For example, you could put a dam in the middle of the tank.

举个例子,在水槽中间放个屏障。

麻省理工公开课 - 固态化学导论课程节选

If this distance is large in comparison to the wavelength, this dam simply casts a shadow.

如果这个距离,比波长更长，水槽中的屏障就会投下阴影。

麻省理工公开课 - 固态化学导论课程节选