"The strange story of the quantum begins with the humble electric light bulb."
Max Planck was advising the German Bureau of Standards on how to make light bulbs more efficient: maximum light with least amount of power.
Knowing Maxwell's theory that light consists of electromagnetic waves, each wavelength describes a different colour, Planck had to figure out how much light of each colour a hot object emits. After many unsuccessful attempts, he worked the data backwards and inferred that light waves could only accept energy in packets, or 'quanta' (later known as photons in the case of light). This became Planck's Constant, and the beginning of quantum physics.
The changing colours of an increasingly hot body are due to the changes in the wavelengths of light emitted.
Maxwell's unified theory of electricity, magnetism, and light (electromagnetism) predicted the speed of light.