1. 18.4 Seeing Light Pg

2. The Human Eye Your eyes allow you to sense light
The eye is a complex structure with many parts
You see objects when a process occurs that involves both your eyes and your brain

3. Light Enters the Eye Light enters the eye through the cornea
The cornea is a transparent surface that protects the eye
Cornea also acts as a lens to help focus light rays

4. The Pupil After passing the cornea, light enters the pupil
The pupil is the part of the eye that looks black
It is an opening through which light enters the inside of the eye
In Dim light:
Pupil becomes larger to allow in more light
In Bright Light
Pupil becomes smaller to allow in less light

5. The Iris
The iris is a ring of muscle that contracts and expands to change the size of the pupil
The iris gives the eye it’s color
Green, blue, brown ( the most common) , or hazel

6. An Image Forms
After entering the pupil, the light passes through the lens
The lens is a convex lens that refracts light to form an image on the lining of your eyeball

7. The Ciliary Muscles
Muscles called the Ciliary Muscles, hold the lens in place behind the pupil
Seeing Far Away:
The ciliary muscles relax, making the lens thin
Seeing Up Close
The ciliary muscles contract, making the lens thick

8. The Retina
When the cornea and lens refract light, an upside down image is formed on the retina
The retina is a layer of cells that line the inside of the eyeball

9. Rods and Cones
The retina is made up of tiny light-sensitive cells called rods and cones
Both help change images on the retina into signals that can travel to the brain
Rods are cells that contain pigment that respond to small amounts of light
They allow you to see in dim light
Cones are cells that respond to color
May detect red, green, and blue light
Works best in bright light

10. Rods and Cones

11. A Signal Goes to the Brain
Rods and Cones send signals to the brain along a short, thick nerve called the optic nerve
Optic Nerve begins at the blind spot
the blind spot is an area of the retina that has no rods or cones
The brain interprets the signals as an upright image
Also combines the images from each of your eyes into a single three dimensional image

12. Eye Diagram

13. Correcting Vision
If the eyeball is too long or too short, the image on the retina is out of focus
Wearing glasses or contacts can correct this type of vision problem
Concave lenses are used to correct nearsightedness
Convex lenses are used to correct farsightedness

14. Nearsightedness
A Nearsighted person can see nearby things clearly, but objects at a distance are blurred
The eyeball is too long and the lens focuses the image in front of the retina
Concave lenses in front of the eye spread out light rays before they enter the eye
Results in the image forming on the retina

15. Farsightedness
A farsighted person can see distant objects clearly, but nearby images may seem blurry
The eyeball is too short, so the image that falls on the retina is out of focus
Convex lenses correct this by bending light rays towards each other before they enter the eye
Results in an image that focuses on the retina