At the turn of the 20th century, new discoveries about the nature of light contradicted old models, creating controversy among physicists. During those tumultuous years, scientists such as Max Planck and Albert Einstein developed a modern theory of light. It not only showed that light behaves as both a wave and a particle, but also led to new ways of thinking about the entire Universe.

According to the modern theory, light has a dual nature. Because it has waves, sunlight passing through a distant rainstorm makes a rainbow. However, when light strikes a solar cell, it delivers energy as a series of very small bursts. Particles of matter have names such as the proton, electron and neutron. Particles of light are called photons; each is a tiny, discrete bundle whose energy is determined by the light wavelength: the shorter the wavelength, the greater the energy.

In 1905, Albert Einstein discovered that light is fundamental to the structure of the Universe, connecting it to space, time, energy and matter. Although you don’t experience it directly in everyday life, objects contract and become heavier as they move near the speed of light. Also, for very fast objects, time slows down for them compared to the rest of the Universe. And with his famous Equivalence Principle, E = mc squared, Einstein showed that all objects contain enormous energy; to find the amount of energy, you multiply an object’s mass by the speed of light, squared.