Why does light has wave particle duality?
Photons of light exhibit properties of both pure energy and of matter.
One example of how light behaves as waves is the interference of light waves. They do not push each other out of the way; they simply cross over each other.
An example of how light behaves as matter is the photoelectric effect. A beam of light can loosen electrons in a metal IF the light photons have enough energy. This implies a quantitative difference in photons of light. Red light, no matter how intense, does not have enough energy to remove electrons. Violet light, no matter how dim, can remove these electrons. The electron flow can be measured with an ammeter in current.
A great website that has visuals you can manipulate is Phet, which is maintained by the University of Colorado at Boulder. It is awesome!!