Waves Flashcards
(110 cards)
1
Q
What do waves do?
A
transfer energy without transferring matter
2
Q
What is a crest?
A
The highest point on a wave above the equilibrium, or rest, position
3
Q
What is a trough?
A
The lowest point on a wave below the equilibrium, or rest, position
4
Q
What is amplitude?
A
The maximum distance from the equilibrium to the peak or trough of a wave. It represents the energy carried by the wave, so a wave with high amplitude carries a lot of energy.
It’s measured in meters and has symbol A
5
Q
What is wavelength?
A
The distance between a point on one wave and the same point on the next wave. In a transverse wave, the wavelength can be measured from one peak to the next peak. In a longitudinal wave, the wavelength can be measured from the centre of one compression to the centre of the next. It is measured in meters and symbol is λ(lambda)
6
Q
What is period?
A
The time taken for one vibration to complete or for one wave to travel past a point. Measured in seconds and unit T
7
Q
What is frequency?
A
The number of waves passing a point in a second or number of vibrations per second. Measured in hertz (Hz) with symbol f
8
Q
Formula for frequency
A
frequency = 1/period, f = 1/T
9
Q
What is wave speed?
A
The distance travelled by a wave each second. Unit m/s and symbol v
10
Q
Formula for wave speed?
A
wave speed = frequency × wavelength, v = f λ
11
Q
What is wavefront?
A
Lines along peaks, compressions, or rarefactions of the wave are aligned
12
Q
What is the space between each wavefront?
A
wavelength
13
Q
What is wave motion?
A
Wave motion is the transfer of energy from one point to another. Wave vibrations can be shown onropes(transverse) andsprings(longitudinal).
14
Q
What is a ripple tank?
A
A ripple tank is a shallow tray of water with a light source shining down through it. This light illuminates the wavefronts making them visible. A straight dipper (wooden bar fastened using elastic bands) is vibrated and creates a series of parallel wavefronts. The screen below the ripple tank is used to observe the wavefronts of the water wave. The video camera is used to make the slow motion of the wavefronts on the screen. To make circular waves a small sphere dipper is used.
15
Q
What is a transverse wave?
A
Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer
16
Q
Examples of transverse waves
A
electromagnetic radiation, water waves and seismic S-waves (secondary)
17
Q
What is the direction of vibration compared to propogation in transverse wave?
A
the direction of vibration is at right angles to the direction of propagation (direction of wave travelled)
18
Q
What is a longitudinal wave?
A
Waves where the points along its length vibrate parallel to the direction of energy transfer
19
Q
What is a compression?
A
Where wave particles are close together and high pressure or high density
20
Q
What is a rarefaction?
A
Where wave particles are far apart and low pressure or low density
21
Q
Examples of longitudinal waves
A
sound waves, seismic P-waves
22
Q
What is the direction of vibration compared to propogation in longitudinal wave?
A
The direction of vibration is parallel to the direction of propagation (direction of wave travelled)
23
Q
What is the reflection of a wave?
A
A wave hits a boundary between two media and does not pass through, but instead stays in the original medium.
24
Q
What changes and what remains constant in reflection?
A
The direction of the wave propagation changes but the speed, wavelength and frequency remain constant
25
How can reflection be shown in a ripple tank?
by placing a plane (straight) surface, such as a wall or mirror in the ripple tank. If the bar is placed at an angle to the wavefront the reflected waves can be seen to obey the law of reflection.
26
How do you draw reflection?
1. Draw direction of wave fronts (90-degree angle to wave)
2. Draw a normal that is a perpendicular angle to the obstacle
3. Measure the angle between wavefronts and normal, this is the incidence angle
4. Make another angle of that size on the other side of the normal and draw a line, this is the reflective direction
5. Draw wavefronts with the same wavelength at a 90-degree angle from the reflective line
27
What is refraction?
Change in speed of a wave as it moves between mediums
28
What happens when the wave goes from deep water to shallow water?
From deep water to shallow water, the wave will slow down because shallow water has more friction. If the waves slow down, the waves will bunch together, causing the wavelength to decrease. The waves will also start to turn slightly towards the normal
29
What happens when the wave goes from shallow water to deep water?
From shallow water to deep water, the wave will speed up because there is less friction in deep water. If the waves speed up then they will spread out, causing the wavelength to increase. The waves will also turn slightly away from the normal
30
How can you observe refraction in a ripple tank?
This boundary behavior of water waves can be observed in a ripple tank if the tank is partitioned into a deep and a shallow section. If a pane of glass is placed in the bottom of the tank, one part of the tank will be deep and the other part of the tank will be shallow. Waves traveling from the deep end to the shallow end can be seen to refract (i.e., bend), decrease wavelength (the wavefronts get closer together), and slow down (they take a longer time to travel the same distance). When traveling from deep water to shallow water, the waves are seen to bend in such a manner that they seem to be traveling more perpendicular to the surface. If traveling from shallow water to deep water, the waves bend in the opposite direction.
31
How to draw refraction?
1. Draw a normal perpendicular to the boundary
2. Draw a line that represents the direction of the wave in the different medium depending on the wave speed. If the wave is slowing down, it bends towards the normal and if the wave is speeding up it bends away from the normal.
3. Draw construction lines at the end of the wavefronts before and draw another construction line parallel to the direction of the new wavefronts
4. Draw new wavefronts making sure they are 90 degrees to the direction line and depending on if the wave’s speed the wavelengths will need to adjust. If the wave is speeding up, the wavelength increases and if the wave slows down the wavelength decreases
32
What is diffraction?
Spreading out of waves when it passes through a gap or an edge
33
What remains constant after diffraction?
Speed, wavelength and frequency remain constant after diffraction
34
How does diffraction occur on an edge?
When a wave goes past the edge of a barrier, the waves can curve around the edge. Larger wavelengths diffract more i.e. bend more around the edge of an obstacle compared to shorter wavelengths.
35
How can diffraction be shown in a ripple tank?
by placing small barriers and obstacles in the tank. As the water waves encounter two obstacles with a gap between them, the waves can be seen to spread out. It can also occur with one barrier
36
What are the factors that can affect diffraction?
1. Gap size
The smaller the gap, the greater the diffraction (wave curving). If the wavelength is larger than gap size then the wave curves more and as the gap gets bigger, the effect gradually gets less pronounced until, in the case that the gap is very much larger than the wavelength, the waves no longer spread out at all.
2. Wavelength
As long as the gap is constant, the longer the wavelength the greater the diffraction and vice versa
37
What are light waves?
Light waves are the transverse waves and electromagnetic waves, so light can transfer energy through the vacuum
38
What is the speed of light in a vacuum?
Speed of light in the vacuum is 3 x 10^8 m/s which is approximately equal to the speed of light in the air
39
Laws of reflection
1. The angle of incidence is equal to the angle of reflection
2. The incident ray, reflected ray and the normal all lie on the same plane
40
Name all the parts of the ray diagram of reflection
Incident ray - the ray of light falling on the reflecting surface is called the incident ray.
Reflected ray - the light ray bouncing back in the same medium as incident ray, after striking the reflecting surface is called as reflected ray.
Angle of incidence - the angle between the incident ray and the normal is the angle of incidence.
Angle of reflection - the angle between the normal and the reflected ray is called angle of reflection.
Normal - it is an imaginary line, perpendicular to the reflecting surface at the point of incidence.
41
How to draw a plane mirror image?
1. Draw 2 rays of light from a point at different angles to the mirror
2. Then draw the reflected ray such as the angle of reflection of each ray is equal to the angle of incidence. Do this by drawing a normal perpendicular to the plane mirror and measuring the angle between the incident ray and mirror. Then draw another angle of the same size on the other side of the normal.
3. Trace the two reflected rays backwards behind the mirror until they meet. The point where they meet is the top of the image of the object
42
What will the image be in a plane mirror?
- The same size as the object
- The same distance behind the mirror as the object is in front of it
- Virtual (formed by the divergence of rays from the image, and cannot be projected onto a piece of paper because the rays don’t actually go through the image)
- Laterally inverted (left hand infront of mirror is right hand)
43
What is the refraction of light?
Refraction is the bending of light when it moves from one medium to another. This is due to the change in speed of the light as it moves from one medium to another
44
Name the parts of a refraction ray diagram
Incident ray- The light arriving to the boundary of the two mediums
Refracted ray- Light ray leaving the boundary
Normal- Perpendicular line on the boundary where the ray strikes
Angle of incidence- Angle between the incident ray and the normal
Angle of refraction- Angle between the refracted ray and the normal
45
What are the changes in direction happening during refraction?
- From less dense to more dense (e.g air to glass), light bends towards the normal
- From more dense to less dense (e.g. glass to air), light bends away from the normal
- When passing along the normal (perpendicular) the light does not bend at all
46
What is refractive index?
- The ratio of the speeds of a wave in two different regions
- Refractive index (n) = speed of light in vacuum or air (c)/speed of light in medium (V).
- The refractive index is a number that is always larger than 1 and is different for different materials (more dense objects have higher refractive index)
47
What is snell's law
- Relationship between angles of incidence and angles of refraction
- n= sin i/sin r
n = the refractive index of the material
i = angle of incidence of the light (°)
r = angle of refraction of the light (°)
48
What is a critical angle?
The angle of incidence when the light ray is travelling from a more dense medium to a less dense medium and the angle of refraction is 90 degrees. Symbol: θc
49
What is total internal reflection? What are the conditions?
Reflection in a more dense material where there is no refracted ray
OR All light in a more dense material is reflected
Conditions for TIR:
- The angle of incidence > the critical angle
- The incident material is denser than the second material
50
What is the formula for refractive index
n = 1/ sin c or n= sin i/sin r
51
How do optical fibres use internal reflection?
- Optical fibres are cables made from high-quality glass or plastic. When light enters one end of an optical fibre cable, it undergoes total internal reflection until it reaches the end of the cable. Digital signals are emitted as pulses of light (either Visible Light or Infrared Light) and reflected along the cable until they reach their destination. This method allows the transmission of information at high speeds. When Infrared is used, the cables are lined with glass. When Visible Light is used, the cables are lined with plastic. The Visible Light and plastic combo is cheaper but lower quality – the different colours in visible light will have slightly different critical angles, so the signal becomes more distorted the longer it travels. Also, at a molecular level, plastic will have more irregularities than glass, which affects the direction that the rays will be reflected which is why the glass and IR combo is always used when the information has to be transmitted over long distances.
- They are used for telecommunications, medicine (endoscopes) and decorative lamps
52
What are some applications for internal reflection?
Prisms are used in a variety of optical instruments, including:
- Periscopes:
A periscope is a device that can be used to see over tall objects. It consists of two right-angled prisms and the light totally internally reflects in both prisms
- Binoculars
- Telescopes
- Cameras
53
What is the principal axis?
A horizontal line which passes through the centre of a lens
54
What is the focal point?
The point at which the parallel rays of light intersect the principal axis and converge
55
What is the focal distance?
Distance between the centre of the lens and focal point
56
What is a real image?
Formed when light rays from an object converge and meet each other and can be projected on a screen
57
What is a virtual image?
Formed when diverging rays are extrapolated backwards and does not form a visible projection on a screen
58
What is a converging lens?
A converging lens causes the light rays that are travelling parallel to its principal axis to refract and cross the principal axis at a fixed point called the focal point.
59
What happens in a diverging lens?
In a diverging lens, parallel rays of light are made to diverge (spread out) from a point. The focal point is now the point from which the rays appear to diverge from
60
How to draw a real image using a converging lens
1. Pick a point at the top of the object and draw three incident rays travelling towards the lens
2. Draw one ray so that it passes exactly through the focal point on the way to the lens
3. Draw the second ray so it is parallel to the exact centre of the lens
4. Draw the third ray so it travels to the exact centre of the lens
Note: A real image is only formed if the object is at a distance greater than F as the position of object from the lens affects image size. If the object is placed closer to the lens than the focal length then a virtual image will be formed and the converging lens ray diagram will be drawn in the following way:
61
What will the image look like if the object is placed beyond 2F?
- real
- diminished
- inverted
- between F and 2F
62
What will the image look like if the object is placed at 2F?
- real
- same size as object
- inverted
- at exactly 2F
63
What will the image look like if the object is placed between F and 2F?
- real
- enlarged
- inverted
- beyond 2F
64
What will the image look like if the object is placed at F?
- The image can't be focused as the rays are parallel and don't meet
65
What will the image look like if the object is placed between F and the lens?
- virtual
- enlarged
- upright
- image is formed on the same side as the object
66
How to draw a ray diagram for a diverging lens?
1. Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel in a straight line
2. Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly upwards away from the axis
3. Draw a dashed line continuing this ray downwards to the focal point, *f*
4. The image is the line drawn from the axis to the point where the above two rays meet
- In this case, the image is:
- Virtual: the light rays appear to meet when produced backwards
- Diminished: the image is smaller than the object
- Upright: the image is formed on the same side of the principal axis
67
What is the use of a single lens?
- When viewed from the right-hand side of the lens, the emerging rays appear to come from a point on the left. This point can be found by extending the rays backwards (creating virtual rays) and a virtual image will be seen at the point where these virtual rays cross
- This image is virtual, enlarged and upright which allows it to be used as a magnifying glass which is why when using a magnifying glass, the lens should always be held close to the object
68
How are lens used to correct short-sightedness?
- People who are short-sighted have eyes that are 'too large'. This means they cannot see things that are far away, and only see things that are close to them
- This is because the eye refracts the light and brings it to a focus before it reaches the retina. In other words, the focus point is in front of the retina at the back of the eye
- This can be corrected by using a concave or a diverging lens
69
How are lens used to correct long-sightedness?
- People who are long-sighted have eyes that are 'too small'. This means they cannot clearly see things that are close, and can only clearly see things that are far away
- This is because the eye refracts the light rays and they are brought to a focus beyond the retina. In other words, the focus point is behind the retina at the back of the eye
- This can be corrected by using a convex or converging lens
70
What is the dispersion of light?
Dispersion of light is the spreading out of white light into the colours of the spectrum. This is done using a prism and is done by refraction
71
What happens when light enters the prism?
When light enters a different medium, the change in wavelength and speed depends slightly on the frequency of the wave. The higher the frequency, the higher the change in speed. We call this effect dispersion. White light is made up of the seven colours of the rainbow – red, orange, yellow, green, blue, indigo, violet – listed in order of increasing frequency. When white light is passed through a prism, it refracts and disperses into those seven colours. The colour with the highest frequency, violet, refracts the most and the colour with the lowest frequency, red, refracts the least.
72
What are the colours in order of longest to shortest wavelength
Red
Orange
Yellow
Green
Blue
Indigo
Violet
73
What are the colours in order of highest to lowest frequency?
Violet
Indigo
Blue
Green
Yellow
Orange
Red
74
What are electromagnetic waves?
transverse waves that transfer energy from the source of the waves to an absorber
75
Electromagnetic spectrum in order of longest to shortest wavelength
- Radio waves
- Microwaves
- Infrared
- Visible (red, orange, yellow, green, blue, indigo, violet)
- Ultraviolet
- X-rays
- Gamma rays
76
How fast do all electromagnetic waves travel in space?
All electromagnetic waves travel at the same high speed in a vacuum. The speed of electromagnetic waves in a vacuum is 3.0 × 10^8m/s and is approximately the same in air
77
What are the uses of radio waves?
Communications- to transmit the signal of radio and television, radio frequency identification, astronomy- radio telescopes and bluetooth
78
What are the uses of microwaves?
To transmit the signals of satellites for television, mobile phones and radar systems. Also to heat food in microwave ovens
79
What are the uses of infared?
Electric grills, short range communications such as remote controllers for televisions, intruder alarms, thermal imaging and optical fibres
80
What are the uses of visible light?
Vision, photography, videography and illumination
81
What are the uses of ultraviolet?
Security marking, detecting fake bank notes, sterilising water and tanning
82
What are the uses of x-rays?
Medical scanning and security scanners
83
What are the uses of gamma rays?
Sterilising food and medical equipment, detection of cancer and its treatment
84
What are the harmful effects of electromagnetic waves?
- As the frequency of electromagnetic (EM) waves increases, so does the energy
- Beyond the visible part of the spectrum, the energy becomes large enough to ionise atoms (remove electrons from atoms or molecules). This is harmful to cells and tissues and can even cause cancer
- As a result of this, the danger associated with EM waves increases along with the frequency
85
How to keep ourselves safe from the harmful effects of electromagnetic waves?
Protect yourself by limiting exposure, staying away from sources, wearing protective clothing
86
Which wave has no known danger?
Radio waves
87
What is the danger of microwaves?
Internal heating of body cells
88
What is the danger of infared?
Skin burns, heating effects in tissues
89
What is the danger of ultraviolet?
Damage to surface cells and eyes, leading to skin cancer and eye conditions
90
What is the danger of X-rays and gamma rays?
Mutation or damage to cells in the body
91
How does communication with satellites happen?
Communication with artificial satellites is mainly by microwaves:
- Some satellite phones use low orbit artificial satellites (Polar, or low orbit, satellites orbit around the Earth’s north and south poles)
- Some satellite phones and direct broadcast satellite television use geostationary satellites (Geostationary satellites orbit above the Earth’s equator)
92
What systems of communication rely on long wave electromagnetic radiation?
- Mobile phones (cell phones) and wireless internet use microwaves because microwaves can penetrate some walls and only require a short aerial for transmission and reception
- Bluetooth uses low energy radio waves or microwaves because they can pass through walls but the signal is weakened on doing so
- Optical fibres (visible light or infrared) are used for cable television and high-speed broadband because glass is transparent to visible light and some infrared; visible light and short wavelength infrared can carry high rates of data
93
What are the two types of signals in electromagnetic waves?
Analogue (Analogue signals vary continuously - they can take any value) or digital (A digital signal can only take one of two (discrete) states. These are usually referred to as: 1s and 0s, Highs and lows, or Ons and offs.) Sound waves can be transmitted as a digital or analogue signal
94
What are the advantages of digital signaling?
- The signal can be regenerated so there is minimal noise
- Due to accurate signal regeneration, the range of digital signals is larger than the range of analogue signals (they can cover larger distances)
- Digital signals enable an increased rate of transmission of data compared to analogue
- Extra data can be added so that the signal can be checked for errors
95
How do sound waves work?
Sound waves are longitudinal waves produced by vibrating sources. When an object vibrates, it causes movement in surrounding air molecules. These molecules bump into the molecules close to them, causing them to vibrate as well. This makes them bump into more nearby air molecules. This “chain reaction” movement, called sound waves, keeps going until the molecules run out of energy.
96
What does sound travel fastest in?
Sound travels faster in solids than in liquids and faster in liquids than in gases
97
What do sound waves require?
Sound waves require a medium to travel through. This means that if there are no molecules, such as in a vacuum, then the sound can’t travel through it
98
What is the range of frequencies audible to humans?
20 Hz to 20 000 Hz
99
What is the speed of sound in air?
330–350 m/s
100
What is a compression in a sound wave?
A compression is a region of higher density i.e. a place where the molecules are bunched together
101
What is a rarefaction in a sound wave?
A rarefaction is a region of lower density i.e. a place where the molecules are spread out
102
How can you find the speed of sound in air?
speed of sound = distance travelled/time taken
103
How do you measure the speed of sound between two points?
1. Two people stand a distance of around 100 m apart
2. The distance between them is measured using a trundle wheel
3. One person has two wooden blocks, which they bang together above their head
4. The second person has a stopwatch which they start when they see the first person banging the blocks together and stops when they hear the sound
5. This is then repeated several times and an average value is taken for the time
6. The speed of sound can then be calculated using the equation: speed of sound = distance travelled/time taken
104
How do you measure the speed of sound using echoes?
1. A person stands about 50 m away from a wall (or cliff)
2. The person claps two wooden blocks together and listens for the echo
3. A second person has a stopwatch and starts timing when they hear one of the claps and stops timing when they hear the echo
4. The process is then repeated 20 times and an average time calculated
5. The distance travelled by the sound between each clap and echo will be (2 × 50) m
6. The speed of sound can be calculated from this distance and the time using the equation: speed of sound = 2 x distance to the wall/ time taken
105
What is the frequency of a sound wave related to?
- The frequency of a sound wave is related to its pitch
- Sounds with a high pitch have a high frequency (or short wavelength)
- Sounds with a low pitch have a low frequency (or long wavelength)
106
What is the amplitude of a sound wave related to?
- The amplitude of a sound wave is related to its volume
- Sounds with a large amplitude have a high volume
- Sounds with a small amplitude have a low volume
107
What is an echo?
The reflection of a sound wave
108
What can echo sounding be used to do?
- Echo sounding can be used to measure depth or to detect objects underwater
- A sound wave can be transmitted from the surface of the water
- The sound wave is reflected off the bottom of the ocean
- The time it takes for the sound wave to return is used to calculate the depth of the water
- The distance the wave travels is twice the depth of the ocean
- This is the distance to the ocean floor plus the distance for the wave to return
109
What is ultrasound?
Ultrasound is a sound with a frequency higher than 20 kHz
110
What are the uses of ultrasound?
- When ultrasound reaches a boundary between two media, some of the waves are partially reflected. The remainder of the waves continue through the material and are transmitted
- The time taken for the reflections to reach a detector can be used to determine how far away a boundary is. This is because ultrasound travels at different speeds through different media. We can use the speed, distance, time equation to do that.
- In medicine, ultrasound can be used:
- To construct images of a foetus in the womb
- To generate 2D images of organs and other internal structures (as long as they are not surrounded by bone)
- As a medical treatment such as removing kidney stones
- In industry, ultrasound can be used to:
- Check for cracks inside metal objects
- Generate images beneath surfaces
