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physics as > Ch3 > Flashcards

Ch3 Flashcards

1
Q

what is a transverse wave

A

a wave where the vibration is at right angles to the wave’s direction of travel

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2
Q

examples of transverse waves

A

em waves, ripples on water and waves on ropes

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3
Q

draw the two ways of drawing transverse waves

A

one s against d and one s against time

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4
Q

what is a unpolarised light

A

a light wave which is vibrating in more than one plane

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5
Q

what is polarised light

A

where light vibrates in one plane only

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6
Q

what does the polariser do

A

it allows light waves through only if the plane of polarisation of the waves is in a certain direction. the light transmitted by the polariser is therefore polarised

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7
Q

why does polarising occur

A

because the molecules in the filter align with each other and they only transmit light waves that are polarised in the same direction as the molecules.

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8
Q

what does the analyser do

A

it cuts out all the light from the polariser if the filters are aligned at 90 degrees to each other

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9
Q

what happens as a result of rotating the analyser by exactly one-half turn

A

the intensity of light rises and falls to zero

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10
Q

polaroid sunglasses

A

they cut out glare due to reflected sunlight because light is polarised when it is reflected from the water.

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11
Q

why can’t sound waves be polarised

A

because polarisation can only happen for transverse waves and sound waves are longitudinal- vibrations are in the same plane.

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12
Q

how do long chain molecules in a filter let light through

A

if they are vertical then they will absorb light in the vertical direction and let light through horizontally

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13
Q

draw a diagram to explain how unpolarised light becomes polarised

A

look in folder

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14
Q

explain how light becomes partially polarised when reflected

A

if you direct a beam of unpolarised light at reflective surface then view the ray through a polarising filter, the intensity of light leaving the filter changes with the orientation of the filter. the intensity changes because light is partially polarised when reflected.

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15
Q

what are digital signals represented by

A

binary numbers

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16
Q

what do the values of a digital signal depend on

A

the number of bits used eg one bit signal can only take the values 0 and 1, but a one byte signal can take 256 different values

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17
Q

what are analogues signals not limited in

A

the values they can take-they vary continuously

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18
Q

what happens when you transmit an electronic signal

A

it will pick up noise from electrical disturbances or other signals. the receiver needs to be able to reconstruct the original signal from the noisy signal if they’re to get an accurate representation of what was sent

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19
Q

how can analogue signals be digitised

A

it is called digitising the signal. u take the value of the signal at regular time intervals, then find the nearest digital value. each digital value is represented by a binary number, so you can convert the analogue values to binary numbers.

20
Q

what does the quality of a digitised signal depend on

A

its resolution- the difference between the possible digital values and the sampling rate

21
Q

the higher the resolution

A

the more closely the digitised signal will match the original

22
Q

what is resolution determined by

A

the number of bits in the binary numbers representing the digital values- the greater the number of bits, the greater the resolution

23
Q

how many bits used to digitise CDs

A

16 bits used

24
Q

what does noise limit

A

the number of bits used for sampling

25
Q

maximum number of bits=

A

log2(total variation/noise variation)

variation is measured in volts

26
Q

minimum sampling rate=

A

2 x maximum frequency of signal

27
Q

why does the sampling rate have to be high

A

the ensure that all the frequencies within its spectrum are transmitted accurately

28
Q

what can a low sampling rate create

A

low frequency signals called aliases that weren’t there in the original signal at all.

29
Q

what are the 4 main advantages of digital signals over analogue signals

A

1) digital signals can be sent, received and reproduced more easily than analogue signals because they can only take a limited number of values
2) d signals are resistant to the effects of noise
3) d signals can be used to represent different kinds of info in the same way eg images and sounds can both be represented as string of bits
4) d signals are easy to process using computers since they are digital devices too
BUT d signals can never reproduce analogue signals exactly-some info will always be lost.

30
Q

signals are made up of lots of different…

A

frequencies

31
Q

what are the frequencies that make up a signal called?

A

its spectrum

32
Q

if you want to reconstruct a signal

A

you need to know all of the frequencies within it as they carry information so if you lose any of the frequencies, you won’t get all the information from the signal

33
Q

what is bandwidth

A

the range of frequencies within a signal

34
Q

how can you find a signal’s bandwidth?

A

by subtracting the lowest frequency within it from the highest frequency

35
Q

in communications systems what does the bandwidth of each signal determine?

A

how many signals can be sent at the same time

36
Q

how are communications signals transmitted

A

using carrier waves

37
Q

what happens at the radio station

A

the audio signal from the presenter is converted to an electronic signal which is them mixed with a carrier wave and the combined signal is transmitted

38
Q

if your radio is tuned to the right frequency

A

it receives the signal from the radio station- it is able to separate the actual signal from the carrier wave and then convert this back into sound

39
Q

all radio stations are given what

A

a particular carrier frequency to broadcast their signal on.they have to be all different so they don’t interfere with each other

40
Q

what does bandwidth limit

A

the number of signals that can be transmitted

41
Q

the larger the bandwidth

A

the larger the gap must be to stop signals at neighbouring carrier frequencies overlapping

42
Q

what are the carrier f’s for radio stations at least

A

0.2 MHz apart and the signals are filtered

43
Q

what does filtering a signal mean

A

very high or low frequencies are removed to make sure they don’t exceed the carrier f range

44
Q

how do u work out the number of stations?

A

highest f-lowest f/ carrier f gap(0.2MHz)

45
Q

rate of transmission=

A

samples per second x bits per sample

46
Q

why must bits per sample be high

A

so that the transmitted signal closely matched the original, but not so high that it is negatively affected by noise

physics as (7 decks)

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