What am I? The confused life of a light particle

Remember I told you about Isaac Newton when we were talking about gravity?

“The apple man and his laws of motion. I remember him.”

Well he is just as famous for working out where the colours from a rainbow come from. He discovered that white light was made up of lots of different colours.

“How did he do that?”

Well the strange thing was that everyone in the local town market knew it already. They sold triangular bits of glass, as novelties. This is what Newton wrote in one of his books – ‘I procured me a triangular glass prism to try therewith the celebrated phenomena of colours’.

“Strange way of talking. What does that mean?”

This was how they spoke three hundred years ago. It means he bought a prism to look at the colours it makes. He worked out that raindrops must act in the same way to turn sunlight into a range of colours.

"So if everyone knew about these colours why is Newton famous rather than the person who first made these prisms?”

He did something much more important than discovering about prisms and colours, he examined it and worked out why a prism makes colours. By experimenting with these prisms Newton worked out a set of rules for how light could be split up into different colours and recombined again. He started by showing how light could be split up by a prism into different colours.

“But we knew that already”

Yes but people thought it was something magical about the glass not a feature of light. He also showed that these individual colours couldn't be split any more by passing the light through another prism. So the prism must separate light into its individual components like unravelling a rope made from lots of different coloured threads. What no-one suspected was you make white light from mixing all the colours back together again.

“Was that an important discovery? It doesn’t seem as big an issue as gravity.”

Not on a galactic scale but closer to home colour televisions use Newton’s discovery.

“Really?”

Oh yes. Go up close to an old fashioned television and you’ll see that all the colours including white are made up from coloured dots. You can’t see the individual dots on some modern televisions but every colour is still made up of a mix of three single colours. But what was much more important than television was that all this inspired Newton to produce an idea or theory about what light is.

"And the answer was?"

Newton concluded that light was made up of little particles, corpuscles as he called us, of different types. Each different type represents a different colour across the rainbow, or spectrum as scientists call it, from red to violet. White light is an equal mixture of all the different types. Every hue imaginable can be made by mixing the different types in varying proportions.

“But you said from the start that we were light particles or photons.”

I did because on this point I agreed with Newton. But for the next two hundred years, this was one thing that people thought Newton got wrong. Christiaan Huygens a Dutch man and French man Augustin Fresnel, argued that light was entirely wave-like spreading like ripples across water and most scientists started to believe that light was a wave.

“But you said near the start that we couldn’t be waves because if we were we couldn’t get through empty space.”

Very true but all that took a lot of working out and it just shows that scientists spend as much time getting things wrong as they do getting them right. The wave idea seems odd to most people because light casts shadows and so appears to always travel in straight lines. Waves on the sea spread a little around objects like boats, rocks and harbour walls. Sound waves carry sounds around corners.

“I can hear around corners but I can’t see around corners. So light and sound must be different.”

Yes, but in fact light does go around corners.

“No, it doesn’t.”

It does but only by a tiny amount and you have look carefully to see it. Just after Newton died another clever Englishman Thomas Young started investigating shadows. Although the edge of a shadow looks like a sharp line between dark and light, if you look closely you can detect faint bright lines running alongside the main light-dark line in the areas that should be dark.

"You're really telling me that light spreads a very tiny amount around corners?"

As strange as that sounds it's true. Young wasn't the first person to see strange things in shadows but he was the first to be able to explain it. In 1803 Thomas Young looked at the shadows made by shining light through two narrow slits that were very close together. In the middle of the shadow cast by the solid bar between the two slits was a faint line of light. Some of the light must have been bent around the solid edge in the same way that waves in the sea can bend around the edges of boats or harbour walls. There were also fainter lines of light, fringes, each side of this central line. All this only makes sense if light are like waves.

“It doesn’t quite make sense to me I’m afraid Albert.”

It’s a bit complicated but here is how it works. Young worked out that these fringes depend on the light from one slit ‘interfering’ with light from the other.

"What does ‘interfering’ mean?"

Interference is science-speak for where two waves collide. It doesn't matter what type of wave you are thinking about; sound waves, light waves and even waves on the ocean can all produce interference. Thomas Young worked out that the lines of light appeared where the peak of the two light waves from the two slits combined and added together. Imagine the peak of a wave hitting the two slits at the same time. At the two slits only a small amount of the wave gets through, just enough to make two smaller waves which are in exact synchrony because they are starting off at the same time. If these little waves travel the same distance then they will remain in sync with each other. So at the centre of the bar's shadow they will have travelled exactly the same distance and all the peaks of the little waves will all line up. The peak of one wave will meet the peak of the other making a wave that is twice as big. This is called constructive interference and that’s how you can get a light of light in the middle of a shadow. At all the places where this interference happens you get a fringe of light. Here is a website that shows what happens (CLICK HERE).

“So light is a wave after all.”

Well it certainly can certainly act like a wave. Young also found that different coloured lights had fringes that were different distances apart, fringes from red light being further apart than fringes from blue light. From this he worked out the light of different colours had a different wavelength.

"What's a wavelength?"

It's how long a single wave is, from the peak of one wave to the peak of the wave following immediately behind it. In the sea there might be 10 metres between waves so a sea-wave has a wavelength of 10 metres. A sound wave will have a wavelength of a bit less than a metre depending on the pitch. Young worked out from the spacing of the fringes that light had a wavelength too but it was very very small. He calculated that red light must have a wavelength of 700 billionths of a metre and blue light 400 billionths of a metre.

"How big is my wavelength?"

Tiny, about 500 billionths of a metre. You also have a frequency; if you could stay at one point and let another light wave pass you by, then the number of peaks that pass you in a second is the frequency of the wave. It also means how many times a second you are vibrating. Frequency and wavelength are related. The smaller the wavelength, the more bunched together the waves and the more waves that will go past in a second and so the higher the frequency.

"So if our wavelength is so small then our frequency must be very large."

It certainly is, our frequency is 600 trillion. In other words in our beam of light we are vibrating 600 trillion times each second. Sound waves from a note somewhere in the middle of a piano keyboard will only vibrate a few hundred times a second. Light of different frequencies and wavelengths have different colours but the whole range of colours is squeezed into a surprisingly small range of wavelengths. If you think of a piano keyboard, which only covers a fraction of the range of possible pitches of musical notes, then you could fit the whole range of visible light into just one octave or a scale of eight notes.

“But I can see millions of different colours, not just eight.”

The high frequency or long wavelength light on this keyboard would look red and be like the low pitched notes in sound terms. The short wavelength light would be blue and be like the high pitched sounds with the whole of the rainbow in between. There appears to be so many colours because the eyes is much better at detecting fine variations in light than ears are at detecting differences in the pitch of a sound, unless you are a bat of course.

"So if I made my wavelength shorter and shorter, would I get bluer and bluer then disappear completely?"

No you'd still be a photon; it's just that humans couldn't see you.

"What would I be?"

An ultraviolet photon. If you were a really short wavelength you might be an X-ray like the ones that black holes make.

"And if I got stretched into a longer and longer wavelength, what would I be then?"

An infrared photon, that's the sort of photon that carries the heat and warmth across space. And if you kept on being stretched you would change into a microwave photon, then a radio wave.

"Could I be stretched or squeezed into a sound wave?"

Never. Sound waves are just vibrations in air. Light is a completely separate breed altogether. After all no matter how much you stretched or compressed a dog, it would never become a cat.

“And all this could be worked out from looking at shadows?”

Amazing isn't it. Most importantly none of this made sense if light was just like small billiard balls travelling in straight lines.

“So are we travelling on a beam of light particles or on a beam of light waves?”

For most of the rest of our journey scientists on earth will be arguing about that with each side convinced that they are correct and the others are wrong.

"Will they have made up their minds by the time we arrive?"

Well sort of, but it won't be a clear victory for either side of the particle-wave argument.

"Why not?"

By then both sides will have conclusive proof that they were right all along. So by saying that light must be a particle and a wave at the same time they reach some sort of agreement.

“That sounds a bit weird.”

Oh, there are weirder things than that ahead of us. I’m only just getting you prepared for the really strange stuff like quantum mechanics and my theory of relativity.