15-06-2013, 02:58 PM
USES OF LENS!!
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HISTORY of LENSES
The word lens comes from the Latin name of the lentil, because a double-convex lens is lentil-shaped. The genus of the lentil plant is Lens, and the most commonly eaten species is Lens culinaris. The lentil plant also gives its name to a geometric figure.
The oldest lens artifact is the Nimrud lens, which is over three thousand years old, dating back to ancient Assyria.[3] David Brewster proposed that it may have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight. Assyrian craftsmen made intricate engravings, and could have used such a lens in their work. Another early reference to magnification dates back to ancient Egyptian hieroglyphs in the 8th century BC, which depict "simple glass meniscal lenses".
The Human Eye and Corrective Lenses
A greatly simplified view of the human eye is shown below. The pupil is a little hole which allows light to pass into the eye. Behind the pupil lies the eye's lens. Muscles in the eye control the size of the pupil and the shape of the lens, thereby adjusting the amount of light that enters they eye and the focus of the lens. The retina is a sensitive layer of nerves at the back of the eyeball; incident light upon the retina is translated into a coherent image by the brain.
Many people do not have perfect vision; that is, a lot of people have eyes whose lenses do not focus light properly on the retina. Two well-known vision problems correctible via eyeglasses are nearsightedness (picture (a) above) and farsightedness (picture ©). Nearsightedness focusses rays of light in front of the retina, while farsightedness focusses rays behind the retina. A diverging lens can correct nearsightedness by bending incoming light rays outwards, so that the eye's lens (which usually bends incoming rays too much) focusses the light closer to the retina (picture (b)). A converging lens similarly corrects farsightedness (picture (d))
Magnifying Glasses
In our study of lenses, we saw that if the source was placed within a focal length of a converging lens, the lens yielded a magnified image on the same side of the lens as the source. This is, of course, the detective's best friend, the magnifying glass.
Cameras
Cameras, unsurprisingly, work on similar principles as the eye.
The aperture, which lets light into the inside of the camera, corresponds to the pupil. The system of lenses in a camera performs the same function as the lens of the eye. However, whereas the lens of the eye changes shape to change focus, glass lenses are not very forgiving of shape changes. Instead, the lens system can be slid along its optical axis in order to focus on the film. Of course, the film plays the role of the retina. In addition, cameras have a shutter, which opens and closes quickly so that the film does not get inundated with light. This produces a more or less clear image of the instant that the photographer shoots
Microscopes
Simple microscopes use lens systems to magnify very small objects, as illustrated in the following diagram.
An object (S1) too small to examine with the naked eye is placed just outside the focus Fo of the objective lens in the microscope above. Tracing the infinite and central rays as we have before, we can find the location of the image I1. We know that a source within the focal length of a converging lens will result in an enlarged virtual image; hence if we position the eyepiece of the microscope so that its focus overreaches I1, the final image seen (I2) will be a magnified image.
The shrewd reader will notice that our simple microscope gives an inverted image, which could be inconvenient for certain situations. This problem can be solved by using a different lens arrangement, as shown below.