THE HUMAN EYELights and Lenses

Explore: How does the eye focus an image?• Procedure:

-Position yourself so you can clearly see an object across the room.-Close 1 eye. Hold up your

index finger and bring it as close toyour open eye as you can while keeping the

finger clearly in focus.-Keeping your finger in place, look just to the side at the more distant object and focus your eye on it.-Without looking away from the more distant object, observe your finger.

What Do You Think?

-Write an explanation in words of what happens

-Draw a picture of what you see.

What might be happening in your eye to cause this change in the nearby object?

The Purpose of the Eye• The main function of the eye is to work with

the brain to provide us with vision. The eye and brain translate light waves into a sensation we call vision

VISION

• Light is REFLECTED off of an object, into your eye. The cornea and lens of the eye BEND the light (REFRACTION) and focus the light on the RETINA. The photoreceptors in the RETINA send signals to the brain through the OPTIC NERVE.

THE IRIS

The Iris is the colored part of the eye. The Iris controls theamount of light in the eye.

THE CORNEA

• The Cornea is a powerful refracting surface that provides 2/3 of the eye’s focusing power.

PUPIL

• The Pupil is the opening in the center of the Iris.

LENS

• The purpose of the lens is to focus light onto the RETINA.

RETINA• Contains more than 125

million photoreceptors and support neurons.

Macula

• Structures in the macula are specialized for high acuity vision. Within the macula are the fovea and foveola which contain a high density of cones (photoreceptors with high acuity).

RETINAA layer of tissue called the RETINA lines the inside of your eye. The RETINA has a layer of cells that detect light (PHOTORECEPTOR CELLS). As the cells absorb light, they start a signal that goes to the brain. In the retina are two types of light-detecting cells: RODS & CONES.

The RETINA- A CLOSER LOOK @ PHOTORECEPTOR CELLS

ROD CELLS-Long, thin cells that let you see in dim light.

CONE CELLS-Cone-shaped cells that let you see color.

RODS-The signals they send tell your brain only how bright the light is.

• There are very few rod cells in the very center of your retina.

• You may not see dim light when you look straight at it. If you look slightly to one side, you may see better.

Distribution of Rods and Cones on the Retina

CONES-Detect high levels of light and let you see color. There are three types of cones:

“Blue” Cones absorb the blue end of the spectrum best.

“Green” Cones absorb well from yellow through cyan. They absorb green light best.

“Red” Cones absorb RED lightbest, but also frequencies ofyellow and green light.

Color Blindness

Test Plate 1: Those with normal color vision should read the number 8.Those with red-green color vision deficiencies should read the number 3.Total color blindness should not be able to read any numeral.

Test Plate 2: Normal color vision should read the number 5. Those with color vision deficiencies will not read the number or read it incorrectly.

Color-BlindnessTest Plate 4: Normal colorvision should read thenumber 15.

Red-Green deficienciesshould read the number 17.

Total color blindnessshould not be able to read anynumeral.

Test Plate 5: Both normal and those with allcolor vision deficiencies shouldread the number 12.

If a person had no green cones, would he or she be able to tell the difference between red and yellow? Explain.

Mixing Light:Equal parts of green light and

red light make yellow light. If a person’s green cones

were not functioning, they would not be able to see green light and they would not be able to tell the difference between red and yellow.