Light behaves like a wave and a particle, and can even be described as both a wave and a particle at the same time. This dual nature of light has confused some people, but it shouldn't. It is actually a good thing that light can behave both as a particle and a wave at the same time, because it allows us to test phenomena such as the double slit experiment.
Whenever a wave front encounters something while propagating, such as a focal point or matter, it can lose (attenuate) some of its energy or magnitude. This is because the "positions" of the particles in the wave front are virtual, and these positions can be lost as a function of the distance from the focal point. When this happens, the wave distribution changes. This is called the law of diffraction. You can see this effect as a "smearing" of the image of an object as it moves away from the viewer. This is what gives rise to the term "wave-particle duality".
