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The Physics of Corrective Lenses
�A Physics Website Created at Miami University in Oxford, Ohio�
Introduction
Approximately $15.4 billion is spent on perscription eyewear in the United States every year. In fact, nearly 150 billion Americans require eyewear to correct refractive disorders, such ad myopia. So many people wear corrective lenses, but few actually understand how they work. This website serves to provide information on this topic. It will discuss how the human eye functions, as well as how corrective lenses improve vision. Sections of this site will explore and help explain various terms such as focal length, converging lenses, diverging lenses, concave lenses, and prisms which are key to corrective lenses. A preview to the future of autofocus lenses will also be covered. Hopefully this website will present how optics is present in the everyday lives of Americans and in turn, develop a greater understanding of and appreciation for optics.
Discussion of Physics
How the Eye Works and Why Eye Problems Occur:
The human eye can be compared to a camera. Like a camera the human eye gathers and focuses light through a lens to make an image. This image is created on the retina. The retina is a thin layer of cells located at the back of the eye. The lens�s job is to bend the light when it enters the eye. The clear covering on the front of your eye is the cornea. The cornea also helps in focusing the light onto the retina. Eye problems occur when light entering the eye is not properly focused onto the retina.
The Problems of Astigmatism and Presbyopia:
Astigmatism occurs when the cornea distorts the light entering the eye. Corrective lenses are usually able to fix this problem with a cylinder lens. The cylinder lens has two refractive powers on one lens. One power is placed over the entire lens and the other is placed in one direction. This creates one focal point on the retina.
Presbyopia occurs when the eye�s focusing power and ability to adjust the focus on the distance between the person and the object is lost. Simple magnifying lenses often easily fix this disorder.
Focal Length:
The focal length=the distance from the lens to the focus
(not actually stated in the prescription of glasses)
In eyeglasses, the focal length must be adjusted so as to focus the image directly onto the retina of the eye, creating a clear image for the person looking through the lense. An incorrect judgment of focal length would cause the image to be focused before or beyond the retina resulting in a blurred image.
Converging (Convex) and Diverging (Concave) Lenses:
Converging lenses = positive focal length and convex
(more commonly used than diverging lenses)
Converging lenses are normally convex. Convex lenses bring the focus beyond the lenses. A series of prisms bend or refract the light into a single point of focus. This effect is furthered in glasses by a gradual strengthening of the prisms in the lenses as one moves closer to the external border of the lenses.
Hyperopia or farsightedness is blurred vision that occurs when light focuses behind the retina because the eye is small or short. Convex lenses fix this problem.
Diverging lenses = negative focal length and concave
(magnify the eyes of the wearer)
Diverging lenses are commonly concave. Concave lenses have two parallel lines, creating a focus inside or out in front of the lenses.
Myopia or nearsightedness is blurred vision that occurs when the eye brings light to a focus in front of the retina. This happens because the eye is elongated. Concave lenses fix this problem.
Refractive Power:
Refractive power is measured in diopters. It is also the reciprocal of the focus
length, which is usually measured in meters. This measure is what is used in prescribing lenses. It expresses how much the light bends or refracts as it passes through the lense.
diopter = 1/f(meters)
(This formula shows that the greater the focal length, the smaller the refractive power.)
The Future of Eyeglass Physics:
The next generation of eyeglasses hopes to incorporate autofocus lenses, which will allow people to see clearly up close or far away. Presbyopia, which is a condition that causes a loss in the flexibility of the lens inside the eye, affects 93% of the world�s population over the age of 45. This problem is currently corrected with multifocal lenses, i.e. bifocal and trifocal lenses. The wearers of these must tip their heads up or down for clear distance or near vision because the eyeglasses are fixed to a certain prescribed thickness to bend light so that it focuses on the eye�s retina. However, variable focus lenses would shoot an infrared beam to the object at which a person is looking, and a microprocessor would calculate the power needed to bring the object into focus. The shape of the autofocus lens would not change, but computer programmed electroactive materials would bend incoming light to compensate for eye problems.
Answering Questions
What new ideas or issues did the articles teach us?
Eyeglasses, as mentioned in the introduction, are an important and present part in the lives of billions of Americans. Eyeglasses� ability to improve vision is often taken for granted. One never thinks to think about how or why their glasses work, they simply benefit from the fact that they do. These articles help to illustrate the incredible complexity which goes into the development of eyeglasses and the improvement of vision.
Eyeglasses work through one basic optics concept: focus. The lenses in glasses manipulate where the light which passes through them is focused so as to correct for problems with the lenses of a person�s eyes. These articles have demonstrated how, through the shapes of different lenses and their refractive properties, glasses manipulate the point of focus. In general, the articles have taught us how and why eyeglasses work and have, in turn, given us a greater respect for eyeglasses themselves.
How could this effect society?
Through a better understanding of how eyeglasses work, scientist have recently been able to improve immensely on eyeglass construction. Eyeglasses have become more powerful, efficient, affordable, and, through the research of new refracting materials, lighter and more comfortable. The future, if research continues, promises astounding new steps for the billions of Americans who suffer with vision problems, such as the development of auto focus lenses.
As understanding in the field of optics as it relates to eyeglasses grows, the social development and progress is not limited simply to the creation of new and better eyeglasses. Many scientists, now having a more efficient eyeglass to work with, are making steps to apply the same general principals and constructions to greater scale endeavors. A perfect example of this is the Eyeglass Space Telescope, created to view distant celestial bodies with the same technology sitting on a nearsighted schoolboy�s nose. In conclusion, eyeglasses, in being necessary aids for so many Americans, play an everyday role in modern social life. The future uses of this integral technology may also then expand its social impact even more broadly as understanding increases.
Links for Further Reading
Katherine Blodgett - See a biography of the inventor of modern eyeglasses.
Pictures of Lenses - To get a better picture of how concave or convex lenses look or work, check out these hand-drawn diagrams.
Eye to Eye - A web site which provides a good commercial example of eyeglass science.
McNelly Optical - Helps connect common eyeglass terms and models to the technical physical properties of different lenses.
The Diffractive Optics Group - Information on the new Eyeglass Space Telescope.
What to Expect - Amazing things some companies are looking to do with eyeglasses.
Conclusion
In summary, eyeglasses serve to correct deficiencies in the lenses� of the eyes ability to focus images. They do this by altering the focal length and point of focus. This alteration is controlled by the use of different lenses� shapes. The different shapes create different refraction patterns and, in turn, change the point of focus to be either before or beyond the eyes� lenses. Using these optical technologies, people can then improve their vision substantially and, as a result, improve their lives. As the applications of eyeglass science expand, who knows how much the effect of eyeglasses themselves have on American society will grow? A technology which already influences billions of Americans is on the threshold of becoming something even greater and the possibilities become more and more endless as the understanding of eyeglasses develops.
Bibliography
Broten, Bob. �How Corrective Lenses Work.�2000. How Stuff Works. http://www.howstuffworks.com/lenses
This article thoroughly describes how the human eye works and focuses along with the procedures for making sunglasses particular to certain individuals. It also illustrates common eye problems such as myopia, hyperopia, bifocal lenses, and astigmatisms.
Eye Glasses at HowStuffWorks.com
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Kaur, Ramanjit. �Focal Length of Eyeglasses.� 2002. The Physics Factbook.
23 Sept. 2002. http://www.hypertextbook.com/facts/2002/RomaKaur.shtml
This article discuss the basic physics involved in the use of lenses. Expresses how different types of lenses function in regard to optics. The article also relates focal length to the different types of lenses which may be found in eyeglasses.
Discussion of Lenses at HyperTextBook.Com
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University of Arizona. �Optical Scientists to Develop Eyeglasses With Autofocus.� 2001. 23 September 2002.http://www.sciencedaily.com/releases/2001/07/010730080847.htm
This article discusses the next generation in eyeglass technology. Within the next three or four years, optical scientists at the University of Arizona expect to develop new autofocus lenses: lenses that automatically vary in focus with the varying distances that people need to see. The article discusses what scientists need to do in order to create such lenses, and it explains the convenience that these lenses will provide for future eyeglass wearers.
AutoFocus Lenses at Science Daily
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�The Human Eye, Its Functions, and Visual Impairment.� American Foundation for the
Blind. http://www.afb.org/info\_document\_view.asp?documentid=201.
This site is useful in explaining how the human eye works. It also addresses the most common eye problems and gives an explanation of why each problem occurs. It then briefly explains how eyeglasses fix these problems.
American Foundation for the Blind
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�Refractive Errors Correction.� Vision Channel. <http://www.visionchannel.net/
refractivecorrection/>.
This site goes into detail about what lenses (convex or concave) fix certain eye problems such as myopia, hyperopia, astigmatism, and presbyopia. It also has information about the function of new multifocal lenses. There is also a brief discussion on which materials are best for making sturdy, longer-lasting eyeglasses.