6.1 La Vision Humaine (Sciences 8) PDF

# Sciences 8 - 6.1 Notes: La vision humaine ## The entrance of light into the eye: * The light enters the eye through the **pupil**, the dark-looking opening. * The **iris** is the colored circle (grey, blue, brown, or hazel) of muscles that surround the **pupil**. * When the light is weak, t...

# Sciences 8 - 6.1 Notes: La vision humaine ## The entrance of light into the eye: * The light enters the eye through the **pupil**, the dark-looking opening. * The **iris** is the colored circle (grey, blue, brown, or hazel) of muscles that surround the **pupil**. * When the light is weak, the iris dilates the pupil to allow more light to enter. * When the light is bright, the iris contracts the pupil to reduce the amount of light that enters. * The **cornea** is a transparent tissue covering the iris and pupil. * Surrounding the iris is white opaque tissue called the **sclera** (or sclerotic). * Behind the pupil is a soft, biconvex lens called the **crystallin**. * Light rays pass through the lens and are focused on the **retina**. * The image is formed here: **Photoreceptor cells** in the retina detect the image. * Retina cells convert light rays into electrical signals that are transmitted to the brain via the **optic nerve**. ## The cornea-crystallin-retina system: * The focusing system is comprised of the cornea, the crystallin, and the spaces in the eye filled with clear fluid. * The fluid between the crystallin and cornea, the **aqueous humor**, supports the cornea and the crystallin and diffuses nutrients from the **choroid** to the cornea, which has no blood vessels. * Why does the cornea not have blood vessels? * Behind the crystallin, the **vitreous humor** gives the eye its shape and supports the crystallin. * Focusing of light rays begins when they enter the cornea. * The cornea refracts the rays so that they converge on the retina. * The cornea is responsible for most of the focusing, and the crystallin takes care of the rest. ## The crystallin: * The crystallin is able to adjust focus by changing its shape automatically: * When tiny muscles contract, the crystallin becomes thicker. * A thicker crystallin focuses on closer objects, like text in a book or a cell phone screen. * When these muscles relax, the crystallin becomes thinner. * A thinner crystallin focuses on objects further away. * You can feel this intense focus when you hold your finger close to your eye and try to see it clearly. ## The blind spot: * The point where the **optic nerve** (optic disc) and the retina connect has no photoreceptor cells. * This is called the **blind spot**. * To find your blind spot: 1. Hold this page at arm's length. 2. Cover your right eye with your hand. 3. Look directly at the X while slowly moving the paper closer to your face. * The X should disappear and then reappear as the image comes into and then exits the blind spot. ## Photoreceptor cells: Rods and cones: * Once light rays are focused on the retina, the two types of photoreceptor cells begin to work: * **Rods** are long and cylindrical and are specialized for detecting dim light. * **Cones** absorb bright light; they are rounded in shape. * **Rods** are very sensitive to light and are the basis of black and white vision (shades of light and dark). * They help us distinguish shapes and movement in low light conditions. * **Cones** are less sensitive to light and are far less numerous in the retina. * There are three types of cones, each with a different pigment: RGB. * If our brain receives equal amounts of signal from these three cones, we see an object as white. ## The path of light rays from the source to the brain: * Light Source * 1. Cornea * 2. Pupil, Iris * 3. Crystalline Lens * 4. Vitreous Humor * 5. Retina * 6. Optic Nerve * 7. Brain ## Photoreceptor Cells: | Cells | Description | | --------- | --------- | | **Rods** | Used for night vision. Very sensitive to light. One type of pigment. Slow reaction to light. Smaller than cones. About 100 million in a human eye. Found outside the center of the retina. | | **Cones** | Used for bright lights. Less Sensitive to light. Three types of pigments. Fast reaction to light. Larger than rods. About 6 million in the human eye. Found in the center of the retina. | ## Correcting focus problems: * Many people have difficulty focusing clearly at some point in their lives. * As children grow, especially during adolescence, the shape of their eyes changes, affecting focus, which may require temporary glasses. * In adulthood, the crystallin becomes less flexible, which makes it difficult to focus on nearby objects. * Most focus problems can be corrected with lenses that are **concave** or **convex**: **A. Normal Vision** - The light from distant and near objects is focused on the retina. **B. Myopia (Nearsightedness)** - The image is focused in front of the retina. - The eye is longer than normal. - A **concave** lens is used to correct the vision to allow light to reach the retina. **C. Hyperopia (Farsightedness)** - The image is focused behind the retina. - The eye is shorter than normal. - A **convex** lens is used to correct the vision to allow light to reach the retina. ## Correcting astigmatism: * A normal cornea is spherical in shape. * Astigmatism happens when the cornea is misshapen, which causes the image to be focused at multiple points on the retina, resulting in blurry vision. * This can be corrected with corrective lenses, contact lenses, or laser surgery to reshape the cornea. ## Blindness: * A visual impairment that prevents someone from carrying our daily activities. * Can involve partial or complete inability to see light. * Some people can only see a small part of their field of vision; others can see the edges of their field, but not objects within it. This can be caused by **Macular Degeneration (AMD)**, which is age-related. **Macula:** * The macula is the area of the retina with the highest concentration of cones (the area where details are seen most clearly). * The fovea is the center of the macula, located in a depression with only cones. ## Other vision conditions: * **Poorly-formed images:** Some people can only distinguish between light and dark and cannot form images. * **Night Blindness:** This condition makes vision difficult or impossible in low-light settings due to the loss of rods' ability to detect light. * It can happen at birth, or develop due to illness or malnutrition. * **Snow Blindness:** This painful temporary condition occurs when the eyes are overexposed to sunlight, especially at high altitudes. * Resting the eyes in a dark room, and sunglasses help prevent this condition. * **Daltonism:** A difficulty distinguishing colors (mainly shades of grey for those most affected), affecting only 8% of males and 1% of females. * **Color blindness:** This condition typically affects the ability to distinguish between red and green, with these two hues often seen as shades of yellow. ## Dissection of a sheep eye (LAB 6-4), pp 212-213: * **Dissect:** To cut apart for study. * **Safety Glasses:** to protect the eyes from sharp edges. * **Prod (aiguillon):** A tool used to gently push tissue during dissection. * **Eyeball (globe oculaire):** The globe-shaped organ containing the lens and retina. * **Dissecting Tray:** A flat surface for working on. * **Yellowish:** This describes the color of the vitreous humor. * **Extraocular muscles:** These control the eye movements. * **Stray light:** Extra light that is not part of the image. * **Iridescent layer:** A surface that shows many colors when light is reflected from it (the iris). ### After the dissection, do the following: 1. Place all the eye parts in a plastic garbage bag. 2. Clean the dissection tools and surface with 10% bleach solution. 3. Remove and dispose of gloves. 4. Wash your hands thoroughly with warm, soapy water.