An Idiot’s Guide to The Most Beautiful Demonstration of Relativity, The Einstein Ring

The end result looks a little something like this:

This photo was taken as part of a joint effort by NASA, the ESA and SLACS service team.* The “halo” is actually a solid non disc shaped object, many billions of miles away behind the bright object in front.

*Credits for all photographs at the bottom of this article. Read them, you can use them to get more pictures!

So how does this work? When where they discovered? Why should we care? Read on dear reader, read on!

How a Normal Lens Works

In order to understand the importance that Einstein rings have, and how they work, we first need to understand the principle of a lens. As you probably well know, a lens works by bending light. If you need eye glasses, the lens on your eye does not bend the light properly and so your vision is blurred. Eye glasses compensate for this by bending the light a little extra one way or the other to compensate for the eye.

They do this using a simple method, and anyone who’s ever driven a car can relate to this. Imagine you’re driving after wet weather. The roads are mostly dry, but there are a few big puddles here and there (What’s your road tax paying for anyway!?). Suddenly, the right hand side wheels of your car hit a puddle, while the left ones stay on dry land. What happens? Your car is pulled to the right, and if you’re driving in Britain, you’re on the wrong side of the road. If you’re driving anywhere else, You’re on the pavement! The sudden extra resistance means one side of your car is moving faster than the other, so it turns, just like a tank!

The exact same thing happens when light hits a lens. As a lens is curved, one “bit” of the beam of light hits it a moment before the other, and so slows down. As the light is travelling in a beam (At this point we get into wave-particle duality, but let’s not worry about that now), the other side must curve towards the lens to keep the beam straight.

Oh and when your science teacher told you that the speed of light was constant, he didn’t add the clause ‘in a vacuum’ – light can be slowed down by passing through differing densities of matter.