Light and Your Eyes

You have five human senses that help you function. These senses work together to give you a complete picture of your environment. One of the most important senses is your sense of sight. It has been said that a picture is worth a thousand words. This is because sight is the most powerful sense we possess.

The eye is made up of parts that work together to help you see. The front of the eye is covered with a clear layer called the cornea. The cornea is a protective layer that keeps dirt and bacteria from damaging the inner eye.

The cornea covers the pupil. The pupil is the opening through which light enters the eye.

The iris is a muscle that controls how much light enters the eye. It contracts or expands depending on the amount of light available. When there is too much light it contracts, too little light and it expands.

After light enters the eye it is focused by the lens. The lens is the clear part located behind the pupil. The lens can change shape so the eye can concentrate on an object. The lens works like the autofocus on a camera.

Light entering the eye is focused on the retina by the lens. The image is upside down when it is projected onto the retina. The retina converts the image into electrical impulses.

These impulses are sent to the brain along the optic nerve. The brain then changes the impulses back into a right side up image.

Two other important parts of the eyes are the rods and cones. They are photoreceptor cells. It is their job to sense light and colors.

Cone cells give us our detailed color daytime vision. There are 6 million of them in each human eye. Most of them are located in the central retina. There are three types of cone cells: one sensitive to red light, another to green light, and the third sensitive to blue light.

Rod cells are about 500 times more sensitive to light then cone cells; they give us our dim light or night vision. They are also more sensitive to motion then cone cells. There are 120 million rod cells in the human eye. Most rod cells are located in our peripheral or side vision.

The sclera is the white, tough wall of the eye. It, along with internal fluid pressure, keeps the eyes shape and protects its delicate internal parts

Our eyelids and eyelashes protect our eyes. Small oil-producing glands lining the inner edge of our eyelids lubricate our eyes. These oils mix with tears when we blink, keeping the eye moist and clean.

The vitreous humor is a jelly like liquid that fills most of the eye (from the lens back). As we age it changes from a gel to a liquid and gradually shrinks separating from the retina. This is when people start seeing floaters, dark specs in their vision. This is a normal sign of aging, but in a few cases the retina can become detached as the vitreous separates.

The aqueous humor is the thin, watery fluid that fills the space between the cornea and the iris. It is continually produced by the ciliary body, the part of the eye that lies just behind the iris. This fluid nourishes the cornea and the lens and gives the eye it's shape. dical-terms- glossary.com/A a-Az/aqueous- humor.html

Light that shines on a material that blocks the light and forms a shadow is called an opaque material.

Some material may allow the light to pass through unaffected. This type of material is called a transparent material.

A material may allow only some light to pass through. This type of material is called translucent.

OBJECTIVES Label the main parts of the human eye. Learn the main parts of the eye.

IN ACTIVITY You will need to work with a partner for this activity. 1. Stand face to face. I will darken the room. Watch what happens to your partners eyes. 2. When I turn the lights back on watch their eyes again. 3. Record what you observed when the lights were turned off and when they were turned on.

VOCABULARY Translucent – some light can pass through Transparent – all light can pass through Opaque – no light can pass through

vitreous humor aqueous humor

NOTES Cornea - protects the eye Pupil – lets in light Iris - regulates light Lens – focuses light Retina – captures light and turns into signals to send to brain Optic nerve – carries signals to brain Rods and cones – detect light and color Sclera – wall of the eye

OUT QUESTION Everyone in this room has the same color eyes. Can you guess what color they are and why?

OUT QUESTION The answer to the question is that we all have brown eyes. That’s the only color of pigment in the iris. The different colors are the result of more or less pigment.