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A-level Physics Aqa > Waves > Flashcards

Waves Flashcards

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1
Q

define a progressive wave

A

A progressive wave is a moving wave that carries energy from one point to another without transfering matter.

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2
Q

Define cycle

A

A cycle is one compleat oscillation of the wave.

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3
Q

Define displacement(x)

A

Displacemnt is the distance a point on the wave has moved from ints equlibrium position. It is mesured in m and is a vector quantity so has a +/-

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4
Q

Define amplitude (A)

A

Amplitude is the maximum magnitude of displacment. It is mesured in m and is a scalar quantity.

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5
Q

Define wavelenth (λ)

A

A wavwlength is the length between two ajacent points moving in phase. E.g from crest to crest or trough to trough. It is mesured in m and is a scalar quantity.

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6
Q

Define period (T)

A

A period is the time taken in seconds for a whole cycle to compleate or to pass a given point.

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7
Q

Define frequnecy(f)

A

Frequency is the number of cycles per second produced, or passing a given point. It is mesured in Hz

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8
Q

Define phase

A

Phase is a mesurement of the position of a certain point along the wave cycle.

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9
Q

Define phase differance

A

Phase differancs is the amount one wave (or one point on a wave) lags behind another.

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10
Q

What are phas and phase differance mesured in?

A

Both phase and phase differanca re mesured in degrees, radians or fractions of a cycle.

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11
Q

Give the equation for frequency and period

A

f = 1/T

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12
Q

Give the Wave speed equation

A

c=fλ

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13
Q

What is a transverse wave?
Give examples

A

The oscillations of the particles/feild are perpindicular to the direction of wave travel/energy transfer.

e.g. EM waves, Water waves, Waves on a string, S waves

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14
Q

What is a Longitudinal wave?
Give examples

A

The oscillations of the particles/feild are parellel to the direction of wave travel / energy transfer.

e.g sound, (ultra sound, infra sound)

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15
Q

Define an unpolarised wave

A

An unpolerised wave has oscillations in all planes that are perpindicular to the direction of wave travel.

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16
Q

Define a polarised wave

A

A polarised wave has oscillations in only one plane that is perpindicular to the direction of wave travel.

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17
Q

Which sort of waves can be polerised and why?

A

Only transvers waves can be polerised.
Longituduinal waves only have oscilations parallel to direction of wave travel so can’t be polerised.

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18
Q

How are waves polarised?

A

Waves can be polarised by passing them through a polariser(polarising filter). This only transmits oscillations in a certain plane(transmission axis). Oscillations in all other planes are absorbed.

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19
Q

How does polarisation affect wave intensity?

A

The intensity of the wave is reduced by 50%

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20
Q

Describe how two identical polerisers can be investigated

A

Place polariser A and B infront of a unpolerised light sorce. When they are set up whith the light transmission axes in parallel the light intensity is at a maximum. When polarisers A and B are set up with there transition axis perpindicular the light intensity is at a minimum(0).

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21
Q

How are waves polerised by a surface?

A

When waves are reflected from a reflective surface they undergo partial plane polarisation.
E.g. if the surface is horisontal the wave will oscillate morein the horisontal plan than the vertical.

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22
Q

Give 3 applications of polarisation

A

> Polarised sunglasses. They do not allow horisontally polarised light to pass through
Polarised photography. these work the same as the sunglasses
Radiowaves and microwaves. They are either vertically or horisontaly polerised therfore the arial needs to be orianted in the correct direction to the transmitter its conncting to.

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23
Q

What is the principal of superposition?

A

When two or more waves arrive at a point they intefear and the resultant displacment at that point is equal to the vector sum of displacments of each wave.

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24
Q

When can superposition take place?

A

Super position works for any type of wave, both longitudinal and transverse however for two waves to superpose they must be the same type (Em-Em or sound-sound)

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25
Q

When will observable fixed inteference pattern take place?

A

They must be COHERANT
>they have the same frequency and wavelength
>they have a fixed path differance between them

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26
Q

When does constructive inteferance occur?

A

Two coherant waves must be in phase.
The resultant amplitude is larger, equal to the sum of the two wave amplitudes.

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27
Q

When does destructive inteference occur?

A

Two coherant waves are in anti-phase
The resultant amplitude is smaller, equal to the differance between the two wave amplitudes.

28
Q

Define stationary waves

A

Stationary waves do not move. They do not transfer energy, instead they store energy.

29
Q

How are staionary waves formed?

A

A sationary wave is the superposition of two progressive waves travelling along the same line:
> with the same speed
> with the same frequency/wavelength
> with similar amplitude
> moving in opposite directions

30
Q

Give examples of stationary waves

A

Transverse waves:
> Microwaves reflect of the wall and superpose with the original wave forming a staionary wave
Longitudinal waves:
>Sound waves in a tube reflect of the wall and superpose with the original wave forming a sationary wave.

31
Q

What are the features of a stationary wave.

A

> all oscillating points have the same frquency and time period
anti-nodes (Points of maximum amplitude) constructive intefearance, waves in phase atp
nodes (Points of minimum amplitude, zero displacment) destructive inteferance, waves in antiphase atp
wavelength = 2x differance between ajacent nodes/antinodes

32
Q

What is a resonant frequency?

A

It resonant frequencies an exact number of half wavelengths fit on the string. At the lowest resonant frequency the first harmonic is produced.

33
Q

Give the 3 factors affecting resonant freqency of a string

A

> The length of string (L)
The mass per unit length of the string(μ)
The tension in the string (T)

34
Q

Define path differance

A

Path differance is defined as the differance in the diststance traveled by two waves from the sources to the point where they meet.

35
Q

Define fringe spacing

A

The distance between the center of two ajacent bright fringes (or two ajacent dark fringes)

36
Q

What is youngs double slit experiment

A

The intefreance of two light sources can be shown by passing light through a double slit. The light diffracts through the slits and intefears to produce a pattern on a screen. The light source must be coherant and monochromatic. This produces bright and dark fringes witheaven spacing that are parallel to the slits. The intensity decreases from the center.

37
Q

How can monochromatic light be made coherant?

A

The non coherant light must be passed through a single slit. This produces a point source of coherent light that can be used in Young’s double slit experiment. The distance from the single slit must be equal each double slit.

38
Q

Give the equation for fringe spacing

A

W=λD/s
All units (m)

This equation is only valid if slit to screen distance(D) is&raquo_space; than slit separation(s)

39
Q

What happens to fring spacing when using light of different wavelengths?

A

Fringe spacing (W) is directly proportianal to wavelenth (λ) so the greater teh wavlenght the greather the fringe sepparation. e.g Red light has a greater fringe spacing than violet light.

40
Q

What happens when white light is used in Young’s double slit experiment?

A

As white light is composed aof all visible light each wavelength produces its own intefearance pattern. The central fringe is bright white. The outer bright fringes are a spectrum from violet on the inside to red on the outside. The outer fringes are wider so may merge as you go way from the center and the dark fringes may no longer be visible.

41
Q

What precautions hsould be taken when using lasers?

A

> Never look directly at a laser or its refection
Don’t shine them towards a person
Wear saftety goggles
Stand behind the laser

42
Q

How can two source inteferance of sound waves be demonstrated?

A

Connect two speakers to the same signal genarator. Sound waves are longitudinal so are made of compressions and rarefractions which.Constructive intefreance occurs when they line up and the sound appears louder. Destructive intefereance occurs when compressions line up with rarefractions and vice verca. This produces a series of loud maxima and quiet minima. Young’s double slit equation can be applied.

43
Q

What is two source inteferance for microwaves?

A

This can be detected by a movable microwave detector. Either a double slit or two transmitters conected to the same signal geneator cause constructive inteferance (detector picks up maximum intensity panth differance of nλ and in phase) and destructive inteferance (detector picks up a minimum intensity path differance (n+1/2)λ and in antiphase). Young’s double slit can be applied to determine sepperaion of maxima and minima)

44
Q

Define diffraction

A

The spreading out of waves as they pass thorugh a narrow slit or go around an object.

45
Q

How does wavlength and gap size affect diffraction?

A

> When the wavelengs is much greater than the gap the diffraction is hardly notiacble
When the wavelenght is several times bigger than the gap diffraction start to become noticable.
when the wavelength is the same size as the gap diffraction is at its greatest.
When the gap is smaller than the wavelength the waves are mostly reflected/ absorbed.

46
Q

How do you observe single slit diffracton pattern?

A

Use a monochromatic coherant light source. The wavelength of light must be about the same size as the slit to get a diffracton pattern due to inteferance.

47
Q

Define monochromatic

A

One wavelenght of EM radiation

48
Q

Define coherant

A

Same frequency and wavelength in fixed phase differance.

49
Q

What are the key features for single slit diffraction.

A

> There is a central bright fringe with alternating light and dark finges.
The central bright fringe is double the width of the outer fringes.
The central brigh fringe has a much higer intensity .
The intensity of the outer fringes decreases with distance from the center.

50
Q

Single slit diffraction
How does the wavelength of light affect the amount of diffraction that occurs?

A

For a greater wavelength there is a greater diffraction for the same size slit.
e.g Red light has a greater wavelength so the diffraction is more spread out than blue light.

51
Q

What is the relationship between intensity in fringes and number of photons?

A

> the central bright fringe in a single slit diffraction pattern is the brightes.
The intensity of light is the highest in the center as the intesnsity is the power per unit area
For monochromatic light, all photons have the same energy so this must mean that there are more photons per unit area each second.

52
Q

Single slit diffraction
What factors affect the width of the central bright fringe?

A

Increasing the slit width
>The amout of diffraction decreases
>The central maximum is narrower
>The intensity is greater as the same amount of enrgy is concentrated over a smaller area.
Increasing the wavelength
>The amount of diffraction increases
>The central maximum is wider
>The intesity is lower as the same amount of energy is concentrated over a larger area.

53
Q

Define diffaction grating

A

A diffraction greating is a plate with hundreds of parallel slits per millimeter.
When monochromatic light is passed through one you get a similar pattern to Young’s double slit experiment. However the bright maxima are brighter and narrower, and the dark areas are darker and wider as there are so many beams from each of the slits reinforcing the pattern.

54
Q

Describe the diffraction grating for monochromatic light

A

When monochromatic light is incident on a diffraction grating, light difracts as it passes through each slit. each of theses waves superpose with the ajacent waves. Bright maxima are formed by constructive inteferance (in phase path differance nλ). The line of maximum brightness at the center is called th zero line with path differance = 0. The lines either side of the zero order line are called the 1st order lines with path differance = λ and so on.

55
Q

Give the equation for difraction grating

A

dsinθ=nλ

56
Q

What does the equation
sinθ=nλ/d
tell you?

A

If the wavelength of light λ is increased
>sinθ increases
>so θ increases
>so the pattern is more spread out
If the distance between the slits d is increased
>sinθ decreases
>so θ decreases
>so the pattern is less spread out

57
Q

What is the diffraction grating pattern for white light?

A

> The zero order maxima is white
All other orders are a spectrum of violet light on the inside and red on the outside

58
Q

What are the applications of diffraction gratings?

A

Identifying elements
>Astronomers and chemists often need to study soectra to hekp identify elements
>E.g. astronomers may analyse light from stars to determine their composition. They use these instead of a prism as thry are more accurate
X-Ray crystallography
>The wavelength of x-rays is simillar in scale to the spaceing in atoms in crystaline solids. This means x-rays form a diffraction pattern when directed at thin crystal
>This can be used to find the spacing inbetween atoms. This technique was used oy find the structure of DNA

59
Q

Define refraction

A

Refraction is when a wave crosses a boundry between to mediums which causes it to change direction. This only happens if they meet the boundry at an angle.

60
Q

What does F.A.S.T stand for

A

Faster-away, Slower-towards

61
Q

What is refractive index?

A

Light travels fastest in a vacum and is slowed down in every other material as particles interact. The more opticaly dense the more light slows down. Refractive index n, is a mesure of optical density.

62
Q

Give the refractive index equation

A

n=C/Cvs

63
Q

give the equation for Snell’s law

A

n1sinθ1=n2sinθ2

64
Q

What is total internal reflection?

A

When n1>n2 light rays are reflected away from the normal. Using Snell’s law as θ1 increases θ2 increases and will reach 90°. The angle of θ1 is = θc .Beyond this law of reflection now applies angle of incidence now = angle of reflection.

65
Q

Give the equation of the critical angle

A

sinθc=n1/n2

A-level Physics Aqa (9 decks)

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