1. The Human Eye

2. The Human Eye A convex lens is the type of lens found in your eye.
The lens takes light rays spreading out from objects and focuses the light, through the process of refraction, on the retina.

3. Light enters the eye and passes through the cornea.
As light passes through the cornea it is refracted.
Light continues through the lens, where it is refracted even more.

4. The Human Eye:
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5. Normal, Near and Far
Normal Eyesight: Light refracts through the lens onto a light sensitive area at the back of the eye called the retina.
Near-Sighted: In this case the image forms in front of the retina. This causes difficulty in seeing distant. Vision is corrected with convex lenses.

6. Far-Sighted: In this case the image has not completely formed by the time the light reaches the retina. This causes difficulty in seeing close objects. Vision is corrected with concave lenses.

7. Vision Problems:
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8. Parts of the Eye
Sclera: The white part of the eyeball. The sclera is made of a tough material and has the important job of covering most of the eyeball.
The retina uses millions of special cells called rods and cones to process light.

9. Rods: Rods see in black, white, and shades of gray and tell us the form or shape that something has.
Cones: Cones sense color and they need more light than rods to work well. Cones are most helpful in normal or bright light. Cones can sense combinations of light waves that enable our eyes to see millions of colors.

10. Comparing the Eye and the Camera
Both the human eye and the camera use a convex lens to gather light from an object and produce an image of the object.
The camera must use a set of lenses to achieve the same effect as the eye.

11. Putting it in Focus...
If an object moves closer to a camera, the lens must move farther away in order to keep focus.
The lens of an eye cannot move farther away from the retina so instead ciliary muscles change the shape of the lens.

12. Changing the Lens Shape
When you look at objects that are far away, your eye can focus clearly and the ciliary muscles are relaxed and the lens is normal shaped.
As objects move closer, the ciliary muscles force the lens to become thicker keeping the object in focus.

13. Bringing in the Light
In a camera the diaphragm and shutter can be adjusted to allow the correct amount of light.
In the eye the iris, the coloured ring, functions like the diaphragm of a camera.

14. When there is not enough light the iris increases the size of the eye’s opening, the pupil, to let more light in.
When there is too much light the iris decreases the size of the pupil to reduce the amount of light let in.

15. Seeing the Image
In a camera the image is produced on a digital device.
In the eye images are formed at the back of the eye on the light sensitive area called the retina. The retina converts the image into a nerve signal that is sent to the brain through the optic nerve.