  Coherence length is an important feature of a real light wave. From the
Michelson interference of He-Ne laser beam, it is found that the coherence
length of the beam decreases with the decrease of intensity when the laser beam
passes through a nonselective absorption filter and the intensity becomes low
enough. This is a novel experimental phenomenon in optics, which verifies the
characteristic of discrete wavelet structure of light for the first time.
