A person blows into a tube of length L that is open at one e

A person blows into a tube of length L that is open at one end and closed at
A person blows into a tube of length L that is open at one end and closed at the other.What is the wavelength of the first harmonic of the tube?
一道SAT2物理题，答案是4L不太明白，为什么....
能仔细讲下么？