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

Brief Explanations:

The photoelectric effect provided evidence. When light shines on a metal, electrons are ejected. The fact that the ejection depends on the frequency (not intensity) of light suggests light has particle - like behavior. Also, phenomena like Compton scattering further support the particle nature of light, where photons interact with electrons like particles.

Answer:

Phenomena such as the photoelectric effect and Compton scattering provide evidence for the particle - like nature of light.