Newton believed that light is a particle because: It travels in straight lines. It can travel through a vacuum.

Particles always travel in straight lines and it was observed that light also travels in straight lines. Waves however are able to bend around obstacles. Light cannot diffract after going around an obstacle or through an opening. So if we look through a series of small holes at a light source we will only be able to see the light if all the holes are inline. If one hole is shifted we will no longer be able to see the light.

 In Newton's time, the only waves that anybody knew much about were mechanical waves, which need a substance to move through (e.g. sound waves travel through air).  Since they also had a pretty good idea that the space between the earth and the sun was a vacuum, how could light reach earth if it was a wave ?  If light is a particle it would have no trouble moving through a vacuum.

 Newton came up with the idea of light consisting of small ‘packages’ of energy called corpuscles (particles of matter) which are emitted in all directions from a source.  Albert Einstien later put forth the idea of a quantum of light. A quantum is just a small, indivisible bundle of energy. In essence, he was thinking of light as a particle. Light wasn't continuous, but came in little specks.  The quantum of light is called the photon.  Higher frequencies of light are composed of higher energy photons. This is expressed in equation: E=hf Where E is energy, h is Planck’s constant and f is frequency

 Today we say that light has the properties of both waves and particles.  Light behaves as a particle in any experiment that tries to prove its particle nature. However, it behaves like a wave in any experiment that tries to prove it is a wave.