Focussing Light onto the retina WALT How light rays are focussed onto the retina How the cornea and lens refract light rays Accommodation of the eye

Controlling light levels Your eye are very sensitive and can be damaged by harsh light. Your iris controls light allowed into the eye by changing the size of the pupil

The colored part of the eye is called the iris.  It controls light levels inside the eye similar to the aperture on a camera.  Radial and circular muscles in the iris control pupil size in response to light The round opening in the center of the iris is called the pupil.   

Control of Pupil Size

Focusing on objects The lens and cornea focus the light on the retina

Focusing The lens job is to make the rays hit the same point The red rays will be out of focus

Bending light Light is refracted and bent to focus it as it passes through the lens Lens thickness can be changed so the amount of bending is changed

Near vision The lens needs to bend the light more to focus it Fat lens needed

Distance vision Rays enter the eye closer together Need less bending Thinner lens needed

Changing lens thickness Ciliary muscles are attached to the lens, when relaxed they pull on the suspensory ligaments causing the lens to be thin When contracted the ciliary muscle cause the suspensory ligaments to slacken and so the lens is fatter

Accommodation When we open our eyes in the morning they are not focussed on near by objects At rest the ciliary muscles relax , puling the lens flat In this state we can focus on distant objects To focus on something near by the ciliary muscles contract reducing the tension on the suspensory ligaments so the lens gets fatter This is called accommodation – changing the shape of the lens to focus on near or distant objects

When the eye looks at an object that is far away the ciliary muscle relaxes which pulls on the suspensory ligaments. These pull on the lens and make it flatter (less convex). This brings the rays of light from the object into crisp focus on the retina.

Distant Objects

With an object closer to the eye, the lens needs to be more convex (fatter). To do this, the ciliary muscles contract making the suspensory ligaments slack and to allow the rubbery lens to return to its naturally rounder shape.

Near Objects

Myopia (short sightedness) the eye is too long and so the image focuses short of the retina

Long sightedness the eye is too short so the image focuses past the retina

Correcting short sightedness a concave lens is used.

Correcting long sightedness- a convex lens is used