How the eye forms an image

You are reading with a camera

Your eye works much like a camera. Light goes in through an opening. A lens makes an image on a screen at the back. A cable takes the picture away. Let us look at the parts, and then follow the light.

The parts of the eye

• The cornea is the clear, curved front of the eye. Light enters here. Most of the bending of light happens at the cornea, because the change from air to the eye is large.
• The aqueous humour is a clear liquid between the cornea and the lens.
• The iris is a coloured ring of muscle. It decides how much light goes in.
• The pupil is the round opening in the middle of the iris. It looks black, because the light that goes in does not come out.
• The lens is a clear, flexible convex lens. It fine-tunes the focus. It is held by the ciliary muscles, which change its shape.
• The vitreous humour is the clear jelly that fills the large space behind the lens.
• The retina is the screen at the back of the eye. It has millions of light-sensitive cells.
• The optic nerve carries the signals from the retina to the brain.

What happens to the light?

Light from an object enters through the cornea and the pupil. The cornea and the lens work together as one convex lens. They bend the rays so that they meet on the retina.

The image on the retina is real, inverted and much smaller than the object. We do not see the world upside down, because the brain learns early in life to turn the picture the right way up.

You should find that the image is only a few millimetres tall, and upside down. Double the distance, and the image halves. That is why far things look small.

A worked example

A friend $1.7\ \text{m}$ tall stands $12\ \text{m}$ away. The retina is $25\ \text{mm}$ behind the lens. How tall is the image?

1. The straight ray. The ray through the centre of the lens does not bend, so the two triangles match: $\dfrac{h'}{25\ \text{mm}} = \dfrac{h}{d}$.
2. Put in the numbers. $h' = 25\ \text{mm} \times \dfrac{1.7\ \text{m}}{12\ \text{m}} = 3.54\ \text{mm}$.
3. So your whole friend fits in about $3.5\ \text{mm}$ of retina, upside down.

The iris and the pupil

In a bright place, the iris makes the pupil small, so less light goes in, and the retina is safe. In a dim place, the iris makes the pupil large, so more light goes in. When you walk from the sunlight into a dark room, you cannot see for a moment. Your pupil needs some seconds to become larger.

The retina has two kinds of cells

• The cones work in bright light, and they let us see colours. There are three kinds, for red, green and blue light.
• The rods work in dim light, but they do not see colours. This is why colours seem to fade at night.

The place where the optic nerve leaves the eye has no cells. It is the blind spot, and we do not notice it, because the two eyes cover each other's blind spot.

Seeing with two eyes

A person with two eyes sees the same scene from two slightly different places. The brain joins the two pictures, and this gives a feeling of depth, so we can judge how far an object is. The field of view with one eye is about $150^\circ$, and two eyes give a little more.

Practise with new numbers

Try it in Python

The program finds the size of the image on the retina for a few everyday objects.

Remember these

• The eye forms a real, inverted and small image on the retina. The brain makes it upright.
• The cornea does most of the bending of light. The lens fine-tunes the focus.
• The iris changes the size of the pupil, to control the light that enters.
• Cones see colour in bright light. Rods see in dim light.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why does the pupil look black?

2. What do the rods and the cones of the retina do?

3. Why do we not see everything upside down, though the image on the retina is inverted?

4. A child $1.5\ \text{m}$ tall stands $6\ \text{m}$ away. How tall is her image on your retina ($25\ \text{mm}$ behind the lens)?