1: Imaging Flashcards
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
Explain refraction
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
