Waves Flashcards
What are the three things that can happen to waves?
All waves can be absorbed, transmitted or reflected
What happens when a wave is absorbed?
The wave is absorbed by a material
This transfers energy to the materials energy stores
What happens when waves are transmitted?
The waves carry on travelling through the new material
Transmission often leads to refraction
What happens when a wave is reflected?
The waves never enters the material
What is the connection between the angle of incidence and the angle of reflection ALWAYS?
They are equal
What is the normal on ray diagrams?
The normal is an imaginary line
It is perpendicular to the surface at the point of incidence.
It is shown as a dotted line
What is the point of incidence?
The point at which a wave hits a boundary
When does specular reflection happen?
When a wave is reflected in a single direction by a smooth surface.
This is because all the normals are in the same direction
It results in a clear reflected image
What is diffuse reflection
When a wave is reflected by a rough surface, and reflected rays are scattered in lots of different directions
This is because the normal is different for each incoming ray
This results in an unclear reflection
What is refraction?
When a wave crosses a boundary between materials at an angle, and changes direction
What determines how much a wave is refracted?
How much the wave slows down or speeds up, which is determined by the density of the materials
The higher the density of a material, the slower a wave travels through it (usually)
Which way does the wave bend if it slows down?
Bends towards the normal
Which way does a wave bend if it speeds up?
Bends away from the normal
What happens to wavelength and frequency when a wave is refracted?
Wavelength changes but frequency stays the same
What happens with speed and direction if a wave is travelling along the normal?
Changes speed, but does not refract, and therefore does not change direction
What is the optical density of a material?
A measure of how quickly light can travel through a material.
The higher the optical density, the slower light waves travel through it
If the material a wave hits at a boundary is optically denser than the first, how big is the angle of refraction in comparison to the angle of incidence?
The angle of refraction is smaller than the angle of incidence, as the wave bends towards the normal.
And vice versa
What type of waves are EM waves?
Transverse waves
How are EM waves’ energy transferred?
All EM waves transfer energy from a source to an absorber
In air or a vacuum, what speed do EM waves travel at?
All EM waves travel at the same speed in air or a vacuum
They travel at 3x10^8 m/s
What sort of spectrum do electromagnetic waves create
They create a continuous spectrum, grouped into seven basic types, based on their wavelength and frequencies
From left to right of the EM spectrum, what happens to wavelength and frequency
Increasing frequency and decreasing wavelength
E.g. radio waves have a low frequency and large wavelength, whereas gamma rays have a high frequency and small wavelength
Why is there a large range of frequencies for EM waves?
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
What is the mnemonic for the EM waves?
Remember My Instructions, Visible Under X-Ray Goggles
(Radio waves, Microwaves, Infrared, Visible light, Ultraviolet, X-rays, Gamma rays)
What are EM waves made up of?
Oscillating electric and magnetic fields
What are alternating currents made up of, and what does this produce?
They are made of oscillating charges, which, as they oscillate, produce oscillating electric and magnetic fields (EM waves)
What is a transmitter?
The object in which charges (electrons) oscillate to create radio waves is called a transmitter
How do you produce radio waves using an alternating circuit?
Using an electrical circuit
1. Electrons are oscillated, producing EM waves in a transmitter
2. These radio waves reach a receiver where they are absorbed
3. The energy transferred by the waves goes to the electrons in the receiver, causing the electrons to oscillate
4. If the receiver is part of a complete electrical circuit, this generates an alternating current with the same frequency as the radio waves that generated it .
How do we know radio waves are useful for communication?
The frequency supplied to produce radio waves (using an ac) is the same as the frequency produced in the receiver
What are the three types of radio wave?
Long-wave, short-wave, very short-wave
How long are long-wave radio waves?
They have wavelengths of 1-10km, and be transmitted and recieved from halfway across the world
How can long-wave radio waves be transmitted and received from such long distances?
The long wavelengths diffract (bend) around the curved surface of the earth. They can also diffract around hills, through tunnels etc.
How long are short-wave radio signals?
They have wavelengths of about 10-100m
They can be received at long and short distances from the transmitter.
How are short-wave radio signals transmitted over long distances?
They are reflected from the ionosphere - an electrically charged layer in the earth’s upper atmosphere.
What type of waves does Bluetooth use?
Short-wave radio waves, sending data over short distances.
What are very short wavelengths used for?
TV and FM radio transmissions. To get reception, you must be in direct sight of the transmitter, as the signal doesn’t diffract or travel far through boundaries
How do waves transfer energy?
When waves travel though a medium, the particles of the medium oscillate and transfer energy between each other without transferring any matter
What is the amplitude of a wave?
The maximum displacement of a point on the wave from its undisturbed position
What is the wavelength of a wave?
The distance between the same point on two adjacent waves
What is the frequency of a wave?
The number of complete waves passing a certain point per second, measured in Hz
What is the (time) period of a wave?
The amount of time it takes for one complete oscillation of a wave to pass a point
Measured using T = 1/f
Define transverse waves:
Waves in which the oscillations (vibrations) are perpendicular to the direction of energy transfer
What are some examples of transverse waves?
EM waves, ripples and water waves, waves of a string
Define longitudinal waves:
Waves where the oscillations are parallel to direction of energy transfer.
Examples of longitudinal waves are…
Sound waves in air, e.g. ultrasound
Shock waves, e.g. some seismic waves
For satellites, what type of electromagnetic wave is required?
Microwaves (the ones that aren’t absorbed by water)
Which type of microwave do satellites work with and why?
Ones that aren’t absorbed by water, because they need to pass through the Earth’s watery atmosphere
How do satellites work?
- A signal from a transmitter is transmitted into space
- A satellite orbiting the earth picks it up
- The satellite transmits the signal back to earth in a different direction
- It is received by a satellite dish on earth
Why is there a time delay between a satellite signal being sent and received?
Because of the long distance the signal must travel
How are there some microwaves that absorb water molecules and some that don’t?
The properties of microwaves on one end of the range differ from the other end, as each EM wave covers a spectrum of wavelengths and frequencies itself
How do microwave ovens work?
- Microwaves that absorb water molecules are used
- The microwaves penetrate up to a few cm into food before its energy is absorbed and transferred to the water molecules
- The water molecules therefore heat up
- The water molecules transfer this energy to the rest of the molecules which heats the rest of the food.
How does the emission of infrared radiation change with temperature?
The hotter the object, the more infrared radiation it gives out
How do infrared cameras work?
- The camera detects the IR radiation and turns it into an electrical signal
- This is then displayed on a screen as a picture
- The hotter an object is, the brighter it appears
How is IR radiation used to heat toast?
- Temperature of the toast increases as it absorbs IR radiation
- IR doesn’t penetrate the surface of food, so the toast is crispy in the outside rather than just generally warm
What EM wave is used for electric heaters?
Infrared
How do electric heaters work?
- They contain a long piece of wire that heats up as a current flows through it
- The wire then emits lots of infrared radiation
- The objects and air in the room absorb the IR radiation, as energy is transferred by the IR waves to the thermal energy stores of the objects
- This causes their temperature to increase
Which EM wave is used for fibre optic cables?
Visible light
What are fibre optic cables used for ?
The transmission of data
How do fibre optic cables work?
- Optical fibres are used, which are thin glass or plastic fibres that can carry data over long distances as pulses of visible light
- They work because of reflection, as the light rays are bounced back and forth until they reach the end of the fibre
Why are fibre optic cables used?
They transmit more info than copper wires
The data is less likely to end up distorted
What type of reflection do fibre optic cables use?
Specular reflection, as the material must completely reflect the light so the data is transmitted correctly
What is fluorescence?
A property of certain chemicals, where UV radiation is absorbed and visible light is emitted. This is why fluorescent colours look so bright - they actually emit light
How do fluorescent light bulbs work?
- They generate UV radiation
- This is absorbed and then re-emitted as visible light
- That is done by a layer of phosphorus inside the bulb
Benefits of fluorescent lights?
Energy efficient so are good to use for long periods of time
Name some uses of UV radiation
- Security pens, as UV light reveals an otherwise invisible ink
- Suntans, found from the sun and used in tanning salons for an artificial suntan
- Sterilises water, as UV kills micro-organisms
Main use of X-rays and gamma rays?
Medicine, e.g. bone scanning, medical tracing and radiotherapy
How does X-ray photography work?
- X-rays pass easily through soft tissue like flesh
- X-rays don’t pass easily through denser material like bones or metal
- It is the amount of radiation that is absorbed or not absorbed that gives you an x-ray image
What is radiotherapy and how does it work?
The use of X-rays and gamma rays to treat people with cancer.
It works as high doses of these rays kill all living cells. They are therefore carefully directed at cancer cells to avoid killing too many healthy cells
How does medical tracing work?
Where a gamma-emitting source is injected into the patient and its progress is followed around the body.
Gamma radiation works with this because it can pass out through the body to be detected
What EM wave is a good alternative for sterilising equipment using boiling water?
Gamma, as it kills microorganisms without causing other damage
How do radiographers protect themselves from X-rays and gamma rays?
They wear lead aprons and stand behind a lead screen or leave the room to avoid exposure
What are the effects of each type of radiation based on?
How much energy the wave transfers
How does energy transfer change going along the EM spectrum?
Low-frequency waves such as radio waves don’t transfer much energy. They normally pass through soft tissue without being absorbed
High frequency waves such as UV, X-rays and gamma rays all transfer lots of energy and so can cause a lot of damage
What is the damage caused by UV radiation?
UV damages surface cells which can lead to sunburn, blindness and increased risk of skin cancer
Which EM waves are types of ionising radiation and what damage does this cause?
X-rays and gamma rays are ionising, causing gene mutation, cell destruction and cancer
What does ionising radiation mean?
They carry enough energy to knock electrons off of atoms (turn them into ions)
What is radiation dose?
A measure of the risk of harm from the body being exposed to radiation
It is NOT a measure of the total radiation that has been absorbed
What is radiation dose measured in?
Sieverts (Sv), but often you’ll see millisieverts (mSv)
