Even more unpleasant dilemmas faced physicists in the early years of this century. Light itself seemed to be misbehaving.

The question as to whether light was made up of waves or particles had seesawed back and forth for several hundred years. Isaac Newton, who discovered in 1672 that light could be split into many colors by a prism, favored a particle theory of light. Others before and after Newton preferred a wave theory to explain the bending of light by lenses, the interference of light passed through adjacent pinholes, and the diffraction of light by gratings.

By the end of the nineteenth century few people doubted the wave nature of light and of all other forms of electromagnetic radiation. James Maxwell developed an elegant theory by which such radiation was thought of as arising from vibrating electric and magnetic fields in space. Visible light was recognized as being just the range of wavelengths of electromagnetic radiation that the eye is sensitive to, with radiation of longer or shorter wavelengths being invisible but present.