Question

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

Answer 1

-- 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 .

Answer 2

Scientists believe that light is made of streams of particles because light exhibits the particle nature. This can be explained on the basis of photoelectric effect observed when light is made to fall on the surface of a metal. when light falls on the surface of a metal, it makes the electron to eject out. This is possible only when light consists of small particle called photons. hence the scientists believe that light is made of streams of particles.