have no plane
- they move in all directions everywhere, when unpolarised

64
Q

What is diffraction?

A

The spending out of waves when they pass obstruction
Huygen’s principle proves this

65
Q

What does diffraction depend on?

A

The amount of diffraction depends on the ratio between the wavelength, λ and the gap, b
The closer λ is to b, the more diffraction
sinθ=λ/b

66
Q

What is Huygen’s principle?

A

Every point on a wavefront is a new secondary source of circular waves
So the waves superpose to make a new wavefront

67
Q

What’s true diffraction?

A

When λ=b

68
Q

What is the pattern diffraction makes?

A

Waves superpose in a pattern after passing through the slits.
They hit the screen, superposing constructively and destructively
This creates a pattern, with the biggest constuctive interference in the centre (central maximum) then gradually goes out and gets smaller and fades
called first order, then second etc…

69
Q

What’s the diffraction grating equation?

A

dsinθ=nλ
d= distance between the slits (like b)
n= number of order
This is because the closer the wavelength to the size of gaps, d the more diffraction

70
Q

Do the number of slits matter?

A

The more slits, the sharper and more defined the lines are
But they all make the same pattern

71
Q

How does the colour of the light effect diffraction?

A

Red light has a longer λ than violet
- so the λ is closer to the slit size, so red diffracts more
- also can superpose for a longer distance
Makes red’s central maximum and orders longer

72
Q

How do electrons behave?

A

They behave like light and have wave like properties
Electrons can also diffract

73
Q

What’s the electron gun experiment?

A

We can diffract the electrons using this experiment
- There is a 6V charge going through the wire
- The wire heats up and emits electrons
- They just hover around, but are instead accelerated towards the anode (+ve) at 6000V
- There is a small hole in the anode, made of graphite, so the electrons go through that and they diffract
- Through the crystalline structure - acting as slits
- The electrons are going too fast to stop, so they hit the fluorescent screen, making a diffraction pattern

74
Q

What’s Plank’s constant, h?

A

h is a constant relating energy of a photon to it’s frequency
E=hf
h = 6.63x10^-34 Js
So where E = the energy of 1 photon
And f = the frequency of the wave

75
Q

What’s a photon?

A

A photon is a massless ‘quantum’ (packet) of EM energy
Made up of all forms of EM radiation
Shows that energy is transferred through small packets as photons

76
Q

What’s de Broglie’s equation?

A

de Broglie discovered very small, fast moving particles, like electrons can behave as waves
- called matter waves
He relates momentum, p to their wavelength
λ =h/p
As p=mv
λ =h/mv

77
Q

What’s the photoelectric effect?

A

The photoelectric effect is the phenomena that electrons are emitted from a metal surface from absorption of EM radiation
Electrons removes are called photoelectrons
This proves that light is quantised
- carried in discrete packets

78
Q

What’s the work function, Φ

A

Φ is the minimum energy required to remove electrons from a surface
E=Φ+Ek
Where Ek = kinetic energy, 1/2 x mv^2
hf=Φ + 1/2 x mv^2
So this shows that there is energy used to remove the electron, then the rest is used for its movement

79
Q

What is f, in hf=Φ +1/2 x mv^2?

A

As Φ is the minimum energy required, f is the threshold frequency (minimum frequency required)
E(Φ)=h x threshold frequency

80
Q

How does intensity of the light effect the photoelectric effect?

A

In order for photoelectrons to be emitted, Φ must be met
if not, then the intensity doesn’t matter

But if Φ is met, then there are more electrons emitted with a higher intensity

81
Q

What’s the gold leaf electroscope experiment?

A

A zinc plate is attached to a gold leaf
- the gold leaf initially is negatively charged, same as the zinc plate
- then uv light is used to emit photoelectrons
- this then causes the repel of the leaf to stop, as the electrons are gone, so the leaf drops back down

82
Q

How do electrons sit in atoms?

A

Electrons will all have a set energy level
The higher the energy level, the higher it is, n
They can gain energy from photons, and move up and back down
Cant go below their set level

83
Q

What’s the atomic spectra?

A

When electrons absorb or emit photons, they release different EM waves
We can tell which atom is what through what they emit
Each frequency is seen as a separate/ discrete line of a different colour

84
Q

What’s the λ of gamma rays?

A

≈1x10^-13

85
Q

What’s the λ of x-rays rays?

A

≈1x10^-11

86
Q

What’s the λ of uv light?

A

≈1x10^-8

87
Q

What’s the λ of visible light?

A

≈1x10^-7

88
Q

What’s the λ of infared light?

A

≈1x10^-5

89
Q

What’s the λ of microwaves?

A

≈1x10^-2

90
Q

What’s the λ of radio waves?

A

≈1x10 - x10^3

91
Q

How do you calculate the wave speed, v on a stretched string?

A

v=root(T/μ)
where T=tension in string (N)
and μ=mass per unit length of the string (kg m^–1)

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