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[PHYSICS] Stroboscope

When viewing a transverse wave through a stroboscope with a frequency 10 Hz, the waves seem to be frozen, but the wavelength does change. If given that the frequency of the waves is within 39 Hz, state two possible frequency of this wave.

我寫5 Hz 同2.5 Hz, 但係佢話係10 同20. 唔明....



But the wavelength does change...

Why the wavelength changes??? I don't understand why the wavelength changes and the wave is frozen.

1 Answer

  • 天同
    Lv 7
    1 decade ago
    Favorite Answer

    The travelling wave wil be viewed as frozen (i.e. stationary) when a crest has just travelled a distance of one wavelength, such that it is now at the position of the crest ahead of it.

    But the time taken for the crest to travel one wavelength forward is one period. Therefore, if the wave period is 1/10 s, equal to that of the stroboscope, the wave will appear as frozen. Thus, 10 Hz is the first wave frequency.

    Suppose the crest travels exactly two wavelengths, it will be at the position of the crest two wavelengths ahead of it. The time required is two periods, thus 2T = 1/10 s or T = 1/20 s , which corresponds to a wave of frequency 20 Hz

    5 Hz is definitely not the answer, as a crest takes 1/5 s = 0.2 s to travel one wavelngth forward. But the stroboscope flashes at intervals of 0.1 s, thus when the stroboscope flashes, the crest only travels half a wavelength, at the position of the trough ahead of it. The wave doesn't appear frozen.

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