Lenses By Micah Massari and Dean Yoder Converging and Diverging Lenses • A lens is a piece of glass that can bend parallel rays of light so they cross or appear to have crossed. • Lenses can cause light to converge to a single point or diverge from a single point. • A converging lens is thicker in the middle than on the outsides and a diverging lens is thinner in the middle then on the outside. • Principal axis of a lens is the line joining the centers of curvatures of its surface. • Focal point is the point at which light converges when beamed parallel to the principal axis. A focal plane is made up of beams that focus above or below the focal plane. • Focal length the distance between the center of the lens and its focal point. Image Formation by a Lens • Far away objects are seen through a small angle of view while closer objects are seen through a larger angle of view. The wider angle allows you to see more detail. • Converging lenses help magnify or let you look at something through a wider angle. • Magnifying glasses can only magnify an object within its focal length. • When an object is farther away than the focal point of a converging lens light does converge and it can be focused on a screen as a real image. • Diverging lenses are used as view finders for cameras the virtual image you see is right side up and smaller than the object. Constructing Images Through Ray Diagrams • Ray diagrams are used to determine the location and size of an image. The image can be located by following light ray paths, there are three: 1. The first ray starts parallel to the principal axis and then is refracted to the focal point. 2. The next path is through the center of the lens and continue in a strait line. 3. The third path is when a ray passes through the focal point of the lens and emerges parallel to the principal axis. – Only two of these rays are needed to find locate an image. – Ray diagrams can also be used for diverging lenses. Image Formation Summarized • When an object is within one focal length it is a simple magnifying glass that produces a magnified , right side up, virtual image. When an object is beyond one focal length it produces a real inverted image. • When an object is viewed with a diverging lens the image is virtual, reduced, and right side up. Common Optical Instruments • The camera- a lens and sensitive film are mounted in a light tight box. The lens forms a real inverted image on the film. • The telescope- a lens forms a real image of a distant object the image is projected into space to be viewed by another magnifying like lens. • Binoculars- are like two telescopes side by side except they have prisms inside to flip images right side up. They are less bright than a normal telescope with two lenses because the light has to pass through all of the prisms making them less bright. Common Optical Instruments • Compound microscope- uses two converging lenses with short focal length. The objective lens is the first lens; it produces a real image of a close up object. The eye piece is the second lens and it forms a virtual image that is even more enlarged. • Projector- a concave mirror reflects light from an intense light source onto a pair of condenser lenses; these direct the light through the slide to a projection lens. The projection lens is mounted so it can be moved back and forth for focusing. The Eye • Iris- regulates the amount of light that can enter your eye and it also surrounds the pupil.Light enters through the cornea then it passes through the pupil and lens and is focused on the retina. • The fovea is the most sensitive part of the retina and allows the view of greater detail. • You see images upside down but your brain has learned to flip it. • To focus an image your eyes accommodate by thickening or thinning your lens by using ciliary muscles that surround the lens. • Vision is sharpest in the eye when the pupil is the smallest. Some Defects in Vision and in Lenses • People with normal vision can see objects from infinity down to about 25 cm. • Farsighted- people with this problem form images behind their retina so they must hold objects more than 25 cm away to see it clearly. To fix this the person may wear glasses with converging lenses. • Nearsighted- the person can see objects nearby very clearly but far away objects are out of focus. This is because they form images in front of the retina. This problem is fixed by wearing glasses with diverging lenses. Some Defects in Vision and in Lenses • Astigmatism- this is caused when one part of the cornea is bent more than the other. For this cylindrical glasses are prescribed. • No lens can give a perfect image combining lenses can reduce aberrations though making clearer images. • Spherical aberration is caused when light rays that pass through a lens focus at different points. This is corrected by adding a diaphragm to the outside or combining other lenses. • Chromatic aberration is caused by the different speeds of different colors of light which causes different refractions. This is fixed by using Achromatic lenses which are lenses combined to correct this effect.
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