Programme four: Light Fantastic

The final episode in this series shows how, just as people began to feel they understood it, light turned out to be a tricky and slippery subject indeed.

Early studies of perception, particularly those aimed at understanding the strange way in which we perceive colour, gave huge insights into the workings of the brain.

No longer did we believe that all we see is real, accurate and a complete map of the world. Rather what we see is a partial, faulty glimpse of reality.

These and other findings aided a young Albert Einstein to totally change our understanding of light and effectively rewrite the laws of physics.

Einstein with his General Theory of Relativity proved to the world that light does not travel in straight lines, and in so doing overturned 3,000 years of thinking.

Whereas light had once been the sign of reliability, a tool for astronomers to find out how the cosmos worked, now it turned out that light was a puzzle, it was a problem, it twisted, it turned, it bent, it strayed.

It was an astonishing intellectual achievement. It declared that humanity's most basic intuition about space, time and light cannot be trusted - the entire universe is an illusion.