Science Questions

I understand x-rays and how they work, but a couple of years ago, my son had a very serious eye injury. Over a period, it improved, but he still has split vision in the right eye. Recently we went to Addenbrooke's and they x-rayed or photographed his eye. She told us to watch they screen so we could see they eye as we went through it. First of all it looked like a sun getting redder and redder, until we got to a white little spot. She said that it was the optic nerve. How do they manage to photograph through the eye? James in Bar Hill

It's actually not as difficult as it sounds. You have this amazing system on the front of your eye ball called the cornea. Just beneath that is the lens, which helps to tweak the process. However, the cornea alone has this amazing focusing ability and is there to focus light precisely onto your retina. All you have to do is look with your camera in the front of the eye and shine a light so you can see what's happening inside the eye. The front of the eye, the cornea and the lens, does the rest of the job for you. It focuses the light straight onto the back of the eye. If it's not quite in focus, you can adjust the camera to make the picture clearer. If you look on the back of the eye, you'll see this white patch. This is where the optic nerve comes into the back of the eyeball and all the nerves from the retina bundle together. The reason they are in one bundle is so that they don't take up that much space in the back of the eye, which would of course create a blind spot. The other way to take a picture of the back of the eye is to inject a glowing yellow dye called fluorescein. When this flows into the blood vessels in the back of the eye, it makes them glow when you shine a certain wavelength of light on them. This shows you where the blood vessels are.

February 2006