Ch3 Flashcards
what is a transverse wave
a wave where the vibration is at right angles to the wave’s direction of travel
examples of transverse waves
em waves, ripples on water and waves on ropes
draw the two ways of drawing transverse waves
one s against d and one s against time
what is a unpolarised light
a light wave which is vibrating in more than one plane
what is polarised light
where light vibrates in one plane only
what does the polariser do
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
why does polarising occur
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.
what does the analyser do
it cuts out all the light from the polariser if the filters are aligned at 90 degrees to each other
what happens as a result of rotating the analyser by exactly one-half turn
the intensity of light rises and falls to zero
polaroid sunglasses
they cut out glare due to reflected sunlight because light is polarised when it is reflected from the water.
why can’t sound waves be polarised
because polarisation can only happen for transverse waves and sound waves are longitudinal- vibrations are in the same plane.
how do long chain molecules in a filter let light through
if they are vertical then they will absorb light in the vertical direction and let light through horizontally
draw a diagram to explain how unpolarised light becomes polarised
look in folder
explain how light becomes partially polarised when reflected
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.
what are digital signals represented by
binary numbers
what do the values of a digital signal depend on
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
what are analogues signals not limited in
the values they can take-they vary continuously
what happens when you transmit an electronic signal
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
how can analogue signals be digitised
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.
what does the quality of a digitised signal depend on
its resolution- the difference between the possible digital values and the sampling rate
the higher the resolution
the more closely the digitised signal will match the original
what is resolution determined by
the number of bits in the binary numbers representing the digital values- the greater the number of bits, the greater the resolution
how many bits used to digitise CDs
16 bits used
what does noise limit
the number of bits used for sampling
maximum number of bits=
log2(total variation/noise variation)
variation is measured in volts
minimum sampling rate=
2 x maximum frequency of signal
why does the sampling rate have to be high
the ensure that all the frequencies within its spectrum are transmitted accurately
what can a low sampling rate create
low frequency signals called aliases that weren’t there in the original signal at all.
what are the 4 main advantages of digital signals over analogue signals
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.
signals are made up of lots of different…
frequencies
what are the frequencies that make up a signal called?
its spectrum
if you want to reconstruct a signal
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
what is bandwidth
the range of frequencies within a signal
how can you find a signal’s bandwidth?
by subtracting the lowest frequency within it from the highest frequency
in communications systems what does the bandwidth of each signal determine?
how many signals can be sent at the same time
how are communications signals transmitted
using carrier waves
what happens at the radio station
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
if your radio is tuned to the right frequency
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
all radio stations are given what
a particular carrier frequency to broadcast their signal on.they have to be all different so they don’t interfere with each other
what does bandwidth limit
the number of signals that can be transmitted
the larger the bandwidth
the larger the gap must be to stop signals at neighbouring carrier frequencies overlapping
what are the carrier f’s for radio stations at least
0.2 MHz apart and the signals are filtered
what does filtering a signal mean
very high or low frequencies are removed to make sure they don’t exceed the carrier f range
how do u work out the number of stations?
highest f-lowest f/ carrier f gap(0.2MHz)
rate of transmission=
samples per second x bits per sample
why must bits per sample be high
so that the transmitted signal closely matched the original, but not so high that it is negatively affected by noise
