A Look Through Lenses
HOW often do we see someone having difficulty reading, perhaps holding a paper at arm’s length? Others may read only when the page is very close to their eyes. Why these differences in seeing? It has a lot to do with the working of the lens in the eye.
Lenses That Bend Light
Light that is reflected from objects all around us passes through the eye lens and forms images on the retina at the back of the eyeball. This activates the nerves leading to the brain, which develops the moving pictures so formed. But these images happen to be upside down or inverted! What a blessing for us that the Designer of the eye also instructed the brain on how to turn these images the right way up!
The inversion of images as they enter the eye occurs because our eye lens is convex, similar in size to an aspirin tablet. And this type of lens has the unusual feature of “twisting” light rays that pass through it to form an inverted image of the object originating the rays.
You can illustrate this with a magnifying glass. Rather like two saucers placed rim to rim, the magnifying glass is thicker in the middle than at the edges. At certain distances this lens can be used to give an enlarged view of something, because of the way it bends light rays passing through it. You may even be reading this page with the aid of such a lens. Magnification, however, will occur only when the object being viewed is held close to the lens, that is, closer than twice the focal length of the lens. Now increase the distance between your eye and the magnifying glass. Hold it out at arm’s length and peer through it at a picture on the wall. You will notice that everything appears upside down. Why? Because of the bending inward of the light rays as they pass through the glass. The image is inverted.
Those light rays that pass through the center of a convex lens do not bend or refract to any noticeable degree. But those striking the lens at a distance from the center are refracted to pass through a definite point called the focal point. The distance between this point and the center of the lens is called the focal length.
Have you ever used a magnifying glass to start a fire? The ancient Greeks and Romans are reported to have used glass containers full of water as “burning glasses.” The sun’s rays would pass through the water, converging to a focus on some combustible material and cause it to burn. To demonstrate this, you can focus the sun’s rays on a sheet of paper by adjusting the distance of the lens from the paper to form a small white spot. This will soon become so hot that the paper will burn, because that white spot is really an image of the sun, appearing at the focal point of the lens. It is obviously a wise rule never to peer at the sun through lenses, especially with telescopes and binoculars, for this could do irreparable damage to the eye.
The other type of lens, called concave, is shaped like two saucers placed base to base, being thicker at the edges than in the middle. This lens diverges or spreads light rays that pass through it. Concave lenses are most often used in combination with convex lenses, and their ability to spread light rays has been adapted as an aid to eyesight.
Glass Lenses Have Their Problems
As you may have noticed, lenses are not pieces of glass like windowpanes, but are usually made of special glass in shapes of carefully measured angles and arcs according to complex lens makers’ formulas. Generally, when used in optical instruments, they are much thinner than the hand reading-lenses.
Simple lenses present several problems, the commonest of which are spherical and chromatic aberrations. If you look closely at an image on a screen formed by a simple lens you will notice what is called spherical aberration. This is a distortion of the image, occurring because light rays from the object pass through the lens at slightly different angles and as a result do not focus sharply at the same spot. We do not have this problem with our eye, nor do we have the falling away of sharpness at the extremity of the lens, which also occurs in man-made lenses.
Neither do we suffer from chromatic aberration. “White light” when refracted sufficiently breaks up into the seven colors of the spectrum (red, orange, yellow, green, blue, indigo and violet), each of which is refracted at a slightly different angle, focusing one in front of another, violet first with red last. This gives that rainbow fringe to the image, called chromatic aberration.
Though it is impossible to correct all known aberrations in man-made lenses, they can be effectively masked by the combination of several precision lenses. These may be cemented together with Canada balsam, a resin from the North American balsam fir tree. Some lenses are coated to prevent “ghosting” or reflections from forming.
Complex systems are also employed in telescopes, binoculars and microscopes. These apply the principle of a convex objective lens for casting an image in the microscope or telescope tube, which is not shown on any screen but is made to fall within the focal length of an eyepiece. The image formed there is then viewed through the eyepiece, giving a magnified view of the object.
The inverted narrate of the image does not really matter in the microscope. (The slide being viewed can be turned upside down first.) But no ship’s captain would be happy with his binoculars or telescope if his next port of call appeared inverted. For this reason a correcting set of lenses or prisms is introduced between the objective lens and the eyepiece to rectify that problem.
Lens making by intelligent men involves a thorough knowledge of optics, refraction-of-light mathematical formulas and then patient skill, learned and accumulated over many years of training by another skilled in these arts. Since this is so, then, to use Isaac Newton’s words when discussing the origin of life, ‘by what sort of reasoning do some persons reach the incongruous conclusion’ that the complex wonders in the natural world have come into existence without an intelligent Creator?
The Superlative Eye Lens
When you look at the “black hole” in your eye, you are really looking through the lens into the dark interior of the eyeball. The tiny lens is held in place behind the colored iris by ciliary muscles and follows the same principles of refraction that man has applied in artificial lenses. The brain, by converting nerve impulses transmitted to it from the retina into full color, three-dimensional moving pictures, gives us an exciting upright view of something larger than the image on the retina but always in optical proportion to our bodies. This is true whether it is a pea or a plate, a vase of lilac blooms or magnificent, snow-clad mountains.
For us to be able to look at a map on our knee one moment and, the next, to view scenery and mountains miles away indicates that the eye lens has been perfectly designed. Instantly it can focus sharply, automatically correcting aberrations that would be found in man-made lenses. How confusing it would be to have a distorted image constantly changing with every movement of the head, with multicolored fringes around each image!
The refractive and focusing agencies of the eye, the lens itself and the cornea (that curved transparent covering of the eye), truly proclaim the intelligent handiwork of the Creator. Even Charles Darwin admitted the absurdity of his natural selection theory when he considered the eye: “To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.”—The Origin of Species, p. 190.
The Spectacle Lens
The eye lens is extremely flexible and can be bent, pulled out or elongated and squashed flatter. It is this ability in association with the refractive power of the cornea that permits quick, accurate focusing without distortion. However, the aging process can harden the lens or the attached ciliary muscles, making adjusting (called accommodation) and clear focusing more difficult. Some have a focusing weakness because of the unusual shape of the eyeball, it perhaps being longer or shorter than the average twenty-four millimeters (about one inch) in length.
Your eye lens is at rest when you view distant objects, but is squeezed thicker by the ciliary muscles to focus on things nearby. Because of this muscular activity we get “eyestrain” when doing work close to the eyes or reading or writing.
If the eyeball is too long, the image is focused short of the retina and appears blurred, causing nearsightedness. This can be corrected by using spectacles of the concave variety, which diverge the light entering the eye and help the eye’s convex lens to focus (form a focal point) on the retina properly.
On the other hand, farsightedness occurs because the eyeball is too small and the image is formed behind the retina. A convex spectacle lens introduced in front of the eye will converge the light rays entering and guide them onto the retina properly.
The formation of an image behind the retina also occurs when the eye’s lens loses its power of accommodation and reaches a point where it cannot assume the deeply curved shape necessary for focusing on nearby objects. Usually affecting those passing middle age, this condition is known as presbyopia, “old sight,” requiring convex segments in the spectacle to correct the weakness.
Great care should be taken with our eyes. Do not poke around in the eye if you get grit in it, or rub it with dirty fingers or cloths. Someone else may be able to lift the foreign body out carefully with a clean handkerchief—perhaps even a physician when necessary. And if you read at night, an evenly, well-lit room will be less of a strain on your eyes than reading under a patch of light.
The Lenses of Other Creatures
If you could look through the lenses of some insects, you would find them useful for quick darting flight or for judging speed. Their eyes are made of numerous lenses that produce individual images. The time lapse between the movement of an image from one segment of the eye to another is used as an indication of its speed.
Vertebrates have paired lenses for vision. Some, such as the horse, have panoramic vision, being able to see almost all around. Others, including man, owls and monkeys have eyes more forward in the head, giving vision in which each eye overlaps the other. Birds’ eyes have most remarkable lenses producing telescopic and microscopic effects. This enables them to have the most acute vision of all creatures. Eagles, vultures and others of their family are able to see tiny things at prodigious distances.
Many natural applications of the principles of refraction and optics have caused man to marvel at their use and to adapt them for his own convenience, intelligently doing so after learning from the handiwork of the Creator.
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The nearsighted eye: the eyeball is too long and the image is focused in front of the retina
Nearsightedness is corrected by use of concave lenses, which help the eye lens to focus properly on the retina