Topic 6 - Waves Flashcards
Waves
> Waves carry energy from one place to another and can also carry information.
When waves travel through a medium, the particles of the medium oscillate and transfer energy between each other.
Only energy is transferred, the particles stay in the same place.
Types of wave
- Transverse
2. Longitudinal
Amplitude - definition
> The amplitude of a wave is the maximum displacement of a point on the wave from its undisturbed position.
Wavelength - definition
> The wavelength is the distance between the same point on two adjacent waves.
E.g. trough to trough.
Frequency
> Frequency is the number of complete waves passing a certain point per second.
It’s measured in Hertz.
1 Hz = 1 wave per second.
Period of a wave
> The period of a wave is the amount of time it takes for a full cycle of the wave to pass a point.
Period = 1 divided by Frequency.
Seconds = 1 divided by Hz.
Wave Speed
> The wave speed is the speed at which the energy is transferred (or the wave moves) through the medium.
Transverse Waves
> In transverse waves, the oscillations are perpendicular to the direction of energy transfer.
Most waves are transverse, inc. all electromagnetic waves (e.g. light), ripples and waves in water and a wave on a string, spring wiggled from side to side.
Mechanical Wave - definition
> A mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium.
Water waves, shock waves and waves in springs and ropes are all examples of mechanical waves.
Longitudinal waves
> In longitudinal waves, the oscillations are parallel to the direction of energy transfer.
If you push a spring you get a longitudinal wave.
Examples: sound waves in air, ultrasound, shock waves e.g. seismic waves.
Longitudinal waves show areas of compression and rarefaction.
Rarefaction - definition
> A rarefaction is a region in a longitudinal wave where the particles are furthest apart.
Compression - definition
> A compression is a region in a longitudinal wave where the particles are closest together.
The wave equation
>Applies to all waves. >wave speed = frequency × wavelength v = f λ >wave speed, v, in metres per second, m/s >frequency, f, in hertz, Hz >wavelength, λ, in metres, m
Experiments with waves - measuring the speed of sound - steps
> By attaching a signal generator to a speaker you can generate sounds with a specific frequency.
You can use 2 microphones and an oscilloscope to find the wavelength of the sound waves generated.
1. Set up the oscilloscope so the detected waves at each microphone are shown as separate waves.
2. Start with both microphones next to the speaker, then slowly move one away until the two waves are aligned on the display, but have moved exactly one wavelength apart.
3. Measure the distance between the microphones to find one wavelength.
4. You can then use the wave speed formula to find the speed of the sound waves passing through air - the frequency is whatever you set the signal generator to.
Experiments with waves - measuring the speed of water ripples practical - steps
> Using a signal generator attached to the dipper of a ripple tank, you can create water waves at a set frequency.
- Dim the lights and turn on the lamp - you’ll see a wave pattern made by the shadows of the wave crests on the screen below the tank.
- The distance between each shadow line is equal to one wavelength. Measure the distance between the shadow lines that are 10 wave lengths apart, then divide this distance by 10 to find the average wavelength. This is a suitable method for measuring small wavelengths.
- If you’re struggling to measure the distance, you could always take a photo of the shadows and ruler, and find the wavelength from the photo instead.
- Use the wave speed formula to find the speed of the waves.
- This set-up is suitable for investigating waves, because it allows you to measure the wavelength without disturbing the waves.
Experiments with waves - finding the speed of a wave on string - steps
> You use a signal generator, but this time you attach it to a vibration transducer which converts the signals to vibrations.
- Set up the equipment shown in book, then turn on the signal generator and vibration transducer. The string will start to vibrate.
- You can adjust the frequency setting on the generator to change the length of the wave created on the string. You should keep adjusting the frequency of the signal generator until there appears to be a clear wave on the string. This happens when a whole number of half-waves fit exactly on the string (you want at least 4 or 5 ideally). The frequency you need will depend on the length of string between the pulley and the transducer, and the masses you’ve used.
- You need to measure the wavelength of the wave. Best way to do so accurately is to measure the length of all the half-wavelengths on the string in one go, then divide by the total number of half-wavelengths to get the mean half-wavelength. You can then double this value to get the full wavelength.
- The frequency is whatever you set the signal generator to.
- Find speed using formula.
Experiments with waves - finding the speed of a wave on string - set-up
> This set-up is suitable for investigating waves on a string because it’s easy to see and measure the wavelength and frequency.
Signal generator attached to vibration transducer which is attached to a string and pulley with masses on end of strong.
All on a bench with pulley and masses coming off edge of bench.
Waves - the three actions
> When waves arrive at a boundary between 2 different materials, 3 things can happen:
- Absorbed
- Transmitted
- Reflected
What can happen when waves arrive at a boundary between 2 different materials?
> When waves arrive at a boundary between 2 different materials, 3 things can happen:
1. The waves are absorbed by the material the wave is trying to cross into - this transfers energy to the material’s energy stores. This is how microwaves work.
2. The waves are transmitted - the waves carry on travelling through the new material. This often leads to refraction.
3. The waves are reflected.
What actually happens depends on the wavelength of the wave and the properties of the materials involved.
Rule of reflection
> Angle of incidence = angle of reflection.
>See diagram in book for ray diagram.
Angle of incidence - definiton
> The angle of incidence is the angle between the incoming wave and the normal.
Angle of reflection - definition
> The angle between the reflected wave and the normal.
The normal - defintion
> The normal is an imaginary line that’s perpendicular to the surface at the point of incidence (point where the wave hits the boundary).
Usually shown by a dotted line.
Types of reflection
> Waves are reflected at different boundaries in different ways:
- Specular
- Diffuse
Specular reflection
> Specular reflection happens when a wave is reflected in a single direction by a smooth surface.
E.g. when light is reflected by a mirror you get a nice clear reflection.
Diffuse reflection
> Diffuse reflection is when a wave is reflected by a rough surface (e.g. a piece of paper) and the reflected rays are scattered in lots of different directions.
This happens because the normal is different for each incoming ray, which means that the angle of incidence is different for each ray. The rule of angle of incidence = angle of reflection still applies.
When light is reflected by a rough surface, the surface appears matte (not shiny) and you don’t get a clear reflection of objects.
Refraction - definition
> When light rays are bent when they enter a new media.
>Waves changing direction at a boundary.
Refraction
> When a wave crosses a boundary between materials at an angle, it changes direction - it’s refracted.
How much it’s refracted by depends on how much the wave speeds up or slows down, which usually depends on the density of the 2 materials.
If a wave crosses a boundary and slows down, it will bend towards the normal. If it crosses into a material and speeds up it will bend away from the normal.
The wavelength of a wave changes when it is refracted, but the frequency stays the same.
If the wave is travelling along the normal it will change speed, but it’s NOT refracted.
What is the optical density of a material?
> The optical density of a material is a measure of how quickly light can travel through it - the higher the optical density, the slower the light waves travel through it.
Density and wave speed
> The higher the density, the slower a wave travels through it.
Constructing a ray diagram to show refraction
- First, draw the boundary between your 2 materials and the normal.
- Draw an incident ray that meets the normal at the boundary. The angle between the ray and the normal is the angle of incidence. If you are given this angle use protractor.
- Now draw the refracted ray on the other side of the boundary. If the material is optically denser than the first, the refracted ray bends towards the normal. The angle between the refracted ray and the normal is smaller than the angle of incidence. If the second material is less optically dense, the angle of refraction is bigger than the angle of incidence.
>See book.
Investigating light - background
> Light experiments need to be done in a dim room so you can clearly see paths of the rays of light.
They also both use either a ray box or a laser to produce thin rays of light. This is so you can trace the paths more accurately, meaning more exact angle measurements.
Investigating light - investigating refraction - steps
> The boundaries between different substances refract light by different amounts. You can investigate this by looking at how much light is refracted when it passes from air into different materials.
1. Place a transparent rectangular block on a piece of paper and trace around it. Use a ray box or a laser to shine a ray of light at the middle of one side of the block.
2. Trace the incident ray and mark where the light emerges on the other side of the block. Remove the block, and with a straight line, join up the incident ray and the emerging point to show the path of the refracted ray through the block.
3. Draw the normal at the pint where the light ray entered the block. Use a protractor to measure the angle between the incident ray and the normal (angle of incidence) and the angle between the refracted ray and the normal (angle of refraction).
4. Repeat this experiment using rectangular blocks made from different materials, keeping the incident angle the same throughout.
You should find that the angles of refraction changes with different materials as they have different optical densities.
Use transparent materials.
Investigating light practical - comparing how different surfaces reflect light - steps
> How light reflects depends on the smoothness of the surface.
- Take a piece of paper and draw a straight line across it. Place an object so one of its sides lines up with this line.
- Shine a ray of light at the object’s surface and trace the incoming and reflected light beams.
- Draw the normal at the point where the ray hits the object. Use a protractor to measure the angle of incidence and the angle of reflection and record these values in a table. Also make a note of the width and brightness of the reflected light ray.
- Repeat this experiment for a range of objects.
Investigating light practical - comparing how different surfaces reflect light - results
> You should see that smooth surfaces like mirrors give clear reflections (the reflected ray is as thin and bright as the incident ray).
Rough surfaces like paper cause diffuse reflection which causes the reflected beam to be wider and dimmer (or not observable at all).
You should also find that the angle of incidence always equals the angle of reflection.
Electromagnetic Waves - speicfication
> Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber. E.g. a hot object transfers energy by emitting infrared radiation, which is absorbed by the surrounding air.
Electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air.
The waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency. Going from long to short wavelength (or from low to high frequency).
The groups are: radio, microwave, infrared, visible light (red to violet), ultraviolet, X-rays and gamma rays.
Our eyes only detect visible light and so detect a limited range of electromagnetic waves.
Why is there such a large range of frequencies of EM waves?
> There is such a large range of frequencies because EM waves are generated by a variety of changes in atoms and their nuclei. E.g. changes in the nucleus of an atom creates gamma rays.
This also explains why atoms can absorb a range of frequencies - each one causes a different change.
Because of their different properties each one has different purposes.
Uses of EM waves
> Electromagnetic waves have many practical applications. For example:
• radio waves – television and radio
• microwaves – satellite communications, cooking food
• infrared – electrical heaters, cooking food, infrared cameras
• visible light – fibre optic communications
• ultraviolet – energy efficient lamps, sun tanning
• X-rays and gamma rays – medical imaging and treatments
Electromagnetic spectrum. Order + wavelength
- Radio waves. 1m-10^4m.
- Micro waves. 10^-2m.
- Infra red. 10^-5m
- Visible light. 10^-7m.
- Ultra violet. 10^-8m.
- X-rays. 10^-10m.
- Gamma rays. 10^15m.
> As you go down, frequency increases and wavelength decreases.
Radio Waves - spec brief
> Radio waves can be produced by oscillations in electrical circuits.
When radio waves are absorbed they may create an
alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit.
Making EM waves.
> EM waves are made up of oscillating electric and magnetic fields.
Alternating currents are made up of oscillating charges. As the charges oscillate, they produce oscillating electric and magnetic fields, i.e. electromagnetic waves.
The frequency of the waves produced will be equal to the frequency of the alternating current.
Making radio waves
- You can produce radio waves using an alternating current in an electrical circuit. The object in which charges (electrons) oscillate to create the radio waves is called a transmitter.
- When transmitted radio waves reach a receiver, the radio waves are absorbed.
- The energy transferred by the waves is transferred to the electrons in the material of the receiver.
- This energy causes the electrons to oscillate and, if the receiver is part of a complete electrical circuit, it generates an alternating current.
- This current had the same frequency as the radio waves that generated it.
>Diagram in book.
Radio Waves - uses
> Radio waves are EM radiation with wavelengths longer than about 10cm.
Long-wave radio waves (wavelengths of 1-10 km) can be transmitted from one place and be received half way around the world.
That’s because long wavelengths diffract (bend) around the curved surface of the Earth. Long-wave radio waves wavelengths can also diffract around hills, into tunnels and all sorts.
This makes it possible for radio signals to be recieved even if the receiver isn’t in line of sight of the transmitter.
Short-wave radio signals (wavelength 10m-100m) can, like long-wave, be received at long distances from the transmitter. That’s because they are reflected from the ionosphere.
Bluetooth uses short-wave radio waves to send data over short distances between devices without wires.
Medium-wave signals can also reflect from the ionosphere, depending on atmospheric conditions and the time of day.
The radio waves used for TV and FM radio transmissions have v. short wavelengths so to get reception, you must be in direct sight of the transmitter - the signal doesn’t bend or travel far through buildings.
Ionosphere - definition
an electrically charged layer in the Earth’s upper atmosphere.
