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AS Physics PiA Spec > Imaging and Signalling > Flashcards

Imaging and Signalling Flashcards

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1
Q

Refraction

A

Occurs at a boundary when a wave changes speed. Bent towards/away from the normal. If light is slowing down (less optically dense to more optically dense), bends towards normal

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2
Q

Optical density

A

The more optically dense a material is, the slower light will travel in it

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3
Q

Absolute refractive index=

A

c/v (where c speed of light in a vacuum, and v is speed of light in the material)

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4
Q

What do lenses do?

A

Change the curvature of wavefronts by refraction.

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5
Q

Uncurved waves through a lens

A

If waves are uncurved when passing through a lens, and pass through at 90deg to the lens (parallel to the lens axis), they will be given spherical curvature, centred on the focal point of the lens

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6
Q

Focal length

A

Distance between lens axis and focal point.

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7
Q

Power of a lens=

A

1/f (f is the focal length. Power measured in dioptres, D. According to equation, more powerful - thicker - lenses have a shorter focal length, as they curve the wavefront more)

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8
Q

Power of a lens

A

How much curvature a lens adds to a wave passing through it

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9
Q

Curvature of a wave=

A

1/radius of curvature

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10
Q

Lens equation:

A

1/v = 1/u + 1/f (where v is the image distance, u is the object distance and f is the focal length)

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11
Q

What must you remember when using the lens equation?

A

Measure distances to the left of the lens axis as negative, and the distances to the right of the lens axis as positive - cartesian convention. Always measure distances from the lens axis

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12
Q

Linear magnification=

A

size of image/size of object = v/u

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13
Q

Wave equation:

A

v=f(lamda). Derived from v=d/t, where d is (lamda) and t is 1/f (speed is velocity)

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14
Q

Transverse waves

A

Medium oscillates at right angles to the direction of travel. E.g. EM waves

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15
Q

Longitudinal waves

A

Medium oscillates parallel to the direction of travel. E.g. sound waves

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16
Q

Bit

A

A single binary digit

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17
Q

Byte

A

8 binay digits - 8 bits

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18
Q

Number of alternatives=

A

2^number of bits (N=2^I)

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19
Q

Number of bits=

A

log(2)(number of alternatives) (I=log(2)(N)

20
Q

Pixels

A

Pixels are a string of binary numbers, coding for a specific colour/shade. Depending on how many bits there are in each pixel, there will be a varying number of shades/colours in the image

21
Q

Brightening an image

A

Adding a fixed positive value will brighten the pixels

22
Q

Improving contrast

A

Multiplying by a fixed positive value that is greater than 1 will improve the contrast

23
Q

Smoothing edges

A

Replace each pixel with the mean of its values and that of its neighbours

24
Q

Removing noise

A

Replace each pixel with the median of its value and those of its neighbours

25
Q

Finding edges

A

Multiply a pixel by 4, then subtract the N,S,E and W (directly above/below/to either side) neighbours - the Laplace rule

26
Q

False colour

A

Changing what a pixel codes for - e.g. a value of 2 may have been a dark grey, but could now be a deep orange. Can highlight important features in bright colours this way.

27
Q

Digital signals

A

Represented by binary numbers. Number of levels depends on number of bits. Limited in the values they can take

28
Q

Analogue signals

A

Vary continuously

29
Q

Advantages of digital

A

Sent, received and reproduced more easily than analogue (limited number of values)
Resistant to effects of noise
Can represent images and sound in the same way - a string of bits
Easy to process using computers (computers are digital devices too)

30
Q

Digitising analogue signals

A

Sample the signal at regular time intervals, and find the nearest digital value - round to the nearest level.

31
Q

Resolution of a digital signal

A

Determined by the number of bits. The difference between the possible digital values. How close he analogue signal is to the digital signal - how much the level changes

32
Q

Disadvantages of a high resolution

A

If resolution is too high, then noise in the signal will be picked up, which is not useful

33
Q

Maximum number of bits=

A

log(2)(Total variation/Noise variation) (b=log(2)(Vtotal/Vnoise))

34
Q

Sampling rate

A

If you sample too infrequently, details of high frequency will be lost. Low sampling rates create aliases - frequencies that weren’t in the original signal

35
Q

Minimum sampling rate=

A

2 x maximum frequency of signal

36
Q

Disadvantage of digital

A

Can’t reproduce analogue signals exactly, some information will be lost

37
Q

Spectrum

A

The frequencies that make up a signal. Spectra graphs are line charts (bars that are lines), with the height of the voltage

38
Q

Analogue bandwidth

A

Range of frequencies within the signal. Highest frequency - lowest frequency

39
Q

Carrier waves

A

How communication signals are transmitted. Have to separate actual signal from carrier signal to receive information

40
Q

Limits of bandwidth

A

If there is only a limited amount of frequencies to transmit on (which there is), the bandwidth of each frequency limits how many channels can be transmitted. Signals can’t overlap.

41
Q

Rate of transmission=

A

samples per second x bits per sample (rate in bits s^-1)

42
Q

Polarised wave

A

Only oscillates in one direction (perpendicular to plane on travel)

43
Q

Polarising a wave

A

Passing a wave through a filter will polarise it. Only transverse waves can be polarised. 2 polarising filters at right angles will completely block a wave, as all directions of vibration will be blocked. Reflecting a wave/passing it through cloudy water will polarise it (can’t oscillate in direction of travel).

44
Q

Uses of polarised light

A

Radio and TV signals are polarised
Communications satellites use different polarisations for signals of same frequency bandwidth, reducing interference
Bees use polarised light to navigate
CD players use polarising crystals

45
Q

Polaroid filters

A

A line of horizontal rods will absorb horizontally polarised waves, as the wave will cause electrons in the rods to oscillate. However, vertically polarised light can pass through.

46
Q

Information in an image=

A

number of pixels x bits per pixel

47
Q

Digital bandwidth

A

1/2 rate of transmission or rate of transmission

AS Physics PiA Spec (3 decks)

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