Imaging and signalling Flashcards
How is information transferred?
Waves
How does medical scanning use waves?
Ultrasounds scans build up an image of a foetus by detecting reflected ultrasound waves
What is a progressive wave?
A wave that carries energy (and usually information) from one place to another without transferring any material
How can you tell a wave carries energy?
EM waves cause things to heat up
X-rays and gamma rays knock electrons out of their orbits causing ionisation
Sound waves make things vibrate
What happens to the source of waves?
Loses energy as the waves are carrying energy
What is the displacement of a wave?
How far a point on the wave has moved from its undisturbed position
What is the amplitude of a wave?
The max. displacement of a wave from its undisturbed position
What is wavelength?
The length of a whole wave (distance from one point on a wave to its identical point on the adjacent wave)
What is the period of a wave?
The time taken for a whole vibration
What is frequency?
The no. of waves passing a given point per second
What is phase difference and what is it measured in?
The amount by which one wave lags behind another wave
degrees or radians
What is the wave equation?
v=fλ
What are electromagnetic waves?
Transverse waves which consist of two perpendicular planes, a magnetic and an electric field
What is a transverse wave?
A wave where the vibration is at right angles to the wave’s direction of travel
What is a longitudinal wave?
A wave where the vibrations are along the wave’s direction of travel
What is intensity?
The rate of flow of energy per unit area at right angles to the direction of travel of the wave
(measure of how much energy a wave is carrying)
Intensity equation (waves) and unit?
intensity = power/area
Wm-2
What is a polarised wave?
A wave that only oscillates in one direction
What do all electromagnetic waves have in common?
Transverse waves
Can be polarised
Travel at the speed of light in a vacuum
What is a polarising filter?
Something that only transmits waves oscillating in one direction
What happens when you shine light through two polarising filters at right angles?
No light will be transmitted as all directions of oscillation will be blocked
How do you know if a wave is transverse?
If it can be polarised
What happens when two polarising filters are at 45°?
Intensity getting through the second filter will be exactly half of that transmitted through the first
What are some examples of polarising filters?
3D filed use polarised light to create depth, the filters in each lens are at right angles to each other so each eye gets a slightly different picture
polaroid sunglasses use polarising filter to help prevent glare
Why don’t polarising filters work on microwaves and what do we used instead?
wavelengths too long so use a metal grille
Why do you only need one metal grille when investigating polarisation of microwaves?
Microwave transmitters transmit polarised microwaves
What equipment do you need to investigate polarisation of microwaves?
You need a metal grille between a microwave transmitter and a microwave receiver which is connected to a voltmeter
When is the intensity of microwaves passing through a metal grille at a maximum?
When the direction of the vibration of the microwaves and the wires on the grille are at right angles
What happens when the wires of the metal grille are aligned with the direction of polarised microwaves?
No signal will be shown on the voltmeter
Why is no signal detected by the microwave receiver when the metal grille wires are aligned with the direction of polarisation?
Because the grille is absorbing the energy
The vibrating electric field of the microwave excites electrons in the metal grille so the grille is absorbing the energy and then re-emitting it in all directions, only a few of those re-emitted waves are vibrating in the same direction of the microwave receiver
Why is there still a drop in intensity when the wires of the metal grille are at right angles to the oscillations of the microwaves?
Some electrons in the grille are still excited so there is still a small drop
Why doesn’t the microwave receiver pick up the re-emitted microwaves?
Only receives microwaves in one plane
What is refraction?
When a wave changes speed at a boundary
Why does a wave change speed at a boundary?
Some of its energy is reflected back into the first medium and the rest is transmitted through into the second medium
How does density of a material affect speed?
The more optically dense a material is the more slowly light travels in it
What does the amount of refraction depend on?
The wavelength of light
So the focal length for a given lens will change depending on the wavelength
What happens when light travels from a less dense material to a more dense material?
Slows down so bends towards the normal
What do converging lenses do and how do they do it?
Change the curvature of wavefronts by refraction
What does a lens do to a wave as it passes through it?
Adds curvature
How does a converging lens curve the wavefronts?
It slows down the light travelling through the middle of the lens more than the light at the edges of the lens - all points of the wavefront take the same amount of time to get to the focus point
What is the focal length?
Distance between the lens axis and the focus
What does a more powerful lens means?
It is thicker so will curve the wavefronts travelling through it more, meaning it has a shorter focal length