Today let me introduce a word ‘coherence’ to all of you. Coherence means how similar you are.

Formal Definition

In physics, two wave sources are perfectly coherent if they have a constant phase difference and the same frequency, and the same waveform.

Coherence is an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference.

It contains several distinct concepts, which are limiting cases that never quite occur in reality but allow an understanding of the physics of waves, and has become a very important concept in quantum physics.

More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets.


I have two friends who come from China. They didn’t know each other until meet by chance in Asian Art Museum. After that, they talked about art, culture, and went hiking in Redwoods, had a lot of fun in SFMoMA. Of course, the authentic food in China Town. Yes, they truly fall in love with each other.

In this example, each person is a light beam. Asian Art Museum is the spatially constraint. The similar age, similar life experience and the most importantly, the similar philosophy of life are the temporally constraint.

It make sense to me that they will fall in love with each other. That’s how it works with laser, all photons has same frequency, same waveform(propagation direction) and constant phase, or at least similar ones.

Other case, my other couple of friends, more like hippies. Extremely free spirit is what they want. They will meet others, talk with the new meet, flirt with the news, and then leave them. They never settle down. They are random light beams. All of these properties are the personalities of a person. It can happen to everyone, however, however, once they meet the right one with them, it would be the final date. I know it’s hard for them, optically, we make use of incoherence, to make extreme conditions for them to be interfered with others. Once the inference fringes occurs, we extract them and reconstruction processing will get the high resolution images we want.


That’s what I think of coherence, hope you can be coherent with this word.