unit 2 wave ( doppler efffect ) Flashcards
doppler effect
change of apparent frequency of wave caused by an observer moving relative to the source
red shift
- doppler effect
- apparent decrease in frequency due to the separation of the source and the observer
- the wavelength appears shifted towards the red end of spectrum is because the distance travelled by each successive wave has increased
blue shift
- doppler effect
- apparent increase in frequency due to the approaching of the source and the observer
- the wavelength appears shifted towards the blue end of spectrum is because the distance travelled by each successive wave has decreased
doppler
sound ultrasound electromagnetic wave radar speed gun remote sensing
pg 43, 44, 45,46.47
doppler effect
pulse echo technique ( ultrasound )
method describe
a signal pulse is sent towards the object and reflected wave is analysed
doppler effect
pulse echo technique ( ultrasound )
we can detect… from
time of reflection
change in wavelength
time reflection
- find the distance of boundary from the source
- distance = 1/2 speed of wave time time of reflection
change in wavelength
- the speed of object reflecting the waves
doppler effect
pulse echo technique ( ultrasound )
why transmitted in pulses
- one pulse must return before the next is sent
- otherwise it is hard to tell which bit of reflected sound originated with which bit of emitted sound
- no overlap btwn pulses
- no interference btwn pulses
doppler effect
pulse echo technique ( ultrasound )
how to avoid overlapping btwn pulses?
time btwn pulses being emitted must be greater than the time interval btwn sending out pulse and receiving its echo
doppler effect
pulse echo technique ( ultrasound )
why shorten the interval btwn pulses being emitted when near to the object
- the distance is shorter so time taken to receive echo is shorter
- time btwn pulses being emitted can be shorten and still able to avoid overlapping btwn pulses
- more frequent pulses allow rapid motion to be preceived
- allow position to be determined precisely / accurately
doppler effect
pulse echo technique ( ultrasound )
explain how ultrasound pulses can be used to build up the image of the fetus in the uterus
- pulses reflected at the boundary
- time for echo pulse to return measured
- distance calculated from time to return
- ultrasound travels as pulses so that one pulse is detected before the next pulse is sent
- probe moved to diff positions
- many pulses are sent
- distance and image of fetus can be plotted
doppler effect
pulse echo technique ( ultrasound )
image quality
sharpness of each reflection depends on
the duration of the transmitted pulse
if the pulse is too long , echo will overlap
if the pulse is too short, energy is small and reflections may be too weak to be detected
best pulse length is the balanced btwn the two
doppler effect
pulse echo technique ( ultrasound )
image quality
resolution =
the smallest details detected
half the length of a pulse
pulse length = speed times time pulse is on
resolution = 1/2 times pulse length
doppler effect
pulse echo technique ( ultrasound )
image quality
shorter pulse =
smaller thickness
different in distance can be measured
doppler effect
pulse echo technique ( ultrasound )
image quality
diffraction will occur if
the wavelength used is too large
resolution will decrease
ultrasound will just resolve details of the same size as its wavelength
resolution = wavelength
doppler effect
pulse echo technique ( ultrasound )
image quality
resolution of ultrasound is higher in air than water
why
- ultrasound speed in air is lower than speed in water
- with same ultrasound frequency, wavelength in air is shorter than in water
- echolocation will detect smaller targets due to less diffraction
