Topic 4: Lenses and Vision Lens a curved transparent material through which light passes (transmit) Ex) glass, plastic
Double Concave Lenses Are thinner and flatter in the middle than around the edges. Light passing through the thicker, more curved outer edges refracts more then the middle. Light is spread out (diverges) after passing through the lens.
Double Convex Lenses Are thicker in the middle than around the edges. Light comes together (converges) after passing through the lens. The more curved, the convex lens, the closer the focal point (the point where the light rays converge after passing through a lens).
Images formed by a convex lens are inverted (upside down), like in the human eye
The Human Eye Has a convex lens that refracts light and focuses them. That is, it brings the light rays entering the eye to a single focal point. This focal point is on a light-sensitive area at the back of the eye called the retina. The images form on the retina and is sent to the brain for processing
Vision Problems: Near-sighted (myopia) Some people have eyeballs that are elongated, making the focal point not on the retina, but in front of it. They have trouble seeing far away. A concave lens in front the eye will correct the vision by making the light rays refract twice and form the image further back onto the retina This is the most common vision problem rays diverge first after going through the lens rays then converge on to the retina
Far-sighted (hyperopia) Some people have eyeballs that are too short, making the focal point behind the retina. That is, the image does form on the retina. They have trouble seeing objects up close. Corrected by placing a convex lens in front of the eye Someone with perfect vision is 20/20, meaning that you clearly see an object from 20 feet away what other people also see from 20 feet away. 20/15 means you see something from 20 ft that others see only at 15 (you have better vision). Legally blind is 20/200. A hawk can have 20/2 vision! rays converge twice: first time after the lens, then through the eye lens
Focusing The lens in the eye constantly adjusts shape to keep objects in focus on the retina. This process of changing the shape of the lens to adjust for different object distances is called accommodation As the lens ages, it loses its flexibility and cannot change shape to adjust for seeing objects. They then have to wear glasses. Near point the shortest distance at which an object is in focus. Adults~25cm, babies ~7cm Far point is the longest distance away from the object but is still in focus. Usually infinity
Human Eye Parts and Function cornea light first passes through this thin, transparent layer covering the front of the eye before going through the pupil iris- the colored ring that surrounds and controls the opening and closing of the pupil. The iris reflex is the rapid automatic adjustment of the pupil from lighted areas to darkened areas. pupil-the dark centre that is actually a hole through which light passes.
lens - Light refracts through the double convex lens and forms an image on the retina. Optic nerve- bundle of nerve cells that carry the electrical impulses from the retina to the brain. Where the light does not strike the retina is called the blind spot. retina- light-sensitive back of eye that receives light rays and sends electrical impulses along the optic nerve to the brain for processing. The retina contains special cells called rods to detect light, and cones to detect colors.
Sclera the white, thick layer that forms the outside of the eyeball Vitreous humour fluid that fills the interior of the eye. It gives it support so the eye does not collapse inward and also help with refraction of light. Ciliary muscles- controls the shape of the lens. Changes the lens shape to adjust for focusing. Aqueous humour- fluid that fills up the layer between the cornea and the iris
Camera vs. Human Eye Camera diaphragm controls the opening and closing of the hole to let light in. Similar to the iris. Aperture is the opening in the diaphragm that allows light in. Similar to the pupil. Shutter limits the amount of light in. Similar to the eyelid. Both have lens to refract light. Camera has a focusing ring for focusing. Eye has the ciliary muscles that focuses by changing lens shape. Camera film is where image forms. Like the retina