The development of optical devices using lenses dates to the sixteenth and seventeenth centuries, although the earliest record of eyeglasses dates from the late thirteenth century. To be a little more specific, history records state that the first spectacles were made between 1268 and 1289.[1] Around 1284 in Italy, Salvino D'Armate is credited with inventing the first wearable eye glasses.[2]
There are two types of lenses; converging and diverging lenses. The converging lens is thicker in the center than in the edges and makes parallel rays converge to a point. On the other hand, the diverging lens is thinner in the center than at the edges and makes parallel light diverge. These lenses are used for many purposes but are very useful as corrective lenses for humans with eye defects.
Some of the eye defects discussed in class were; myopia, hyperopia, presbyopia, and astigmatism. Myopia, also known as nearsightedness, is when the eye can focus only on nearby objects. It is usually caused by an eyeball that is too long or that the curvature of the cornea is too great. In either case, the images of distant objects are focused in front of the retina (Figure 1 a.). The lens used to correct this defect is a divergent lens because it allows the rays to be focused at the retina (Figure 1 b.).
Figure 1: Nearsighted eye (a) Image is focused in front of the retina; (b) image is located on the retina with the use of a divergent lens
Hyperopia, also known as farsightedness, is when the eye cannot focus on nearby objects because the near point is greater than the “normal” (25 cm). It is usually caused by an eyeball that is too short or by a cornea that is not sufficiently cureved (Figure 2 a.). The lens used to correct this defect is a converging lens. Presbyopia is similar to farsightedness. It refers to the lessening ability of the eye to accommodate as a person ages, and the near point mores out. It is also corrected by a converging lens (for older people, bifocals are used.
Figure 2: Farsighted eye (a) Image is focused behind the retina; (b) image is located on the retina with the use of a convergent lens
Finally we have astigmatism which is usually caused by an out-of round- cornea or lens so that the objects are focused as short lines, which blurs the image. An astigmatic eye may focus on rays in one plane. Astigmatism is corrected with the use of a compensating cylindrical lens. Lenses for eyes that are nearsighted or farsighted as well as astigmatic are ground with superimposed spherical and cylindrical surfaces, so that the radius of curvature of the correcting lens is different in different planes.
Like I mentioned before, my eyes have multiple defects. These are; myopia, hyperopia, presbyopia, astigmatism, strabismus, and nystagmus. As you can see, I have all the defects discussed in class. But, how can one person be near and farsighted at the same time? Easy, I am nearsighted on the right side and farsighted on the left side. I always got these confused and never knew which one was which, but with the description in class and the book I was able to determine this. So with this we can conclude that I use a divergent lens on the right side and a convergent lens on the left side with superimposed spherical or cylindrical surfaces on each because I also have astigmatism.
Now, when I read my prescription I know what things mean. The plus sign (+) is for my left side because that is the convergent lens that corrects my hyperopia. The minus sign (-) is for my right side because that corrects my myopia. Even though this topic was only one section on the book, I now understand how to read my prescription and what types of lenses correct each defect.
[1] What man devised that he might see; Drewry R.D. Accessed on November 12, 2011.
[2] The History of Eye Glasses or Spectacles; Bellis, B. Accessed on November 12, 2011
Other Resources: Giancoli D. (2008); Physics for Scientists & Engineers Fourth Edition p. 882-884
nice!
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