First suggested the existence of atoms by speculating that matter is composed of elementary particles called atoms.
Proposed the first modern atomic theory
Developed the electromagnetic wave theory (which came to be the classical theory of light).
Experimentally proved Maxwell's electromagnetic wave theory, and discovered the photoelectric effect.
Provided evidence for the existence of the electron and developed what was to be known as the "plum-pudding" (or "blueberry muffin") model of the atom.
Regarded as the founder of quantum theory. He studied the light emitted by hot objects and his experiments led him to hypothesize that energy could be gained or transferred in whole-number multiples.
Determined the charge of an electron.
Disproved Thomson's model of the atom, and established his own more accurate model of the atom, which states that an atom has a nucleus, which is a dense, positively charged atomic centre, that is surrounded by electrons moving around at a far distance.
Developed a quantum model for the hydrogen atom, which became accepted as the most accurate model of the atom (before being proven to be incorrect), known as the Bohr model of the atom.