Absorption is just the opposite of emission, so instead of starting at a high energy level and dropping down, when we absorb light we start low and we absorb energy to bring ourselves up to an n final that's higher.

吸收就是发射的逆过程,与从一个高能量到低能量不同，当吸收光时，我们从低能量开始,吸收能量到一个更高的能量。

麻省理工公开课 - 化学原理课程节选

So, for example, we have this average level, and then it can go high where we have the peak, or it can go very low. We can also discuss sound waves, so again it's just the periodic variation of some property -- in this case we're talking about density, so we have high density areas and low density areas.

例如这是平均位置，在峰的位置水位高，在谷水位低，我们也可以,讨论声波它也是某种,量的周期变化，在这里我们,讨论的是密度,我们有高密度区和低密度区。

麻省理工公开课 - 化学原理课程节选