The Eye

The Human Eye

• The human eye is the optical instrument that helps most of us learn about the external world.

How We See Colour• The retina of the human eye contains two types

of cells that respond to light.• Some cells look like tiny cylinders, called rods, and they

detect the presence of light.• The other cells are cones because they look like cones.

These cells respond to colour.» Some of the cones respond to green, some respond to red,

and the third type of cone responds to blue.

• Signals from the rods and all three type of cones travel along the optic nerve to the brain. The brain interprets the shape and colour of the objects seen.

• Most people believe that we see with our eyes. In reality, the eye acts as a light gathering instrument. We actually see with our brain!

• The cornea-lens combination acts like a converging lens and produces smaller, real, inverted images on the retina.

• Impulses from the retina travel through the optic nerve to the brain. The brain takes the inverted image and flips it so that we see the image as it appears.

• This is how the lens in a normal human eye focuses light rays onto the retina.

• Light refracts through the lens onto a light-sensitive area at the back of the eye called the retina.

• The image you see is formed on the retina.

Myopia – Nearsightedness

•For some people, the eye has a longer shape.

•This means the imaged forms in front of the retina. These people are near sighted (have trouble seeing far objects).

Hyperopia – Farsightedness

•For some people, the eye has a shorter length so the image has not formed by the time it reaches the retina. The image forms behind the retina.

•These people are far sighted and have trouble seeing things close up.

Presbyopia

• Presbyopia is an age-related loss of flexibility of the lens inside the eye.

• With the onset of presbyopia, you'll find you need to hold books, magazines, newspapers, menus and other reading materials farther away in order to see the print clearly.

• Knowledge of how light behaves when it travels through lenses helps eye specialists correct vision problems.

• A convex lens in placed in front of the far sighted eye to help bend the light rays to form an image on the retina.

Eyes and Cameras

• There are many similarities between the human eye and a camera.

• You know that you see objects when light is reflected to your eye, refracted through your lens, and focussed on your retina.

• In a camera, the lens refracts the light and the film senses the light.