Although Tanno and Iguda enter the eye from the back, light enters the eye from the front.
Image credit: SOMSO® model from Adam,Rouilly
This model shows how the whole eyeball sits inside the eye socket. There are muscles around the outside that can move the eyeball from left to right and up and down. You can see this by looking at your eyes in a mirror and moving them around (you might even be able to see the blood supply to the ‘whites’ of your eyes).
The optic nerve runs form the back of the eyeball to the back of the cerebrum in the brain.
Light entering the eye first passes through the transparent surface called the cornea and then through the pupil. The pupil looks like a dark circle in the centre of your eye. The light is then focused by the lens so that it forms a sharp image on the retina, after passing through a thick clear substance called the vitreous humour that fills up most of the eyeball.
Why do we have two eyes?
1) Make a small dot on a piece of paper and place the paper on a table.
2) Sit in a chair about an arm’s length away from the paper and close your left eye.
3) Hold a pencil in your hand and hold your arm straight up.
4) Bring your arm down slowly and try to touch the tip of the pencil to the dot.
How close did you get on the first try? Did you aim too close or too far? How many tries did it take to touch the dot?
This activity shows that we need two eyes to see the world in three dimensions, allowing us to judge depth and distance when the final image is formed.
The retina is packed full of receptor cells that detect the light entering the eyes. Some of these are called rod cells and let your brain know how bright the light is.
The other cells are called cones, and these detect colour, allowing an image to be formed. Remember, Tanno and Iguda found out that the image is formed upside down, and it is the brain that flips it back the right way.
These scans show the blood vessels in the retina, the receptor cells work very hard so need a good blood supply providing a great deal of oxygen.
Image credit: Alex Laude; Newcastle University
This is where Tanno and Iguda join the optic nerve, travelling in the opposite direction down the nerve to the back of the eye, the retina.
Looking at the outside of the eye.
This requires a partner (or could be done by looking in a mirror).
1) Ask your partner to close their eyes for ten seconds and then open them.
2) What happens to their pupils (the dark circles in the centre of the eye) when they open their eyes?
3) Now shine a torch into your partners eyes. What happens to the pupils?
4) Now ask your partner to first look at a distant object and then one nearby (without moving their head). What happens to their pupils?
Now switch roles and check again on the other person.
The Blind Spot
Tanno and Iguda arrive at the back of the eye, the retina, where light is detected and the image sent back to the brain. However, where the optic nerve joins, there are no receptor cells, so any light that falls there is not ‘seen’. This is called the blind spot. We can demonstrate:
1) Draw out a picture of two objects onto a piece of paper. It can be any two objects, even Tanno and Iguda, as long as they are approximately 7 cm apart.
2) Hold the paper close to your face.
3) Close your left eye, and look at the X (or the object you drew on the left hand side) with your right eye.
4) Slowly move the picture away from your eyes, keeping your right eye focused on the X all the time.
5) What happens to the O as the picture moves away from you? At some point the light from the O will fall on the blind spot in your right eye.
6) Repeat the experiment this time with both eyes open. What happens, and can you explain the difference?
You can go to the previous PostcardsFromTheBody secret pages by clicking the places below.