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Physics AS > Chapter 11- Waves > Flashcards

Chapter 11- Waves Flashcards

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1
Q

What is a progressive wave?

A

An oscillation which transfers energy through a medium without transferring matter.

The energy is passed through vibrations of a particle in a medium or fields

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2
Q

What do particles do when they vibrate? Energy transfers

A

They move from equilibrium to positive(peak) and negative amplitude(trough). The particle has 0 kinetic energy at peaks and troughs because it is converted to potential energy

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3
Q

What is a transverse wave?

A

A wave where the direction of transfer of energy is perpendicular to the direction of oscillations. Peaks and troughs are maximum displacement from equilibrium

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4
Q

What’s are the 2 types of waves and what is the difference?

A

Mechanical: particles physically move(requires a medium)

EM: are changes in the electric and magnetic field created by the motion of a charged particle

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5
Q

What is a longitudinal wave? How do they travel?

A

A wave where the direction of oscillation is parallel to the the direction of energy transfer.

When travelling through a medium they create compressions and rarefactions. Particles displace and collide with neighbours which transfers energy and also provides a restoring force

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6
Q

What is restoring force?

A

Force felt by a particle to return back to equilibrium

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7
Q

Why does the speed of mechanical waves differ with density?

A

The particles are closer together in a denser medium
So particles don’t need to be displaced as much before interacting with other particles
So energy is transferred faster

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8
Q

Define displacement

A

The distance and direction from the equilibrium position of a particle. It can be positive or negative

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9
Q

Define amplitude

A

Maximum displacement from equilibrium

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10
Q

Define wavelength

A

The distance between 2 adjacent particles/points in a wave vibrating in phase with the same velocity

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11
Q

What is time period? What is units

A

Seconds

The time taken for a particle/point to perform one complete oscillation of its motion

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12
Q

What is frequency and unit?

A

Hz

Number of complete oscillations performed by a particle/point per second

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13
Q

What is wave speed?

A

The speed at which a wave travels through a medium per unit time

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14
Q

How does speed change when there is or isn’t a medium change?

A

In the same medium, the wave travels at a constant velocity so a higher frequency means shorter wavelength

Speed changes when a wave crosses the boundary between 2 media which causes wavelength to change, NOT FREQ

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15
Q

What is the wave equation and the other one?

A

C=f * (lamda) Velocity= freq x wavelength

Freq= 1/time period

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16
Q

What is 1 complete wave cycle?

A

360* = 2pi

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17
Q

What is phase of a particle?

A

The fraction of a complete oscillation that a particle is at at a given point in time

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18
Q

What is phase difference?

A

The difference in phase between 2 points along the same wave or 2 different waves with the same frequency at a given point in time

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19
Q

How is phase calculated?

A

Fraction of completed wave x 2pi(360*)

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20
Q

How is phase difference and distance calculated?

A

Distance between 2 points as fraction of wave/wavelength x 2pi (or wavelength)

Phase difference/ 2pi X wavelength = distance

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21
Q

What is anti phase?

A

When there is a difference in phase of an odd integer of pi radians

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22
Q

How is phase difference and frequency linked?

A

When 2 points with the same frequency meet at a point in time, phase difference is CONSTANT

When they have different frequencies, phase difference is always changing

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23
Q

What are the 4 types of wave phenomena?

A

Reflection, refraction, diffraction, superposition

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24
Q

What is optical density? How is it linked to em waves

A

A measure of the absorbance of a material. EM a waves travel slower through higher optical density while mechanical waves travel faster in higher physical density

25
Q

What is a wave train?

A

An infinite number of waves moving together as a unit

26
Q

What is a wave front?

A

A line of constant phase of a wave trai ( a line that can link the peak of each wave)

Wave direction is 90* to wave fronts

Changing wavefronts(or wavelengths) means a wave phenomena is occurring.

27
Q

Define reflection and it’s law

A

When a wave reverses direction upon meeting the boundary of 2 different media

Angle of incidence=angle of reflection (to the normal)

Wavelength and media DO NOT change

28
Q

Define refraction

A

Refraction is the change in a waves direction as it crosses the boundary between 2 media.

Speed changes which means wavelength changes. FREQ IS CONSTANT

29
Q

Explain how a wave refracts

A

If the incident ray approaches the boundary at an angle from the normal, it’s wavefronts change speed which causes a change in direction

30
Q

What happens when a wave speeds up/slows down and why?

A

When mechanical waves enter a denser medium they bend AWAY from the normal. Wavelength gets LONGER. This is because part of the wavefront meet the boundary and speed up first so the whole wave train bends away

When mechanical waves enter a less dense medium, they bend towards the normal and SLOW DOWN.

31
Q

Why do waves refract at the boundary of shallow and deep water?

A

Deep water is more dense than shallow water. Because there is a change in density hence waves will speed up in higher physical and optical density.

32
Q

Define diffraction

A

The physical phenomena of waves spreading out when passing through a gap or around an obstacle.

There is no change in media

33
Q

Relationship between gap size and wavelength

A

The smaller the gap size relative to the wavelength, the greater the diffraction
Gap size has to be a similar order of magnitude as the wavelength for significant diffraction to occur

34
Q

Why does sound refract through a door but not light?

A

The wavelength of sound is the same order of magnitude as the door so sound waves refract. Light has a smaller order of magnitude

35
Q

What is polarisation?

A

The process of confining the oscillations of a wave to a specific plane

Light from a laser is polarised but light from a lamp isn’t as oscillation occur on many planes

36
Q

Why can’t longitudinal waves be polarised by transverse can?

A

Longitudinal waves have oscillations parallel to the direction of energy transfer so the oscillations are already confined to one plane
Transverse waves can be polarised because the plane of oscillation are perpendicular to the direction of energy transfer so they can be confined to 1 plane

37
Q

Why does the amount of light vary when 2 polarising filters are rotated?

A

When the transmission axis of 2 filters are aligned, the same amount of light passes through both. As you rotate the filter, the amount of light that passes through varies.
The larger the vertical component, the more vertically polarised light passes through.
2 perpendicular filters don’t let any light through
If the 2nd filter is 45 degrees rotated from the first one, half the light will go through (intensity halved). Because the vertical component of the second filter decreases

38
Q

What happens to light reflected off a puddle?

A

It gets partially polarised on the horizontal plane because oscillations can only happen on that plane

39
Q

How does a Polaroid filter work?

A

They only transmit light with e field oscillations and components on a specific plane which decreases the wave intensity.
Used in sunglasses to only let light with the electrostatic field vector oscillating in the vertical plane to pass

40
Q

What happens to light and mechanical waves the further it travels from a source?

A

It spreads out more so the energy(power) transferred becomes less concentrated so it is dimmer.
Mechanical waves decrease in amplitude

41
Q

Explain how a filament lamp works

A

Heat from the lamp is infrared, bulb has argon gas

Electrons oscillating at different rates and directions form all colours to form white light.

42
Q

Define intensity and power

A

Intensity is the radiant power passing through a surface per unit area

Power is energy transferred per unit time

43
Q

What is the formula for intensity

A

Intensity(w/m2) = power(w)/area(m2)

I = P/ 4pi r^2

From a point source power is spread in 3D so surface area of sphere

44
Q

What is the relationship with intensity and distance?

A

Intensity has an inverse square relationship to distance from source
If distance doubles, intensity decreases by a factor of d2

I= 1/r^2

45
Q

What is the relationship between energy intensity and amplitude?

A

Intensity is proportional to amplitude squared

Reason: the further waves travel, amplitude decreases because energy spreads up which means reduced speed. E=1/2 mv^2 so energy is quartered

Double amplitude= quadruple intensity

46
Q

How are EM waves produced?

A

They are produced by the movement of a charged particle which causes the magnetic field vector to oscillate in the Z plane and electrostatic field vector to oscillate in the y plane

47
Q

Why is sunlight unpolarised?

A

Because the electrostatic field vectors oscillate in all planes because charge particles accelerate in all directions

48
Q

What is an em wave?

A

A transverse wave consisting of oscillating electric and magnetic fields which are in PHASE and perpendicular to each other and to the direction of travel

They don not need a medium to travel in and travel in a vacuum at 3x10^8 m/s

Velocity varies with freq

They can be polarised and obey v=f x lamda

49
Q

What is the em spectrum and what are the order of magnitudes of the categories?

A

The EM spectrum is the continuum of all em waves

(Wavelength in metres)

Radio: 10^6  to 10
Micro: 10 to 10^-3
Infrared: 10^-3 to 7x10^-7
Visible: 7x10^-7 to 4x10^-7
UV: 4x10^-7 to 10^-8
X Rays: 10^-8 to 10^-10 to 10^-13
Gamma: 10^-10 to 10^-13 to 10^-16
50
Q

What is absolute refractive index?

A

N: the ratio between the speed of light in a vacuum and the speed of light in a medium

51
Q

What is the formula for refractive index?

A

C/V
Or wavelengthC/wavelengthV

FREQ IS CONSTANT NO MATTER WHAT MEDIUM

52
Q

What is the effect on the speed of light if there is a change in medium?

A

Light is fastest in vacuum and slows down in other materials because it interacts with more particles(more optically dense)

53
Q

What is the relationship between refractive index and the angle of incidence/refraction

A

N= sin i / sin r

54
Q

What is snells law?

A

N1 sin 1 = n2 sin 2

55
Q

What is total internal reflection?

A

When light completely reflects back at the boundary between 2 media when the incident angle is larger than the critical angle

56
Q

What conditions are necessary for total internal reflection?

A

The media in which the ray is incident in must have a lower refractive index. (More to less optically dense)
The incident angle must be greater than the critical angle

57
Q

Formula for critical angle

A

Sin-1(n2/n1)

Or just use snells law

58
Q

Why are diamonds so shiny?

A

They have a high refractive index so there is a small critical angle, so light is totally internally reflected many times. It also dispersed light to form a rainbow effect

59
Q

How do optical fibres work? 3 points

A

Light is incident to the fibre boundary at an angle bigger than the critical angle to light is constantly totally internally reflected. The fibres are transparent so light is absorbed less.

Core is very thin to stop multipath dispersion(where there is a difference in the path length of rays reflecting off sides and going straight through.

Physics AS (6 decks)

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