Annus mirabilis of Isaac Newton

Isaac Newton investigates sunlight and color with a glass prism

Newton expected the patch of sunlight on his wall to be round. The prism stretched it into a long band, and that shape — not the colours — was the puzzle.

A hole in the shutter

In his 1672 account Newton said that at the beginning of 1666 he obtained a triangular glass prism to investigate the familiar phenomenon of prismatic colours. He darkened a room, admitted sunlight through a small hole in the window shutter, and placed the prism at the opening so the refracted light struck the opposite wall.

Under the accepted laws of refraction he expected a roughly circular image. What appeared was a vividly coloured, elongated one.

Not the discovery of colours

Prisms making colours was already well known; Descartes had described prismatic rainbow colours. Newton's contribution was a different argument, worked out through subsequent tests: that white light is heterogeneous, made of differently refrangible rays that carry their own colours rather than acquiring them from the glass.

On that account a prism does not create the colours. It separates rays that already differ.

An approximate year

The exact day is unknown, and the year itself is soft. Newton's beginning of 1666 comes from his retrospective 1672 narrative, and Cambridge University Library scholarship notes that surviving evidence may place his initial optical discoveries in autumn 1665.

Nothing changed publicly. During the period later called his annus mirabilis Newton entered an essay on refraction in his mathematical notebook, but he did not publish the new theory of light until his 1672 letter to the Royal Society.

Sources

Researched 23 Aug 2026 8 sources 2 audit passes Date disputed

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Date disputed. Sources disagree; the research places it at ~1666.

What the sources leave uncertain

The exact day is unknown.
Newton's 'beginning of 1666' date comes from his retrospective 1672 narrative. Cambridge University Library scholarship notes that surviving evidence may place his initial optical discoveries in autumn 1665.
The precise dating of individual experiments, including the two-prism experimentum crucis, is debated; the sheet therefore uses an approximate year.
No credible Newton Project or scholarly source consulted connects the prism experiment to an alchemical concept called 'Lux Dei' or calls 1666 the 'Year of the Morning Star.'