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 colored part of the eye is called the iris.  the iris. 

It controls light levels inside the eye It controls light levels inside the eye similar to the aperture on a camera.  similar to the aperture on a camera. 

Radial and circular muscles in the Radial and circular muscles in the iris control pupil size in response to iris control pupil size in response to lightlight

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

Distant Objects

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

Near Objects

MyopiaMyopia(short sightedness) (short sightedness)

the eye is too long and so the image focuses the eye is too long and so the image focuses short of the retinashort of the retina

Long sightednessLong sightednessthe eye is too short so the imagethe eye is too short so the image

focuses past the retinafocuses past the retina

Correcting short sightednessCorrecting short sightednessa concave lens is used.a concave lens is used.

Correcting long sightedness-Correcting long sightedness-a convex lens is useda convex lens is used