cartoon: "Once and for all I want to know what I'm paying for. When the electric company tells me whether light is a wave of a particle, I'll write my check."

From Sidney Harris,, see also

Actually, in everyday life the distinctions between wave and particle behavior are seldom observed directly, which is why the concept did not become firmly established until early in the 20th century. Then, it was found that not only light but the fundamental particles such as electrons and protons also have wave-particle duality, resulting in behavior that can only be described by the branch of physics called quantum mechanics. Once the theory was established, it was found to unite and explain a huge number of things that do have everyday implications. An example coming up soon is the structure of the atom and why a given type of atom emits and absorbs light only at very specific wavelengths.