P6: Waves Flashcards
1
Q
What are waves?
A
Disturbances which travel through a medium, causing particles to oscillate and transfer energy to each other. Waves transfer energy but not matter.
2
Q
What is the amplitude of a wave?
A
The maximum displacement of a point on a wave from its undisturbed position.
3
Q
What is the wavelength of a wave?
A
The distance between the same point on two adjacent waves (e.g. trough -> trough).
4
Q
What is the frequency of a wave?
A
The number of complete waves passing a point per second.
5
Q
What is frequency measured in?
A
Hertz (Hz). 1 Hz = 1 wave/second.
6
Q
What is wave speed?
A
The speed at which energy is transferred as a result of a wave (the same as the speed the wave is moving at).
7
Q
What equation can be used to find wave speed?
A
8
Q
What is the period of a wave?
A
The amount of time it takes for one full wave to pass a point.
9
Q
The formula for period is given in the exam. What is the period of a wave with speed 8 m/s and wavelength 2m?
A
- Use v = fλ to work out the wave’s frequency: 8 = f x 2 so f = 4 Hz
- Period = 1/frequency = 1/4 = 0.25 s
10
Q
What are transverse waves?
A
Waves whose oscillations are perpendicular to the direction of propogation of the wave (or the direction of energy transfer).
11
Q
What are longitudinal waves?
A
Waves whose oscillations are parallel to the direction of propogation of the wave (or the direction of energy transfer).
12
Q
Name 3 transverse waves.
A
EM waves, water ripples and S-waves.
13
Q
Are electromagnetic waves transverse or longitudinal?
A
Transverse.
14
Q
Name 2 longitudinal waves.
A
Sound waves and P-waves.
15
Q
Are sound waves transverse or longitudinal?
A
Longitudinal.
16
Q
What is the speed of sound in air?
A
≈330 m/s.
17
Q
Describe how you could use an oscilloscope to measure the speed of sound in air.
A
- Attach a signal generator to a speaker.
- Connect 2 microphones to an oscilloscope.
- Place both microphones next to the speaker, slowly moving one away until the waves on the oscilloscope’s display are alligned, but have moved one wavelength apart.
- Measure the distance between the microphones to find one wavelength.
- Use v = fλ to find the wave speed - frequency is what the signal generator was set to.
- Check your result is about 330 m/s.
18
Q
What three things can happen when a wave arrives at a boundary between two mediums?
A
- The wave is absorbed, transferring energy to the medium’s energy stores.
- The wave is transmitted (carries on travelling through the new medium. Most of the time, it is refracted.
- The wave is reflected.
What happens depends on the wavelength of the wave and the properties of the mediums.
19
Q
What rule applies to all types of reflection?
A
Angle of incidence = angle of reflection.
θi = θr
20
Q
When drawing reflection/refraction, what is the normal?
A
An imaginary line perpendicular to the surface at the point of incidence (the point where the wave hits the boundary).
21
Q
What are the two types of reflection?
A
Specular and diffuse.
22
Q
What is specular reflection?
A
Where a wave is reflected by a smooth surface and in a single direction.
23
Q
What is diffuse reflection?
A
Where a wave is reflected by a rough surface, and the reflected rays are scattered in different directions.
This happens because the normal is different for each incident ray. But θi = θr still applies.
24
Q
Why do some surfaces appear matte and some shiny?
A
- Shiny: taking a mirror for example, the surface is smooth, so specular reflection occurs, resulting in a clear reflection.
- Matte: this happens when the surface is rough, so diffuse reflection occurs, resulting in a reflection which is not clear.
25
Draw a diagram of reflection.
θi = θr
26
What is optical density?
A measure of how quickly light travels through a medium. The higher the optical density, the slower light travels.
27
What is refraction?
When light waves are bent when they enter a new medium (which is of a different optical density to the previous medium).
28
If a wave, when it is refracted, slows down, it bends __ the normal. This happens when the second medium is __ than the first.
1. Towards.
2. Optically denser.
29
If a wave, when it is refracted, speeds up, it bends __ the normal. This happens when the second medium is __ than the first.
Away from.
30
What happens to the wavelength and frequency of a wave when it is refracted?
The wavelength changes, but the frequency remains the same.
31
In what circumstances is a wave transmitted without also being refracted?
If a wave travels along the normal of a surface, it will hit the boundary of a new medium face on, so it carries on in the same direction (but at an altered speed, if the new medium is of a different density than the last).
32
Draw a ray diagram to show refraction where the second medium is optically denser than the first.
33
How would you measure the speed of waves in a ripple tank? Why would this setup be suitable for investigating waves?
1. Attach a signal generator to a dipper rod on the tank.
2. Dim the lights and turn on the lamp so that the wave crests create shadows on the screen below the tank.
3. The distance between each shadow line is one wavelength, so measure the distance between 10 and divide by 10 to find the average wavelength.
4. Use s = fλ to find the speed.
*This is suitable for investigating waves because it allows you to measure the wavelength without disturbing the waves.*
34
_The following method can be used to create water waves of a specific frequency in a ripple tank:_
## Footnote
Attach a signal generator to a dipper rod on a ripple tank. Turn on the lamp so that the wave crests create shadows on the screen below the tank. Measure the distance between the shadows of the wave crests to find the average wavelength. Use s = fλ to find the wave speed.
**What would be the greatest source of error for this investigation? Suggest 2 methods to ensure the validity of the results.**
1. Random human error in measurement.
2. You could take more measurements and find a mean, or you could dim the lights so that the shadows are easier to make out.
35
**Practical:**
Plan an investigation to find the speed of waves produced across a taut length of string.
* Create a "tightrope" by attaching a piece of string on one end to a vibration transducer, and on the other to a pulley and weight (see diagram).
* Attach a signal generator to the transducer, then turn on the generator. The string will start vibrating.
* Adjust the frequency of the signal generator until one whole wavelength fits exactly on the string.
* Measure the length of the string to find the wavelength.
* Find the speed of the wave using v = f λ. The frequency is whatever the signal generator is set to.
* Repeat this process, setting the generator so that different numbers of full wavelengths fit along the string. Calculate the speed - should be the same.
36
**Practical:**
Plan an experiment to investigate the refraction of light by different substances.
* Do the experiment in a dim room.
* Place a transparent block onto a piece of paper and trace around it.
* Use a ray box or laser to direct a ray of light at the middle of one of the block's faces.
* Use a ruler to trace the incident and emerging rays.
* Join them up to show the path of the refracted ray through the block.
* Draw the normal to the point where the indicent ray entered the block.
* Use a protractor to measure the angle of incidence and angle of refraction.
* Repeat this process with blocks of different materials, keeping the angle of incidence constant throughout.
* You should find that the angle of refraction is smaller for more optically dense materials.
37
**Practical:**
Plan an experiment to investigate the reflection of light by different types of surface.
* Do this experiment in a dim room.
* On a blank sheet of paper, direct the beam of light from a ray box at one type of surface.
* Draw the normal to the surface and use a protractor to measure the angle of incidence, which should equal angle of reflection.
* Note the clarity and brightness of the reflected ray.
* Repeat this process with a range of different surfaces, making sure the angle of incidence is constant.
* You should observe that smooth surfaces, like mirrors, cause a clear reflected ray - specular reflection. Rough surfaces cause diffuse reflection.
38
What are EM waves?
Transverse waves that transfer energy from a source to an absorber. They are vibrations of electric and magnetic fields, rather than particles.
EM waves form a continuous spectrum; they all travel at the same velocity through a vacuum or air.
39
Why can EM waves travel through a vaccum?
They are vibrations of electric and magnetic fields, rather than of particles (of which there are none in a vaccum).
40
List 3 general properties of EM waves.
1. Transverse
2. Can travel through vaccums
3. They all travel at the same speed, no matter through what medium
41
EM waves form a continuous spectrum, over a range of frequencies. They are grouped into 7 types, based on their wavelength and frequency. Name these types.
*In order of increasing frequency and decreasing wavelength:*
1. Radio waves
2. Micro waves
3. Infrared light
4. Visible light
5. Ultraviolet light
6. X-rays
7. Gamma rays
42
Why is there such a range of frequencies in the EM spectrum?
EM waves are generated by a variety of changes in atoms and their nuclei.
43
Why do our eyes detect a limited range of EM waves?
They can only detect visible light.
44
How are radio waves produced?
By oscillations in electrical circuits.
* An ac causes its oscillating charges (electrons) to produce oscillating electric and magnetic fields - i.e. EM waves.
* Frequency of waves produced = frequency of ac.
45
How are radio waves received?
* Radio waves absorbed by a reciever.
* Causes electrons in the receiver to oscillate.
* This generates an a.c. in the receiver, with the same frequency as the radio waves that generated it.
* So radio waves can themselves induce oscillations in an electrical circuit (which is what they are created by).
46
Give 2 uses of radio waves and explain why they are suitable for these uses.
**TV and radio.** Radio waves are suitable as they are easily generated (by circuits), transmitted and absorbed.
47
Give 2 uses of microwaves and explain why they are suitable for these uses.
* **Satellite communications.** Suitable as some can pass easily through Earth's moist atmosphere to reach satellites.
* **Cooking food.** Suitable because some can be absorbed by water molecules in food, transferring energy to the thermal energy store of the food.
48
Give 3 uses of infrared waves and explain why they are suitable for these uses.
• **Electrical heaters.** These IR radiation, transferring energy to the thermal energy stores of objects/air in the room.
**• Cooking food.** The temperature of food increases when it absorbs IR radiation.
**• Infrared cameras.** These monitor temperature by detecting IR radiation and converting it to an electrical signal. A corresponding image is displayed on a screen.
49
Give one use of visible light and explain why it is suitable for this use.
**Fibre optic communications.** These transmit data by light rays being reflected/bounced back and forth along optical fibres.
They are suitable because they transmit data quickly, and are hard to hack.
50
Give 2 uses of UV waves and explain why they are suitable for these uses.
* **Energy efficient lamps.** Fluorescent lamps generate UV radiation, which absorbed and re-emitted as visible light by a layer of phosphor on the inside of the bulb.
* **Suntanning.** People tan either by sunbathing (sun produces UV radiation) or by using UV lamps to give them an artificial suntan.
51
Give 2 uses of X-rays and gamma rays, and explain why they are suitable for these uses.
• **Both: radiotherapy.** High doses of these rays are carefully directed at cancer cells, killing them.
**• X-rays: medical photographs.** Rays pass easily through flash, but not as easily through bones (denser). So a negative image is created.
**• Gamma rays: medical tracers.** A gamma-emitting source is injected into a patient; rays pass out through the body and are detected by gamma cameras. This allows substances to be tracked.
52
Name the 3 types of EM radiation which can be hazardous to humans.
The high frequency and therefore high energy ones:
## Footnote
**UV radiation, X rays and gamma rays.**
53
Ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue. The effects depend on the type of radiation and the size of the dose. Define "radiation dose".
Radiation dose (in sieverts) is a measure of the risk of harm resulting from an exposure of the body to the radiation.
54
How can UV waves be harmful to humans?
UV waves can cause skin to age prematurely and increase the risk of skin cancer.
They don't cause harm inside the bosy because they can't pass through skin.
55
How can X-rays and gamma rays be harmful to humans?
X-rays and gamma rays are forms of ionising radiation that can cause gene mutations and cancer.
56
What are sound waves? Why can't they travel through a vacuum?
Oscillations in particles. There are no particles in a vacuum; this is why sound waves cannot travel through one.
57
Through which state of matter do sound waves travel the fastest?
Solids.
58
Name 2 processes which convert wave disturbances between sound waves and vibrations in solids.
1. Human hearing.
2. Microphones.
59
How does human hearing work?
Within the ear, sound waves cause the ear drum and other parts to vibrate, causing the sensation of sound.
60
What is the normal range of human hearing?
20 Hz - 20 kHz.
61
Why is human hearing restricted to 20Hz to 20kHz?
* The conversion of sound waves to vibrations of solids works over a limited frequency range, depending on the solid.
* The limits of human hearing are restricted by the size and shape of the ear, and the structures inside it that transfer energy from sound waves.
62
How do microphones pick up sound?
* Sound waves casue a diaphragm to vibrate
* This movement is transferred to an electrical signal
63
How can waves be used to explore/detect structures hidden from direct observation?
You can study the way they behave across the boundaries of these structures, their behaviours relating to:
## Footnote
* velocity
* complete or partial reflection
* refraction
* absorption
64
Give 3 examples of how waves are used to explore/detect structures hidden from direct observation.
* **Ultrasound** waves in medical/industrial imaging
* **Seismic waves** provide evidence of the size and structure of the Earth's core
* **Echo sounding** used in water
65
How are ultrasound waves useful in medical and industrial imaging?
* They have a frequency higher than the upper limit of human hearing.
* They are are partially reflected when they meet a boundary between 2 media.
* The time taken for reflections to reach a detector can be used to determine how far away a boundary is.
* This means they are useful in imaging.
66
What produces seismic waves?
Earthquakes.
67
Name the 2 types of seismic waves and describe their properties.
**P-waves:**
* longitudinal
* travel through liquids and solids, at different speeds
**S-waves:**
* transverse
* can travel through solids, but not liquids
68
How do seismic waves provide evidence for the size and structure of the Earth's core?
* Scientists observe how seismic waves are absorbed or reflected
* This gives information about where the properties of the inside of the Earth dramatically change
* Therefore, evidence is provided about parts of the Earth which aren't directly observable
69
How does a lens form an image?
By refracting light.
70
Explain how images are formed in **convex** lenses.
* Parallel rays of light are refracted and brought to a focus at the principal focus.
* A ray passing through the centre of the lens carries on in the same direction.
* The distance from the lens to the principal focus is called the focal length.
71
Explain how images are formed in concave lenses.
* Parallel rays of light are refracted and diverge outwards.
* A ray passing through the centre of the lens carries on in the same direction.
* The principle focus is where the diverging rays would meet if you joined them up.
* The distance from the lens to the principal focus is called the focal length.
72
How are convex lenses represented in ray diagrams?
73
How are concave lenses represented in ray diagrams?
74
What is meant by the phrases "real image" and "virtual image"?
* **Real images:** the light from an object comes together to form an image on a screen (e.g. your retina).
* **Virtual images:** the light rays from an object are diverging, so the object appears to be in a different place (e.g. a mirror, or magnifying glass).
75
Out of convex and concave lenses, which can produce real/virtual images?
* The image produced by a **convex** lens can be **real** or **virtual** (virtual if object is closer to lens than focal point)
* The image produced by a **concave** lens is always **virtual**.
76
What 3 things need to be considered when describing how an image is formed?
* Real/virtual
* Enlarged/diminished
* Upright/inverted
77
Draw a ray diagram for this lens.
convex
78
Draw a ray diagram for this lens.
concave
79
What is meant by specular reflection and diffuse reflection?
* **Specular reflection** is where light is reflected uniformly, in a single direction, from a smooth surface.
* **Diffuse reflection** is where light is scattered and reflected in multiple directions by a rough surface.
80
What is the difference between transparent and translucent materials?
* **Transparent**: nearly all light is transmitted through.
* **Translucent**: has internal boundaries - light is refracted into many directions
81
Draw the **light** colour wheel.
82
How do colour filters work?
They absorb certain wavelengths and transmit other wavelengths, so that you see objects as only certain colours (or black) when looking through them.
83
What would you see if you looked at a yellow apple and an orange through a green filter?
* **Yellow apple looks green** (yellow light is made of red and green wavelengths, of which only green passes through)
* **Orange also looks green, but darker** (because orange is made of red and green wavelengths, but mostly red, which is absorbed. So a smaller amount of green light goes through. The absence of light is darkness, so the colour is darker)
84
What determines the colour of opaque objects?
* The colour is determined by which wavelengths of light are more strongly reflected.
* Wavelengths that are not reflected are absorbed.
* If all wavelengths are reflected equally, the object appears white.
* If all wavelengths are absorbed, the objects appears black.
85
Objects that transmit light are either __ or \_\_.
transparent or translucent
86
What do all objects do, no matter their temperature?
Emit and absorb infrared radiation. Hotter objects emit more in a given time.
87
What is a perfect black body?
An object that absorbs all of the radiation incident on it, and does not reflect or transmit any radiation.
Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter (reemits all possible frequencies).
88
How is an object's temperature related to how it emits and absorbs rediation?
* A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation.
* The temperature of a body increases when the body absorbs radiation faster than it emits radiation.
89
**Practical:**
Plan an investigation to show how the amount of infrared radiation emitted by a surface depends on its nature.
* Use a Leslie cube with different surfaces, e.g. matt black, shiny black, matt white, shiny white.
* Place the cube on a heatproof mat and fill it with boiling water.
* Wait for it to warm up, then check, by holding a thermometer to each face, that the cube's surface is the same temperature everywhere (a control).
* Place an infrared detector a set distance away from a face and record the amount of IR radiation detected. Repeat for each face at this same distance.
* Do the experiment 3 times to make sure the results are repeatable.
* This should be the _decreasing_ order of the amount of IR radiation emitted from the surfaces: matt black, shiny black, matt white, shiny white.
90
Name factors that the temperature of the Earth depends on.
* rate of absorption of radiation
* rate of emission of radiation
* rate of reflection of radiation
