Imaging Flashcards
How do you calculate Frequency?
Inverse of the period - 1 / T
How do you calculate the speed of a wave?
V = f * λ
Define Intesity of a wave, and hence derive an equation from the definition, giving units.
Rate of flow of energy per unit area
Intensity (Wm^-2) = Power (W) / Area (m^2)
What 2 components make up an EM wave and how do they oscillate relative to eachother?
Electric and Magnetic
They oscillate at a right angle to eachother as well as the direction of travel
What does a polarising filter do and how can we use this to block out all light?
A polarising filter removes one of the EM wave’s components depending on it’s rotation.
If we have 2 filters at a right angle to each other, then no light can pass through as all components of the EM wave are removed.
Describe a setup to observe the effects of polarising filters on light
- Place 2 filters in front of the light soure both vertical.
- Keep the first filter vertical but rotate the second
- The intensity of the light will vary as the filter is rotated
Describe a setup to investigate the polarisation of microwaves
- Place a metal grille inbetween a microwave transmitter and reciever
- As you rotate the grille, the intensity of the wave will decrease, so the voltage reading on the reciever will also decrease
- When the oscillations of the wave and the metal grille are alligned, none of the wave is able to pass, so the signal recieved is 0V.
Describe in detail how a polarisation filter and a grille polarise a wave.
- The energy of the incoming wave is absorbed by the particles of the filter
- Only a small amount of the energy is re-emitted and the direction is random
- this means that any recievers are extremely unlikely to recieve the energy of the waves
- When the filter and the oscillations are aligned, all the energy is absorbed, so the intensity the reciever recieves is 0
- When the filter and the oscillations are at a right angle to each other, a lot of the energy can still pass, so there is only a small drop in intensity
How does refraction work?
When a wave enters a different, denser medium at an angle to the normal, the part of the wave that enters the medium first will slow down, causing the wave to bend towards the normal inside the medium
Explain curvature and how it applies to lenses
As waves move away from the source, the wave fronts become straighter and straighter, as curvature is equal to the inverse of the radius, so as the radius tends to infinity, the curvature tends to 0.
Lenses add curvature to waves, so when uncurved waves enter the lens, they are given curvature centered on the focal point.
What is the focal length, how can the power of the lens affect it and explain why this happens
The focal length is the distance between the lens and where the image comes into focus. The power of the lens changes the distance of the focal length, as the power of the lens is equal the inverse of the focal length, therefore a larger power leads to a smaller focal length.
Explain the equation 1/v = 1/u + 1/f
The curvature of the wave to the image from the lens is equal to the curvature from the lens to the object plus the power of the lens
Explain the 3 special cases in regards to lenses
- If the distance from the lens to the source tends to infinity, the focal length and the distance from the lens to the image is equal
- if the source is at the focus of the lens, it starts with negative curvature, which is cancelled out by the lens, so the wavefronts are flat when leaving the lens and therefore the focal point tends to infinity
- The absolute value of u is equal to v when u is equal to 2 times the focal length |u| = |v| when u = 2f
Explain how to use 1/v = 1/u + 1/f
Invert the sign of u (as u is always negative), then substitute in your values.
What is the formula for linear magnification?
image height / object height
Describe how to find the focal point, and hence magnification, from an illustration
- Draw a normal to the lens
- Draw 3 straight lines from the top of the object to the top, middle and bottom of the lens
- On the other side of the lens, draw all three lines going through the focal length
- Measure the distance from the lens to where they all crossover, that’s the focal point
- The magnification is then the height of the image (from the normal to the focal point) divided by the height of the object
Describe the process to find the power of a lens
- Place a light source a distance away from a screen and then place a lens inbetween them
- Move the screen around until the image is focused, then measure the distance between the lens and the screen, which is v
- Record your results and repeat the test, moving the lens further away from the source each time (I.e. change u )
- Work out the inverse of v and u, then add them to get the inverse of the focal length, which is the power
How could you calculate the uncertainty of the power of a lens
Once you’ve found the point at which the image is focused, move it back and forth to the very edge of which you would consider the image focused, which is your uncertainty for v. Then when you calculate the power of the lens, the uncertainty for v is the uncertainty for the power of the lens.
