Planck constant
During the 18th century, Newton's theory was given the most attention. In 1803, Thomas Young described the "double-slit experiment". In this experiment, light going through two narrow slits interferes with itself. This causes a pattern which shows that light is made up of waves. For the rest of the nineteenth century, the wave theory of light was given the most attention. In the 1860s, James Clerk Maxwell developed equations that described electromagnetic radiation as waves.