A brief history of Light, from the daft Greeks to the brilliant Iraqi Al-Haytham

“Albert, what about us?”

What about us?

“Well, you’ve told me a lot about stars and the universe but not much about light. Aren’t we meant to be imagining this trip of yours across the universe on a beam of light?”

Of course, light is after all one of my favourite topics. A lot has happened in terms of light during the journey so far. When we started out starlight arriving on Earth was greeted by a truly appreciative audience that placed great importance on the twinkling lights in the sky. Mesmerised young faces would gaze up into the stars and ask their parents what stars were.

"So what did they tell them?"

They told them stars were little holes in the floorboards of heaven, where light shone through.

"Lovely idea."

A beautiful notion, but a little wide of the mark scientifically. Those ideas lasted for over a thousand years from the time of the ancient Greeks until the 1600's. That was when natural philosophers, which is what scientists were called back in the year 1605, started to realise that they could abandon the old ideas of the ancient Greeks and think for themselves.

"So what did these ancient Greeks think about light?"

The Greek philosophers had a good think about light and then most of them came up with all the wrong answers. Plato, born in 427 BC, was most famous for his writings on politics, but also dabbled in science and his idea was that light works by sending out 'feeling rays' from the eyes to whatever you happen to be looking. This sounds a bit odd today but as an idea it lasted for almost two thousand years.

"So if all humans went blind overnight, light would cease to exist."

True, but I suppose they thought cats and dogs saw things the same way. Another problem with this great theory is that if light comes from the eyes why can't people see in the dark?

"Of course, why didn’t I think of that?”

Don’t worry, neither did most of the Greek philosophers. More worrying is what this says about progress in politics, no-one takes any notice of Plato now on science but his writings on politics are still highly regarded – most famously Plato’s Republic. The other great greek philosophers weren't much better on light. Have you heard of Aristotle?

“I’ve heard the name but I’m not sure what he is famous for.”

Aristotle was perhaps the best philosopher of ancient Greece and was brilliant in developing logic and studying animals and biology. He wasn’t so good at light and thought that all colours were a mixture of black and white. Another ancient Greek, Epicurus, got the right idea in 300 BC with light coming from objects to the eye, but since he was a small second division philosopher no-one listened to him.

“Why didn’t anyone listen to him? It seems so obviously true.”

Not back then. The notion of light travelling from objects to the eye seemed nonsense to most people- how could the light coming from something big like an elephant fit through the tiny pupil of an eye?

"So how does it all fit?"

Very easily. Light isn't made up of atoms so it doesn't take up any space. You can fit more light than the eye could cope with through even the tiniest pupil.

"So did no-one have a clue about light back then?"

After the Greek and Roman empires collapsed Europe was stuck in the dark ages for a thousand years.

"I guess they weren't likely to work out much about light in the dark ages."

Very true, but it wasn't so bad everywhere. To find someone talking sense about light 1500 years ago, you would have to skip Europe and look towards the Middle East at a time when Arabic culture was leading the world in mathematics, astronomy and science. Iraq was the major science centre back then.

“Iraq?”

It may come as a surprise to many people but Baghdad and Basra a thousand years ago were some of the most cultured places on the planet rather than just another war-zone. Did you know the numbers 0 1 2 3 4 5 6 7 8 9 are called arabic numerals?

"I thought they were invented in Europe.”

Well you’re wrong. The numbers we now use around the world can be traced back even further to Hindu and Indian mathematicians. Imagine if we were still lumbered with Roman numbers, all those X's and V's. And it's not just the numbers, the foundation of modern mathematics was created by arabic mathematicians a thousand years ago. Think of the word algebra.

“That’s a type of mathematics. Equations and stuff.”

It certainly is, but the ‘al’ part also reveals its Arabic origins. The father of this type of mathematics, Muhammad bin Mūsā al-Khwārizimi, was from Persia which would now be part of Iran but he spent most of his life working in Baghdad. Algebra is work that comes his famous book on the subject that contained the word al-Jabr in the title. But the most important Arabic scientist for our story was Abu Ali Muhammad bin al-Hasan bin al-Haytham.

“Wow, long name.”

He’s often just called Ibn al-Haytham or even just Alhazen. He was born in the year 965 A.D. in Basra, in what is now Iraq. He knew all about the writings of the Greek philosophers and thought this ‘feeling ray from the eye idea’ just didn’t make sense. More than that he worked out things that we know take for granted like how light travels in straight lines and how light is reflected by mirrors.

“How did he get his inspiration?”

Al-Haytham had plenty of time to think about light since he spent years under house arrest pretending to be insane after giving up his attempts to stop the Nile flooding. At that time in Egypt the flooding of the river Nile was the most important event in the year. He was invited to Egypt after claiming he could stop this annual flood. The plan involved a dam on the Nile at Aswan. Overwhelmed at the scale of the task he admitted defeat and pretended to go mad as a result. The ruler of Egypt at the time, Fatimid caliph al-Hakim, was pretty upset at this. Since al-Haytham appeared mad it seemed a little unfair to kill him for failing in his mission, so he let him off and simply placed him under house arrest for life. Unfortunately the Caliph had a long memory so al-Haytham had to pretend to be mad until the Caliph finally died years later. Al-Haytham has the consolation that his plan would have worked if he had completed it. A dam was built at Aswan almost a thousand years later and it did stop the Nile flooding.

“So when did he have his great ideas on light?”

In all those years of house arrest he wrote 209 books on science, philosophy, mathematics and astronomy. The most famous and influential of these was his book on light Kitab Al Manazir or the Book of Optics. This amazing book started the science of light. Just about everything he said about light made more sense than the ideas of the ancient Greeks and so he overturned what was thought to be true for over a thousand years and no-one improved on his ideas for another 500 years. When Europe started to escape from the dark ages it was Alhazen’s book that helped to get people thinking sensibly about light.

"So Alhazen was the first ray of light to illuminate the gloom of the dark ages."

You're getting quite poetic in your old age.

"Why thank you, that's your first compliment in over two thousand years."