Ch3

Question 1

what is a transverse wave

Answer 1

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

Question 2

examples of transverse waves

Answer 2

em waves, ripples on water and waves on ropes

Question 3

draw the two ways of drawing transverse waves

Answer 3

one s against d and one s against time

Question 4

what is a unpolarised light

Answer 4

a light wave which is vibrating in more than one plane

Question 5

what is polarised light

Answer 5

where light vibrates in one plane only

Question 6

what does the polariser do

Answer 6

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

Question 7

why does polarising occur

Answer 7

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.

Question 8

what does the analyser do

Answer 8

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

Question 9

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

Answer 9

the intensity of light rises and falls to zero

Question 10

polaroid sunglasses

Answer 10

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

Question 11

why can’t sound waves be polarised

Answer 11

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

Question 12

how do long chain molecules in a filter let light through

Answer 12

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

Question 13

draw a diagram to explain how unpolarised light becomes polarised

Answer 13

look in folder

Question 14

explain how light becomes partially polarised when reflected

Answer 14

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.

Question 15

what are digital signals represented by

Answer 15

binary numbers

Question 16

what do the values of a digital signal depend on

Answer 16

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

Question 17

what are analogues signals not limited in

Answer 17

the values they can take-they vary continuously

Question 18

what happens when you transmit an electronic signal

Answer 18

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

Question 19

how can analogue signals be digitised

Answer 19

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.

Question 20

what does the quality of a digitised signal depend on

Answer 20

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

Question 21

the higher the resolution

Answer 21

the more closely the digitised signal will match the original

Question 22

what is resolution determined by

Answer 22

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

Question 23

how many bits used to digitise CDs

Answer 23

16 bits used

Question 24

what does noise limit

Answer 24

the number of bits used for sampling

Question 25

maximum number of bits=

Answer 25

log2(total variation/noise variation) | variation is measured in volts

Question 26

minimum sampling rate=

Answer 26

2 x maximum frequency of signal

Question 27

why does the sampling rate have to be high

Answer 27

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

Question 28

what can a low sampling rate create

Answer 28

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

Question 29

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

Answer 29

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.

Question 30

signals are made up of lots of different...

Answer 30

frequencies

Question 31

what are the frequencies that make up a signal called?

Answer 31

its spectrum

Question 32

if you want to reconstruct a signal

Answer 32

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

Question 33

what is bandwidth

Answer 33

the range of frequencies within a signal

Question 34

how can you find a signal's bandwidth?

Answer 34

by subtracting the lowest frequency within it from the highest frequency

Question 35

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

Answer 35

how many signals can be sent at the same time

Question 36

how are communications signals transmitted

Answer 36

using carrier waves

Question 37

what happens at the radio station

Answer 37

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

Question 38

if your radio is tuned to the right frequency

Answer 38

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

Question 39

all radio stations are given what

Answer 39

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

Question 40

what does bandwidth limit

Answer 40

the number of signals that can be transmitted

Question 41

the larger the bandwidth

Answer 41

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

Question 42

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

Answer 42

0.2 MHz apart and the signals are filtered

Question 43

what does filtering a signal mean

Answer 43

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

Question 44

how do u work out the number of stations?

Answer 44

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

Question 45

rate of transmission=

Answer 45

samples per second x bits per sample

Question 46

why must bits per sample be high

Answer 46

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