Newton's prism experiment

Newton had been exploring 'optics' and 'a theory of colour' since around 1665. He was fascinated by the observed refraction of sunlight into colours by a prism, and frustrated contemporary claims that this was a 'contamination' of pure white light. 

In Newton's only drawing of his 'crucial experiment' with light, we see how light enters through a slit in a covered window on the right and passing through a lens, before encountering the prism in the middle of the drawing. After refraction it makes an elongated image on a board with 5 small circles, a hole that allows a primary ray to encounter a second prism to the left. On passing through the second prism, the colour and angle of refraction remain the same.

In the schematic animation of a continuous beam of light being dispersed by a prism, we see how the white beam represents many wavelengths of visible light as they travel through a vacuum with equal speeds. The prism causes the light to slow down, which bends its path by the process of refraction. This effect occurs more strongly in the shorter wavelengths (violet end) than in the longer wavelengths (red end), thereby dispersing the constituents. Upon exiting the prism, each component returns to the same original speed and is refracted again.

Nothing Newton did, neither refraction nor reflection, could alter the inherent properties of a ray of light: the colours were not generated by external design, corruption, or intervention, they were only made apparent by processes which separated them from the heterogeneous mixture of white light. This was a significant challenge to the assumption of two thousand years of optical research. Newton published his 'New Theory of Light and Colours' in the Philosophical Transactions of the Royal Society in February 1672, and developed it further in his Opticks of 1704.