Niels Bohr was a physicist whose genius provided the first quantum-based model of the atom. Before Bohr, scientists couldn't explain why atoms didn't just collapse. Bohr proposed that electrons move in fixed orbits and only emit or absorb energy when they jump between these orbits. This explained the 'spectral lines'—the specific colors of light emitted by elements. He also formulated the Principle of Complementarity, which states that subatomic particles can be viewed as both waves and particles, though not at the same time. Bohr's institute in Copenhagen became the world capital of theoretical physics, and he was a central figure in the development of quantum mechanics and nuclear energy.