The ciliary muscles

The idea

Ciliary muscles change the curvature of the eye lens: when relaxed, the lens is thin and its focal length longer, for distant objects; when contracted, it is thicker and its focal length shorter, for nearby objects.

Near and far

Look at your finger held close to your eyes, and then look at a tree far away. Both can be seen clearly in turn.

How can one eye lens focus both well?

• The ciliary muscles change the curvature of the lens
• The retina moves to and fro
• The cornea changes into a different lens

Near and far

Move the object. The numbers come from a simple model of the eye in which the retina is 2.3 cm behind the lens (the eyeball is about 2.3 cm across). The object is always drawn in the same place, so the picture is not to scale.

Thin or thick

For a distant object the muscles relax and the lens is thin. For a nearby object they contract and the lens is thicker. A thicker lens bends light more, so its focal length is shorter.

Ciliary muscles change the curvature of the eye lens. When they are relaxed, the lens is thin and its focal length longer, for distant objects. When they contract, it is thicker and its focal length shorter, for nearby objects.

Notes

Relaxed ciliary muscles: thin lens, longer focal length, distant objects. Contracted: thicker lens, shorter focal length, nearby objects.

Check yourself

When we look at a nearby object, the ciliary muscles…

When the ciliary muscles are relaxed, the lens becomes thin and its focal length…

What can the ciliary muscles modify?

For which object is the focal length of the eye lens shorter?

• contract, and the lens becomes thicker — correct. Yes!
• relax, and the lens becomes thinner. Relaxed muscles make the lens thin, which is for seeing distant objects.
• contract, and the lens becomes thinner. When the ciliary muscles contract, the curvature of the lens increases and the lens becomes thicker.
• decreases, which lets us see distant objects clearly. A thin lens has a longer focal length. The focal length decreases when the lens is thicker, for nearby objects.
• increases, which lets us see distant objects clearly — correct. Yes!
• stays the same. Changing the curvature of the eye lens changes its focal length.
• The size of the pupil. The iris controls the size of the pupil. The ciliary muscles modify the curvature of the eye lens.
• The position of the retina. The ciliary muscles act on the eye lens, changing its curvature and so its focal length.
• The curvature of the eye lens — correct. Yes!
• An object close to the eye — correct. Yes!
• An object far from the eye. For distant objects the lens is thin and its focal length longer.
• Both have the same focal length. The ciliary muscles change the focal length: shorter for nearby objects, longer for distant ones.