Total internal reflection

Have you ever played with a mirror and tried to shine a light on it at a funny angle? If you have, you've probably noticed that sometimes the light bounces off and you can see it, but other times the light seems to disappear entirely. This is called reflection and as long as the angle of the light is right, it'll bounce back off the mirror and we'll see it.

But sometimes, instead of bouncing off the mirror, the light stops at the surface of the mirror and reflects back into the mirror itself. We call this "total internal reflection". Imagine you're a bird flying over a pond. If you look straight down into the water, you can see everything clearly - your reflection, the rocks at the bottom of the pond, and so on. But what if you tilt your head to one side? Suddenly, the water looks like a shiny piece of metal, and you can't see through it anymore! The angle you're looking at the water from is called the "critical angle", and if the angle of the incoming light is greater than the critical angle, the light will stop at the surface of the water and reflect back inwards, creating a mirror-like effect.

Total internal reflection is used in lots of cool ways, such as fiber optic cables that can send light long distances without losing much of it. It's also what makes diamonds and other transparent gemstones sparkle so brightly - the light comes in at just the right angle to bounce around inside the diamond before reflecting back out again, giving the gem its signature shine. So the next time you see something that looks like a mirror but isn't, remember that it might be total internal reflection at work!