Common vision defects are easy to understand, and some are simple to correct. Figure 1 illustrates two common vision defects. Nearsightedness , or myopia , is the inability to see distant objects clearly while close objects are clear. The eye overconverges the nearly parallel rays from a distant object, and the rays cross in front of the retina. More divergent rays from a close object are converged on the retina for a clear image. The distance to the farthest object that can be seen clearly is called the far point of the eye normally infinity.
Farsightedness , or hyperopia , is the inability to see close objects clearly while distant objects may be clear. A farsighted eye does not converge sufficient rays from a close object to make the rays meet on the retina. Less diverging rays from a distant object can be converged for a clear image. The distance to the closest object that can be seen clearly is called the near point of the eye normally 25 cm. Figure 1. This can be caused by the lens of the eye being too powerful or the length of the eye being too great.
This can be caused by insufficient power in the lens or by the eye being too short. Since the nearsighted eye over converges light rays, the correction for nearsightedness is to place a diverging spectacle lens in front of the eye. This reduces the power of an eye that is too powerful.
Another way of thinking about this is that a diverging spectacle lens produces a case 3 image, which is closer to the eye than the object see Figure 2. Then the image produced by a spectacle lens must be at this distance or closer for the nearsighted person to be able to see it clearly.
It is worth noting that wearing glasses does not change the eye in any way. The eyeglass lens is simply used to create an image of the object at a distance where the nearsighted person can see it clearly. Whereas someone not wearing glasses can see clearly objects that fall between their near point and their far point, someone wearing glasses can see images that fall between their near point and their far point.
Figure 2. Correction of nearsightedness requires a diverging lens that compensates for the overconvergence by the eye. The diverging lens produces an image closer to the eye than the object, so that the nearsighted person can see it clearly. What power of spectacle lens is needed to correct the vision of a nearsighted person whose far point is Assume the spectacle corrective lens is held 1.
You want this nearsighted person to be able to see very distant objects clearly. That means the spectacle lens must produce an image An image The image distance is negative, because it is on the same side of the spectacle as the object.
The negative power indicates a diverging or concave lens, as expected. The spectacle produces a case 3 image closer to the eye, where the person can see it. If you examine eyeglasses for nearsighted people, you will find the lenses are thinnest in the center. Additionally, if you examine a prescription for eyeglasses for nearsighted people, you will find that the prescribed power is negative and given in units of diopters.
Since the farsighted eye under converges light rays, the correction for farsightedness is to place a converging spectacle lens in front of the eye.
This increases the power of an eye that is too weak. Another way of thinking about this is that a converging spectacle lens produces a case 2 image, which is farther from the eye than the object see Figure 3. Then the image produced by a spectacle lens must be at this distance or farther for the farsighted person to be able to see it clearly.
Figure 3. Correction of farsightedness uses a converging lens that compensates for the under convergence by the eye. The converging lens produces an image farther from the eye than the object, so that the farsighted person can see it clearly.
What power of spectacle lens is needed to allow a farsighted person, whose near point is 1. When an object is held An image 1. The object is The word "lens" owes its origin to the Latin word for lentils, the tiny beans that have from ancient times been an important ingredient in the cuisine of the Mediterranean region.
The convex shape of lentils resulted in thier Latin name being coined for glass possessing the same shape. Because of the way in which lenses refract light that strikes them, they are used to concentrate or disperse light.
Light entering a lens can be altered in many different ways according, for example, to the composition, size, thickness, curvature and combination of the lens used. Many different kinds of lenses are manufactured for use in such devices as cameras, telescopes, microscopes and eyeglasses. Copying machines, image scanners, optical fiber transponders and cutting-edge semiconductor production equipment are other more recent devices in which the ability of lenses to diffuse or condense light is put to use.
Lenses may be divided broadly into two main types: convex and concave. Lenses that are thicker at their centers than at their edges are convex, while those that are thicker around their edges are concave. A light beam passing through a convex lens is focused by the lens on a point on the other side of the lens. This point is called the focal point. In the case of concave lenses, which diverge rather than condense light beams, the focal point lies in front of the lens, and is the point on the axis of the incoming light from which the spread light beam through the lens appears to originate.
Concave lenses are used in eyeglasses that correct nearsightedness. Because the distance between the eye's lens and retina in nearsighted people is longer than it should be, such people are unable to make out distant objects clearly. Placing concave lenses in front of a nearsighted eye reduces the refraction of light and lengthens the focal length so that the image is formed on the retina.
Convex lenses are used in eyeglasses for correcting farsightedness, where the distance between the eye's lens and retina is too short, as a result of which the focal point lies behind the retina. Eyeglasses with convex lenses increase refraction, and accordingly reduce the focal length. Most optical devices make use of not just one lens, but of a combination of convex and concave lenses. For example, combining a single convex lens with a single concave lens enables distant objects to be seen in more detail.
This is because the light condensed by the convex lens is once more refracted into parallel light by the concave lens.
This arrangement made possible the Galilean telescope, named after its 17th century inventor, Galileo. Adding a second convex lens to this combination produces a simple telephoto lens, with the front convex and concave lens serving to magnify the image, while the rear convex lens condenses it. This is how zoom lenses work. The focused image through a single convex lens is actually very slightly distorted or blurred in a phenomenon known as lens aberration.
The reason why camera and microscope lenses combine so many lens elements is to correct this aberration to obtain sharp and faithful images.
One common lens aberration is chromatic aberration. Ordinary light is a mixture of light of many different colors, i. Because the refractive index of glass to light differs according to its color or wavelength, the position in which the image is formed differs according to color, creating a blurring of colors. A social network can be represented as. The hybrid variety of mango developed from the cross of Neelum and Alphonso is.
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