Have you ever tried to read in the dark? Of course not, it's impossible. In order for us to see, light is necessary. Your eyes see by detecting light, so anytime you see something, it is because light has come from that object to your eyes. Light can start from a source, such as a light bulb, or it can reflect off of an object, such as the page of a book. Whenever light travels from an object to your eyes, you see the object. Your eyes are the only structure in the human body that can detect electromagnetic waves. Light sources send out light waves that travel in all directions. The electromagnetic waves spread out from the light source just as ripples on the surface of water spread out from the point of impact of a pebble.

What do you see when you look into a plane mirror? A plane mirror is a flat surfaced mirror. When looking into these types of mirrors you see your reflection, and your reflection appears upright and seems to be behind the mirror at the same distance you appear in front of the mirror. The image you see is known as a virtual image . A virtual image is an image that appears to be real, but our mind knows its not. In order to see the mirror you must be in the line of sight. Mirrors follow the Law of Reflection and this law states that the anle of incidence must equal the angle of reflection. Look at the animation to the right and follow the light ray. If you are not in the path of the light ray, you will not see the image that reflected the light ray. Once the light ray enters the eye it is focused to a light sensitive structure in the eye known as the retina. The retina sends a signal to the brain where the signal is interpreted. Not all mirrors are flat like plane mirrors are. If the surface of a mirror is curved inward, it is called a concave mirror.If the surface is curved outward, it is called a convex mirror. Look at the animation below. Picture light approaching from left to right. Which mirror is concave?

A concave mirror has an optical axis. The optical axis is an imaginary straight line drawn perpendicular (at a right angle) to the surface of the mirror at its center. There is a point on this optical axis that every llight ray parallel to the optical axis is reflected through called the focal point. The distance from the center of the mirror to the focal point is called the focal length. The image formed by a concave mirror changes depending on where the object is located relative to the focal point of the mirror. If the object is in front of the focal point, the image formed is virtual and enlarged. If the object is behind the focal point, the image formed is real and upside down. A real image appears in front of the mirror and can be shown on a piece of white paper. Concave mirrors and real images are used in motion picture projectors, over-head projectors and film strip projectors.

A mirror that curves outward like the back of a spoon is a convex mirror . Light rays that strike a convex mirror diverge, or spread apart after they are reflected. Convex mirrors form virtual images which are smaller than they actually appear. Because convex mirrors spread light rays outward, they allow large areas to be viewed. In addition to increasing the field of view in places like grocery stores and factories, convex mirrors can widen the view of traffic. Look at the picture to the right, your see a convex mirror. When you look through the passenger mirror of an automobile, you are using a convex mirror.

A lens is a transparent material with at least one curved surface that causes light to ben, or refract as light rays pass through. The image that is formed is dependent upon the curvature of the lens. Look at the two images below and move you mouse over each image. A concave lens scatters light rays, while convex lenses bring light rays together and form a focal point. In convex mirrors the type of image that is formed depends on where the object is relative to the focal point. If the object is beyond the focal point the image is real, small, and upside down. If the object is in front of the focal point the image is upright, and magnified. Magnifying glasses make use of this property. Concave lens is thinner in the middle and thicker at the edges. As with convex lenses, concave lenses are used in eyeglasses, but you will find them also in refracting telescopes.

Your eyes have a built-in convex lens which helps focus light rays to the light sensitive part of the eye, the retina. When you look at an object, light rays are reflected from the object to the cornea, which is where the miracle begins. The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The lens' job is to make sure the rays come to a sharp focus on the retina. The resulting image on the retina is upside-down. Here at the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position! Think of the eye as a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens to refract, or focus the light and a film (retina) on which to focus the rays. If any one or more of these components is not functioning correctly, the result is a poor picture. The retina represents the film in our camera. It captures the image and sends it to the brain to be developed. Sometimes the retina encounters problems with its ability to focus light and vision is affected. Nearsightedness or myopia, occurs when light entering the eye focuses in front of the retina instead of directly on it. This is caused by a cornea that is steeper, or an eye that is longer, than a normal eye. Nearsighted people typically see well up close, but have difficulty seeing far away. This problem is often discovered in school-age children who report having trouble seeing the chalkboard. Near-sightedness usually becomes progressively worse through adolescence and stabilizes in early adulthood. It is an inherited problem. Signs and Symptoms - 1. Blurry distance vision 2. Vision seems clearer when squinting Farsightedness or hyperopia, occurs when light entering the eye focuses behind the retina, instead of directly on it. This is caused by a cornea that is flatter, or an eye that is shorter, than a normal eye. Farsighted people usually have trouble seeing up close, but may also have difficulty seeing far away as well. Young people with mild to moderate hyperopia are often able to see clearly because their natural lens can adjust, or accommodate to increase the eye’s focusing ability. However, as the eye gradually loses the ability to accommodate (beginning at about 40 years of age), blurred vision from hyperopia often becomes more apparent. Signs and Symptoms - 1. Difficulty seeing up close 2. Blurred distance vision (occurs with higher amounts of hyperopia) 3. Eye fatigue when reading 4. Eye strain (headaches, pulling sensation, burning) 5. Crossed eyes in children

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