Chapter 1 Flashcards
What is the equation for the curvature of a circle?
curvature = 1/r
What is the curvature when r = ∞?
0
What are waves are called when they come from a very distance point?
Plane wave-fronts
Light can be thought of as [ ] fronts or [ ]
Wave fronts
Rays
Which way does a ray of light point?
Points along the direction of motion of the wave-front and is always at right-angles to the wave-front.
Draw light from a small source and some from a very distant point. (Ray point of view)
Draw light from a small source and some from a very distant point. (Wave point of view)
See book. Pg 19
https://www.kerboodle.com/api/courses/15026/interactives/109567.html
What is a converging lens?
A lens that focuses light to a point behind it. It can be used to concentrate light rays. In ray terms, the lens bends the parallel light rays focusing them together at the focus. In Wave terms, the lens works by altering the curvature of the waves so that they converge on a focus.
What is the distance between the lens and the focus?
The radius of the wave fronts just after passing through the lens.
Focal Length, f
How much curvature does a converging lens add?
1/f
Lens power (D) =
Lens power (D) = 1 / focal length (m)
Why does a converging lens make an image of the source of light (object)?
Because light from every part of the object goes through the lens
Curvature of waves leaving the lens =
curvature of waves entering it + curvature added by the lens
What does v represent?
The distance from the lens to the image of the source
What does u represent?
The distance from the lens to the source
After passing through thee lens, the waves form part of [ ] centred on a point at distance [ ]. So the curvature of the waves leaving the lens = [ ] and the curvature of the waves entering the lens = [ ]
spheres
v
1/v
1/u
1 / v =
1/v = 1/u + 1/f
(Lens maker’s equation)
NB: ‘u’ is negative like on a number line.
What is the focal point? Focal length?
The point at which parallel waves from a very distance object are brought to focus.
The focal length is the distance from the centre of the lens to the focal point, and is a constant for a particular lens with a fixed shape.
Waves from a nearer object are brought together …
beyond the focal point.
The image distance v is [ ] than the focal length f, except for [ ]
greater
very distant objects
Draw diagrams for a very distance object, an object just beyond the focus and an object at the focus
Pg 23
What does it mean to magnify?
The size of the image appears larger than the original object.
Linear magnification m =
Image height (m) / Object height (m)
or
Image distance v (m) / Object distance u (m)
What does it mean if the magnification is negative?
The object is inverted
Describe where an image is focused in a digital camera
light - sensitive microchip called a charge-coupled device (CCD)
What is a CCD?
A screen covered by millions of tiny ‘picture elements’ called pixels
Explain how pixels work
Each pixel stores electric charge when light falls on it - the brighter the light falling on the pixel, the greater the charge stored on it. The image becomes an array of numbers, which can then be manipulated to edit the image
What is one bit of information?
One pixel simply records ‘high’ or ‘low’, then only one memory location storing a 1 or 0 is needed
What colour colour scale is usually used?
256 grey-scale. From 0 (black) to 255 (white)
What is a group of 8 bits called?
A byte
How many bits do you need for a 256 grey-scale?
8
Work out the binary representation of 5, 9, and 26
5 = 0101 9 = 1001 26 = 11010
What is the equation for working out the number of arrangements of bits N? And working out the number of bits available b?
N = 2ᵇ b = log₂N
Define Resolution
The scale of the smallest detail that can be distinguished.
Resolution =
Width of object in image / No. of pixels across object
Amount of information in an image =
No. of pixels * Bits per pixel
Explain how to change the brightness of an image
A dim image can be brightened by increasing the value on each pixel by the same amount until the brightest pixel in the image is coded at 255
Explain how to remove the noise of an image
Noise can be reduced by smoothing, where the value of each pixel is replaced with the median or mean of its value and those around it.
Explain how to edge detect an image
To enhance edges in an image the average value of the pixel’s neighbours is subtracted from each pixel. This removes uniform areas of brightness and picks out the places where the gradient of the brightness changes abruptly - at the edges
Explain how to change the contrast of an image
An image with little contrast will not use the full range of pixel values. Eg, using the range 76-100. To improve the contrast this range is stretched across the 256 possible values so that the the value 76 becomes 0 and 100 becomes 255.
What is noise in an image?
Noise in images refers to the random speckles across the image
Unwanted interference affecting a signal
Give an example of something with polarising lenses. Why do they have them?
Snow goggles and sunglasses. It dramatically cuts down the glare in bright environments
Waves: Frequency f (Hz) =
1 / T
where T = time period (s)
What is the speed of all electromagnetic waves?
c = 3.00 * 10⁸ m/s
List the electromagnetic waves from largest to smallest and the wavelengths (m)
Radio (>10^6 - 10^-1), Micro- (10^-1 - 10^-3), Infrared (10^-3 - 710^-7), Visible (7-4 * 10^-7), Ultraviolet (410^-7 - 10^-8), X-rays (10^-8 - 10^-10), Gamma rays (10^-13 - <10^-16)
Electromagnetic waves can be polarised. This is a property of [ ] waves.
Transverse
Describe polarised and unpolarised transverse waves
Polarised: They vibrate in one plane only
Unpolarised: Vibrate in randomly changing plane
Electromagnetic waves are waves of [ ] magnetic and [ ]. The two fields are at right-angles to [ ] and to the [ ].
oscillating magnetic and electric fields
Each other and the direction of travel of the wave
Can all electromagnetic waves be polarised?
Yes
What does it mean if a wave is plane-polarised? Unpolarised?
The direction of oscillation remains fixed.
The direction of the oscillation will not be fixed
Unpolarised light is polarised when it is passed through a [ ]
Polarising filter (look at the diagram on pg 31)
How can you detect whether light has been polarised?
By observing it through a single polarising filter and rotating the filter. If the intensity of the light remains constant, the light source is emitting unpolarised light. If the intensity varies as the filter rotates, the source is emitting polarised light.
How can you detect polarised radio waves?
By rotating the receiving aerial. The aerial will pick up the strongest signal when it is set up parallel to the plane of polarisation of the radio waves
EM radiation is made up of …
2 transverse waves vibrating in different directions
Light that has passed through the polarising filter will only be vibrating in …
One direction
Describe microwaves going through a metal grille (Short)
The intensity drops to 0 when the wires are aligned with the direction of polarisation because the grille is absorbing the energy
Describe microwaves going through a metal grille (Extended)
1) The virbrating electric field of the microwaves 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 those re-emitted waves are vibrating in the direction of the microwave receiver
4) The receiver only receives microwaves in one plane so even if the re-emitted waves travels towards the receiver it might not be picked up
5) When the wires and vibrations of waves are aligned, more electrons are excited than when they are at right angles to each other - all the energy is absorbed and the intensity reading drops to 0
6) When the wires and vibrations are at right angles to each other, some electrons in the grille are still excited and so there is a small drop in intensity
How do converging lenses work?
They change the curvature of wavefronts by refraction
It adds curvature.
It curves the wavefronts by slowing down the light travelling through the middle of the lens more than lights at the edges
All points on a wavefront take the same amount of time to reach the focus
What is false colour used for?
Highlight certain features
Used in remote sensing of data
How do you ‘false colour’?
False colour is where you add different values to different colour values in order to accentuate that feature colour
