Did you ever think about how you’re able to see glass? I mean, the light just passes straight through glass, right? So how come I can see the edges, and the glasses I wash-up don’t just disappear from view? Why aren’t glasses invisible?
Well, it’s all to do with how light doesn’t just travel in straight lines.
Imagine, if you will, a long line of people holding hands and walking along. They’re all walking at the same speed, so they keep a fairly rigid line as they walk. But! Second act twist! The right hand side of the line come across a patch of difficult ground! Now, everybody in the line quite likes holding hands – and hey, who could blame them? – so nobody lets go. The right hand side of the line slows down, the left hand side keeps at the same speed, and so the entire line gets pulled to the right.
Now – and this is where we jump from the metaphor to the reality, so hold on to your hats – this is roughly what happens to light. Light doesn’t move at the same speed all of the time; the speed of light through a vacuum is the fastest anything can possibly go, but it doesn’t always go that fast. It entirely depends what it is that light is actually moving through at the time.
So to bring it back to our glass which has just beeen freshly washed-up – as light moves from the air, through the glass, then back into the air again, it changes speed every time it changes what it’s moving through. And, due to the random nature of life, the light basically never enters the glass straight-on – one part of the light always enters first, and so pulls the light around and bends it as it goes through, just like our line of people. And as the lights get bent and changes, it gives a distorted view of what’s behind the glass, so our very clever brains figure out that there must be something there. Thus we ‘see’ the glass.
How much light bends in a given material is related to its ‘refractive index’. We can see the clean glass because the refractive index of the air is different to the refractive index of the glass – so the light travels at different speeds and bends as it goes from one material to another. But if your materials had the same refractive index – say, oil and pyrex – the light would just go straight through with no distortion or bending, and we wouldn’t even know anything was there.
So if you’d like to turn yourself invisible, you essentially need to change your refractive index to that of air. Of course, that would leave you with the minor problem of being blind (As we need light to interact with the back of our eyes in order to see), but the practicalities of it are clearly not really in the forefront of everyone’s mind in our latest adventure…