What is the reason for the Phenomenon happeningin the video clip? Let's check the chart in the video clip again.

The experiment shows that the scale is higher as the length of the test tube without water is shorter. If you remember that the pitch is higher as the string is shorter and the frequency is larger, we can say that the air in the waterless space in the testtube is vibrating and producing the sound.

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## Basic formula of wave

The number of vibrations of the sound source per second is called frequency.The length of one wave is called wavelength-the unit is λ.The formula to calculate the speed of wave is:**v = fλ**

It is pronounced "v equal f ramuda". we assume that one wavelength is 1 and that the frequency is 3(per secomd), the speed or the distance the wave moves per secomd is :3(f)×1(λ)=3(v)

If you refer to the illustration above, it is three waves in one second and each wavelength(λ) is one . So the distance the wave moves in one second is:3×1=3 The formula v=fλ is correct.

Please do the following exercise.

**Question**What is the wavelength of the sound? The frequency is 640Hz and the speed of sound is

**Answer**The formula is v=fλ In this case v=340 and f is 640. 340=640×? The answer is 0.5

A.0.5（m）

**For summary**

・If you poor some water into test tube and blow air into it, the waterless space in the test tube vibrates.

・The formula to calculate the speed of wave or the speed of sound is v=fλ. v is the speed, f is frequency, and λ is wavelength, pronounced "ramuda".

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