What is light?

To most people, "light" refers to something that comes in a range of colours and issomething you can see. But visible light is actually part of a much larger "electromagnetic spectrum" that ranges from long-wavelength radiowaves at one end of the scale, to very short wavelength X-rays and gamma rays at the other. The part of this spectrum we can see sits roughly in the middle. And to complicate matters, sometimes light behaves as though it's a wave, while at other times, it behaves like a stream of tiny particles. One of the pioneers of the study of light was Isaac Newton. Graihagh Jackson went to meet historian Scott Mandelbrote at Cambridge University Library to see some Newton's original notebooks where he documented his attempts to understand this mysterious entity...

Graihagh - As we walk down these aisles, we've walked past Charles Darwin and now that we're in Newton's aisle, I can see Kelvin's papers and a bit further down, Ernest Rutherford's. It's a staggering collection of scientific manuscripts.

Scott - Yes, well the University library holds one of the finest collections of scientific manuscripts in the world and has held Newton's papers since the late 19th century.

Graihagh - The papers I had come to see had been especially laid out in the manuscript reading room. [I'm so excited!]. As you may be able to hear, I was just a tad amazed at the thought of seeing the original notebooks that Sir Isaac Newton himself had handwritten.

Scott - Well, we're now in a room which has a number of Newton's papers, a wonderful collection and some of the most important things that Newton wrote.

Graihagh - And they're in fantastic condition and I'm beginning to see why, because they're beautifully laid out on what looks like individual pillows for each and every single notebook! I wonder if we could focus on this one in particular because it's a sort of an A5 notebook and there's what looks like an eyeball with a stick pointing at it. What's he describing here?

Scott - Well, what he writes is, "I tooke a bodkine & put it betwixt my eye & [the] bone as neare to [the] backside of my eye as I could: & pressing my eye [with the] end of it there appeared severall white darke & coloured circles. Which circles were plainest when I continued to rub my eye [with the] point of [the] bodkine..."

Graihagh - Yes, you did hear that right. Newton peeled back his eyelid and stuck a bodkin or what we call them today, a giant needle, between his eyeball and cheekbone. He also stuck a brass plate in there too and later, stared directly at the sun until he went blind! Fortunately, Newton's eyesight recovered in all incidences. Why bother? You may wonder. Well, Newton was interested in the nature of light and how we see things around us. This particular experiment was designed to see if colour was the product of the outside world or created within the mind. Although sticking a needle in his eye didn't exactly determine this, it was one of many steps that led him to demonstrate how white light is made up of 7 individual colours. Prior to this revelation, it was widely believed that light was pure and that colours were created by the mixing of light and darkness. So, white light is 7 colours combined. But what physically is light? I met Zephyr Penoyre, a trainee physicist at Cambridge University, on the river to see if he could give me any answers.

西风,牛顿是一个大的支持者the idea of light as a particle which he believed in because he saw how light reflected in straight lines off a surface, just like a ball bouncing off a surface.

Graihagh - Thomas Young came along and he said, "No Newton. You're wrong." What did he say?

Zephyr - He came up with a fantastic experiment where you take two small slits in a piece of material and shine light on them. If Newton was right, what we'd see is 2 dots of lights.

Graihagh - I can see that working.

Zephyr - It does seem to make sense but actually, if you do it, you don't see that, you see a series of white and dark bands.

Graihagh - A bit like a bar code then.

Zephyr - Yeah, it looks exactly like a bar code. This is a thing that only waves do. We've got here two poles and a river and I'm going to show what interference looks like.

Graihagh - Okay, let's go for it. We have two punt poles because we are on River Cam and you're going to dip them in and out of the water at the same time.

Zephyr - Yup! So, I'm going to dip them in and out. They're quite heavy.

Graihagh - And we're seeing a series of ripples from both of the poles as you'd expect, like you drop a stone in a pond, you see the ripples cast out. But actually, where the ripples meet, something quite interesting is happening.

Zephyr - What a ripple is, is a series of peaks and troughs moving outwards from a point. But where the two ripples meet, some places, the peaks are together and they add up; some places the troughs are together and add up. But some places, the peaks and troughs meet and they add up to nothing. Because waves have this property of adding up and cancelling out, this is exactly why we see the series of light and dark stripes on the wall. This is why Young's experiments proved that light must be a wave.

Graihagh - Problem solved! That's what I saw in my textbooks when I was 16, light is a wave, right?

Zephyr - Well yeah, light is a wave. But now, the next issue is, how light travels. So, we know that sound waves must move through a solid object or through air. Water waves must move through water. so, what does light move through?

Graihagh - Air? Is it not the same?

Zephyr - Well, we know that light does move through air, because we see light on Earth. But we also see light moving through space. And we know that space is a vacuum. Sound can't travel through space because there's nothing to transfer the vibrations in space. But somehow, light does and that was the next question which really puzzled scientists.

Graihagh - How does light travel then?

Zephyr - Someone called James Clerk Maxwell came along who showed that it's mathematically true that electric and magnetic fields can make waves. And he put this together to show that light is actually variations in electric and magnetic fields. This not only explains how light travels through a vacuum, but also explains things like how light comes in different colours and its different temperatures and energies.

Graihagh - Different wavelengths of light have different properties and that's where your radio waves, microwaves and x-rays come in. It's all light but just in different forms. Light then is a wave and we can all breathe one big sigh of relief. Well, sadly not. Einstein came along with photoelectric effect. He noticed more electrons were emitted from a metal when certain colours of light were shone on it. This is weird because well, waves shouldn't have that effect on electrons. Particles, on the other hand, do. So Einstein said, "Hey! You know what? Light comes in little packets or bundles and I hereby call them photons. Does that mean Newton was right all along when he said light was a particle. Well then Einstein developed his theory of special relativity and said light was a field of waves. Yeah, I know, confusing. So, this takes us right back to square one - what is light? A wave, a particle, or could it in fact be both?

Zephyr - In some ways, Newton was right. Light can behave like a particle, like a photon. But at the same time, Maxwell is completely right and Young, when they said that light behaved like a wave. Light behaves as both. We call this wave-particle duality. And it's something that we see a lot of evidence for in science, but we still can't really explain why light will behave as a particle sometimes and a wave with others.

Graihagh - Does this mean we still don't really know what light is?

Zephyr - It kind of does. We still might find that there's a better explanation for light or it may just be that light is going to remain this confusing thing that we don't quite understand.