How do X-rays work?

In 1895, Wilhelm Roentgen shot off yet another volley of electrons into a vacuum tube. The high voltage used ensured that the electrons reached a tremendous speed as they crashed into the metal surface at the other end. But then a strange thing happened: across the darkened room, a faint green glow (from a fluorescent screen that Roentgen owned) was visible. Now fluorescent things tend to glow when exposed to radiation (such as light), so by itself this wasn't hugely surprising. What was surprising - hugely, hugely surprising indeed - was that there ought not to be any way for the radiation to get to the phosphorescent surface: Roentgen's machine was surrounded by thick black cardboard.

What the hell was going on? Roentgen instantly realised that unlike visible light, this new form of radiation was amazingly passing through both the vacuum tube and the cardboard, and interacting with his fluorescent screen on the other side of his lab. He named this mysterious new form of radiation "X-rays", after the customary algebraic symbol for an unknown thing: x.

As the weeks progressed, it became obvious that X-rays could pass through other things too, like wood, water and human tissues. Happily, X-rays also caused photographic film to be exposed, and the combination of these two facts allowed Roentgen to take the first ever X-ray picture - of his wife's hand (RIGHT). The discovery would net him the Nobel prize for Physics in 1901.

So what were these mysterious X-rays made of? Subsequent investigations proved that they were a form of electromagnetic radiation. Light is the form of electromagnetic energy most familiar to us, but in reality it is best to think of visible light as merely the portion of the electromagnetic spectrum that our eyes can detect. In reality, the spectrum stretches all the way from low energy radio waves to the ultra-high energy gamma waves. For instance, just to the 'higher energy' side of the visible spectrum is ultraviolet light, which a few other animals (like bees) can detect, despite these waves being utterly invisibility to us. The entire spectrum is depicted below:

X-rays, you will note from the diagram, are therefore a form of 'light' (though invisible to us) that lies to the higher energy side of our visual spectrum. I think this is a good place to stop and catch our breaths, and we'll continue our story in the next post. There, we'll discuss how X-rays interact with our bodies, and thus how they have come to be so useful in forming X-ray images.