12.3 - Lens Technologies and The

Human Eye

The Human Eye

Cornea:◦ Tissue that forms a transparent, curved structure

in front of the eye◦ Refracts light before it enters the eye

Retina:◦ A layer of cells (known as rods and cones) that

respond to light and initiate nerve responses Rod cells are very sensitive to light but cannot

distinguish between colours Cone cells detect colour

The Human Eye

Recall: normally, when an object moves in front of a lens, its image moves; moving the lens has the same effect

This won’t work in the eye – images from different distances need to focus on the retina, but the distance between the retina and the lens is always the same

So what happens?

How the Eye Focuses

The cornea refracts light in the same way regardless of the location of the object

The lens, however, can change shape and refract light to a different extent

This allows it to focus light from both nearby and distant objects onto the retina

The ciliary muscles (or ciliary bodies) make the lens shorter and thicker by pulling on it

How the Eye Focuses

This process is called accomodation To focus on a nearby object, the curvature of the

lens needs to be greater because it needs to refract light more

How the Eye Focuses

Cameras are designed very much like eyes

Both have lenses that focus light on a light-sensitive material◦ Retina in the eye◦ Film or CCD in a camera

Lens of the eye changes shape in order to focus on objects at different distances

Lens of the camera must be moved in and out to focus on objects at different distances

Eye has an iris (or pupil) that controls the amount of light entering it

Camera also has an iris (or aperture) to control the amount of light

The Eye vs. The Camera

Myopia is near-sightedness◦ The eyes cannot focus on distant objects

Caused by the eyeball being too long The image forms in front of the retina, not on it By the time light rays reach the retina they have

begun to diverge, and the image looks fuzzy

Correcting Vision Using Lenses - Myopia

Adding a diverging lens spreads out the parallel light rays before they reach the eye

The rays that are separating from each other appear to be coming from an object that is closer to the eye

Due to the higher angle of incidence, when the eye refracts the light, it is focused correctly on the retina

Correcting Vision Using Lenses - Myopia

Hyperopia is far-sightedness◦ The eyes cannot focus on nearby objects

Caused by the eyeball being too short Light rays entering the eye reach the retina

before they converge at a point, causing the image to be blurry

Correcting Vision Using Lenses - Hyperopia

Adding a converging lens bends the rays slightly inward before they enter the eye

The lens of the eye then refracts them more, and causes the rays to be correctly focused on the retina

Correcting Vision Using Lenses - Hyperopia

As a person ages, the lenses of the eye become stiffer, and the ciliary muscles can no longer make them change shape

This condition is called presbyopia People with presbyopia cannot focus on nearby objects, but

this is not farsightedness (which is caused by an incorrect eyeball length)

If someone is already nearsighted and gets presbyopia, they cannot focus on distant OR nearby objects!

To correct this condition, people wear bifocals

Correcting Vision Using Lenses - Presbyopia

Bifocals are lenses with two parts◦ The top part of the lens corrects for nearsightedness◦ A small section of the lower part with a different

curvature helps the eye focus on nearby objects Bifocal contact lenses are also available

Correcting Vision Using Lenses - Presbyopia

Astigmatism is blurred or distorted vision that is usually caused by an incorrectly shaped cornea

Instead of being rounded, the cornea is oval-shaped

Part of an image might be in focus, but the rest of the image is blurry

This is corrected by using lenses that have a cylindrical curvature◦ Refracts incident light rays along one axis only

Correcting Vision Using Lenses - Astigmatism