Here's an experiment you can try in the dark. It lets you see something inside your eyeball.
Caution: this activity does involve a flashing light. If you suffer from a form of epilepsy that can be triggered by flashing lights, it may be a good idea to skip this activity.
You will need
A small torch.
A really bright one doesn't work too well. I found a small torch powered by a single AAA battery worked well.
A dark room or a blank black wall to look at.
What to do
Stand in the dark room
If you wear glasses, take them off
Look straight ahead.
Hold the torch pointing towards you, about 3-4 cm from one eye, but off to one side.
Move the torch around in a small circle, about a centimetre wide.
You will notice the glare from the torch when it shines at your eye. After about ten seconds, you may also start to see branching lines, which look a bit like a river system on a map or branches of a tree. They only last as long as you are moving the torch. These are blood vessels inside your eye!
If you don't get this one to work right away, don't worry. Try slightly changing the angle of the torch or the way you move it until you see them. I find it sometimes helps to close the other eye and to stare off into the distance.
What's happening
Our eyes work by focussing light onto a layer of light sensitive cells on the back of the eye. When the light sensors are triggered, they send a message to the brain, which reconstructs the signals into the image we see.
There is a blood supply to the cells of the eye. Strangely, the blood vessels are placed on top of the layer of light sensors, between the light and the light detectors.
You might be wondering why we can't always see these blood vessels, if they are right in front of the light sensors. It's because when our brain reconstruct the image we are seeing, there are a lot of things that our brain edits out or enhances. These include:
Blood vessels, which we have seen this week.
A blind spot in each eye. (Science by Email 17 May, 2002)
That we only see detail in the very centre of our field of view and only see colour up to about 60 degrees away from the centre. (Science by Email 26 April, 2001)
Even though the signal from our eyes isn't perfect, our brain enhances the images it receives from the eyes, based on its previous experiences. In this way, we see a perfect, clear image.
When we moved the torch around, the shadows cast on the retina by the blood vessels moved too. This is enough to fool the part of the brain that processes the images into thinking it is seeing a moving object, so it lets us see the blood vessels.
This is the view I see from my office.
If my brain didn't improve the image it receives from my eye, my view would look more like this.
Although this may be the first time you have seen the blood vessels in your eye directly, you may have seen them before in a different way. The red-eye effect in photos is caused by light from a camera flash bouncing off the blood vessels at the back of your eye.
Incidentally, if you have ever wondered why your pet's eyes seem to glow yellow or green in photos, instead of red, it's because they have a reflective layer in their eyes behind the light sensors called the Tapetum Lucidum. It bounces light back to the light sensors, giving them a second chance to detect it. The light shining off the Tapetum Lucidum produces the colours you see in your pet's eyes. Since the Tapetum Lucidum is so shiny, you will often see the reflection even in normal light. Humans don't have this reflective layer, so we only get the red eye effect when there is a bright light like a camera flash.
Hold the torch a few centimetres from your eye like this, and move it around in a small circle, but do it in a dark room.
