Why do scientists believe that light is made of streams of particles?

- - When you build an experiment that's designed to detect waves,

light behaves like waves.

- - When you do an experiment that's designed to detect particles,

light behaves like particles.

- - The whole basic question that scientists were trying to answer

about 150 years ago is:

Is light made of particles or waves?

and the answer turned out to be:

Yes.