#### primarygun

If a wave with wavelength 1.5 cm and frequency 8Hz, is viewed through a stroboscope with 6Hz, what's the change of its wavelength?
My answer is not identical to what in my text book.
My thought is :there is no change of the wavelength, as the time given is enough for the wave to move over one wavelength.

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#### primarygun

anyone helps?" Need to help my friend

#### geosonel

primarygun said:
If a wave with wavelength 1.5 cm and frequency 8Hz, is viewed through a stroboscope with 6Hz, what's the change of its wavelength?
My answer is not identical to what in my text book.
My thought is :there is no change of the wavelength, as the time given is enough for the wave to move over one wavelength.
are you sure the problem requests the change in apparent wavelength, and not the change in apparent frequency or apparent velocity?
btw, what answer does the textbook give?

#### primarygun

Thanks for kind response.
It is the observed wavelength and the answer is 0.75 cm, half of the wavelength.

#### geosonel

primarygun said:
If a wave with wavelength 1.5 cm and frequency 8Hz, is viewed through a stroboscope with 6Hz, what's the change of its wavelength?
My answer is not identical to what in my text book.
My thought is :there is no change of the wavelength, as the time given is enough for the wave to move over one wavelength.
from the stated problem, it seems you'd perceive an apparent change in frequency, not in wavelength. it seems like you'd be able to see the physical extent of the wave and be able to measure the wavelength directly during each strobe pulse. thus, you'd not detect any apparent change in wavelength.

on the other hand, the frequency would appear to change. because the strobe frequency of 6 Hz is not aligned (or "synced") with the wave frequency of 8 Hz, the viewer would only see a wave peak return to a given position every 1/2 second (= 3 strobes @ 6 Hz = 4 wave cycles @ 8 Hz). that's the shortest elapsed time for which the 6 Hz strobe can coincide with the 8 Hz wave's peak returning to a given position:
Code:
strobe     wave peak @ given position
(sec)         (sec)
0 **       ** 0
1/6           1/8
2/6           2/8
...          3/8
3/6 **     ** 4/8  ---- wave period would appear to be 1/2 sec
wave period would appear to be 1/2 sec, so freq would appear to be f=(1/Period)=(2 Hz).
thus, problem answer seems to be:
wave frequency would appear to change from 8 Hz to 2 Hz .

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