The Human Eye and the Colourful WorldThe Human Eye

Your eye works very much like a camera. A camera has a lens that bends light and forms an image on a light-sensitive screen at the back, an opening that lets the light in, and a ring that makes the opening bigger or smaller. Your eye has a part for every one of those jobs.

Join each part of a camera to the part of the eye that does the same job.

Now let us meet each of these parts properly, in the order in which light reaches them.

A Tour of the Eye

The eyeball is roughly a sphere, only about 2.3 cm across. Light enters through the cornea, a thin, transparent membrane that bulges out at the front of the eyeball. Surprisingly, most of the bending of light happens right here, at the outer surface of the cornea. The eye lens behind it only provides the fine adjustment of focal length needed to focus objects at different distances.

Behind the cornea sits the iris, a dark muscular diaphragm. The hole in its middle is the pupil: the iris widens or narrows the pupil and so controls how much light gets in. The ciliary muscles hold the eye lens. At the back, the retina acts as the screen: a delicate membrane packed with an enormous number of light-sensitive cells. From the retina, the optic nerve leads to the brain.

Label the eye (the textbook's Figure 10.1): tap a label, then its pin on the diagram.

The Journey of Light

Here is everything that happens between "light enters your eye" and "you see a mango". Drag the steps into the right order, first at the top.

Look at the third step: the image on your retina is upside down, because the focusing parts of the eye act like a convex lens and form a real, invertederectvirtual image. You never notice it, because your brain processes the signals so that you perceive objects the right way up.

The iris adjusts the pupil all day long. Walk out of a dark cinema hall into bright afternoon sun, and your pupils become smallerlarger, letting lessmore light in. Step back into the dark hall and they slowly grow larger again.

Power of Accommodation

Look at the far end of your street, then at the tip of your finger. Both looked sharp, yet the distance from the lens to the retina never changed. A camera would have to move its lens; your eye changes the shape of its lens instead.

The eye lens is made of a fibrous, jelly-like material, and the ciliary muscles can change its curvature. When the muscles relax, the lens becomes thin and its focal length increases: that is how you see distant objects clearly. When you look at something near, the muscles contract, the lens becomes thicker and more curved, and its focal length decreases.

The ability of the eye lens to adjust its focal length is called accommodation. Notice what stayed the same all along: whenever the image was sharp, it formed at the same distance behind the lens, on the retina. The eye keeps the image distance fixed and changes the focal length instead.

But the focal length cannot shrink forever. At 10 cm the lens was already as thick as it can get, so the image fell behind the retina and looked blurred. Hold a printed page very close to your eyes and you will see the blur, or feel the strain.

The Near Point and the Far Point

To see an object comfortably and distinctly, you must hold it about 25 cm from your eyes. The minimum distance at which objects can be seen most distinctly without strain is called the least distance of distinct vision, or the near point of the eye. For a young adult with normal vision, the near point is about 25 cm2.5 cm25 m.

The farthest point up to which the eye can see objects clearly is called the far point of the eye. For a normal eye it is infinity25 cmabout 2 m: you can see the Moon and the stars sharply.

So a normal eye sees clearly everything between 25 cm and infinity0 cm and 25 cm25 cm and 2.5 m.

Speed Round: Which Part?

Each slide describes a job. Pick the part of the eye that does it.

Get five right to move on.