Flashcards in 1: Imaging Deck (44):
What is the defining characteristic of a polarised wave?
It oscillates in only one direction
Why an electromagnetic radiation be polarised?
It is made up of 2 transverse waves, vibrating in different directions - electric and magnetic field.
What happens if you try to pass light through 2 polarising filters at right angles to each other?
No light will get though, all directions of vibration will be blocked
What is an indication that light is a transverse wave?
The fact it can be polarised - only transverse waves can be polarised
Describe how you would investigate the polarisation of light using 2 polarising filters
1) Align the transmission axes of 2 polarising filters so they are both vertical. Shine unpolarised light on the first filter. Keep the position of the first filter fixed and rotate the second one
2) Light that passes through the first filter will be vertically polarised
3) As you rotate the 2nd filter, the amount of light that passes through the second filter varies
State 2 examples of polarising filters
1) 3D films use polarised light to create depth
2) Polaroid sunglasses block out some light to reduce glare
Why can you not polarise microwaves using a polarising filter? How do you polarise them?
Their wavelength is too long.
Use metal grilles
How do you investigate polarising microwaves?
1) Put a metal grille between the microwave transmitter and receiver
2) The intensity of the microwaves is at a maximum when the direction of the vibration of the microwaves and the wires are at right angles to each other
3) As you rotate the grille, the intensity decreases, so the reading on the voltmeter (connected to receiver) decreases
4) When the wires of the grille are aligned with the direction of the polarised waves, no signal will the shown on the voltmeter
Why does the intensity drop to 0 when the wires are aligned to the direction of polarisation of the microwaves?
The grille is absorbing their energy
Explain what happens when microwaves pass through a grille
1) The vibrating electric field of the microwave excites electrons in the metal grille
2) The energy of the incoming microwaves is absorbed by the grille and re-emitted in all directions
3) Only a few of the re-emitted waves are vibrating in the direction of the receiver
4) The receiver only receives waves in one plane, so even if the re-emitted wave travels toward the receiver, it might not be picked up
Explain why how and why the intensity of the microwaves changes when the metal grille is rotated.
1) When wires and vibrations are aligned, more electrons are excited than when they're at right angles to each other - all the energy is absorbed and the intensity reading drops to 0
2) When the wires and vibrations are at right angles, some electrons in the grille are still excited and so there is still a small drop in intensity
When does refraction happen?
When a wave changes speed at a medium boundary
When a ray of light meets a boundary between one medium and another, some of its energy is reflected back into the 1st medium and the rest of it is transmitted through into the 2nd medium
What does the amount of refraction depend on? What does this imply about focal length?
The wavelength of the light - therefore the focal length for a given lens will change depending on wavelength
Describe how density and speed affect/are affected by refraction
If light meets the boundary at an angle to the normal, the transmitted ray is bent/refracted as it travels at a different speed in each medium
The more optically dense a material is, the more slowly light travels in it
What do lenses do? Simple
They change the curvature of wavefronts by refraction
Describe how lenses add curvature to wavefronts
A lens adds curvature to waves as they pass through it. If waves are uncurved before passing though the lens, and parallel to the lens axis, they will be given spherical curvature, centred on the focus (focal point) of the lens
Explain how a converging lens curves wavefronts by changing the speed of he wave
It slows down the light travelling through the middle of the lens more than light at the lens edges. All points on a 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
The more [ ] (thicker) the lens, the more it will [ ] the wavefronts that travel through it - so the [ ] its focal length
Describe the wavefronts of a distant light source. What curvature will a converging lens give them
Flat. The converging lens will give them a curvature of 1/f
Describe the wavefronts if the source is at the focus of the lens
The wavefronts will start off curved w/ negative curvature . This -ve curvature is then cancelled out by the +ve curvature added by the converging lens - so the wavefronts will be made flat
A [ ] (moving) wave carries [ ] and usually information from one place to another without transferring any [ ]
Define Displacement (waves)
How far a point on the wave has moved from its undisturbed position (vertical)
The length of one whole wave, eg crest to crest
The time taken for a whole vibration
The number of whole vibrations per second passing a given point
Define Phase Difference
The amount by which one wave lags behind another wave
Are EM waves transverse or longitudinal?
Describe a transverse wave
The vibration is at a right angles to the waves direction of travel
Describe a longitudinal wave, give an example
The vibrations are along the wave's direction of travel
What is a bit?
A single binary digit
What is a byte?
A group of 8 bits
When an image is stored on a computer, what is represented by binary numbers?
Each pixel is represented by a binary number
Describe 2 meanings of image resolution
1) The length represented by each pixel
2) Number of pixels in the format width * height eg, 1920 x 1080 pixels
or the total eg, 2073600 pixels
In a black and white photo, what colour is a pixel with the value 255?
What does adding a fixed number to each value of a pixel do?
Increases the brightness (if the fixed number is +ve) - each pixel has a higher number therefore has a lighter colour
What does multiplying each value of a pixel, by a fixed number, do?
Increase/Improves the contrast (if the fixed number is greater than 1)
What is noise in an image?
Noise is unwanted interference affecting a signal. In images this is usually bright or dark spots on a picture
Describe how to remove noise on an image
Replace each pixel with the median of itself and the 8 pixels surrounding it. The result is that any 'odd' (very high/low values are removed and the image is made smoother) - you can also do this replacing each pixel by the mean of itself and surrounding pixels. This isn't as good because the 'odd' value affects the new value
What is the Laplace rule used for?
Explain the Laplace Rule
You multiply the value of each pixel by 4 then subtract the value of the pixels immediately above, below, left and right. If the value is -ve, it is treated as 0