Lenses

# Lenses - Lenses Performed: 4/12/07, Due: 4/19/07 Theory:...

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Lenses Performed: 4/12/07, Due: 4/19/07 Theory: Lenses serve as means of refracting light to form images in different places depending on the lens and other conditions. The symmetrical axis of a lens is know as the optical axis. The two common types of lenses that we investigated were converging and diverging lenses. A converging lens is thicker in the middle than at the edge of the lens, with two convex surfaces, a convex surface and a plane (flat) surface, or one convex and one concave with the convex surface having a smaller radius of curvature because it is more sharply curved. Converging lenses cause light to converge on the axis when it passes through the lens. For a converging lens, light rays that travel parallel to the optical axis, and through the lens, are made to converge to a point called the focal point of the lens. The distance from the focal point to the center of the lens is the focal length, f, which is by definition positive for a converging lens. A diverging lens is thicker at the edges and thinner at the center of the lens. It causes light that passes through it to diverge from the optical axis. Light rays that are parallel to the optical axis, and pass through a diverging lens, diverge on the other side of the lens such that they appear to be coming from the a point on the side of the incoming light, this is the focal point of the lens, and is taken to be a negative, and the focal length is taken to be the negative of the distance from the lens to the focal point. Focal length, objects, and their images are related by the basic lens equation, which is: (1/s) + (1/i) = (1/f) where s is the distance from of an object whose light passes through the lens to the center of the lens, i is the distance from the center of the lens to the image of the object produced by the lens, and f is the focal length, again measured from the center of the lens. The side of the lens from which the light approaches is designated by “incoming” and the side of the lens from which the light recedes or goes away from the lens is called “outgoing.” The sign conventions are such that the object distance, s, is taken as positive when the object is on the incoming side, and negative otherwise. The image distance, i, is positive if the image is formed on the outgoing side, and the negative if not. This formula is appropriate for ‘thin lenses’ or lenses that are much thinner than either the object distance, s, or the image distance, i, and is called the “Thin Lens Formula.” In this formula it makes no difference which side of the lens light from the object approaches the lens. If i is positive the image is real, and can be observed on a screen; whereas if i is negative the image is a virtual image, and cannot be seen on a screen no matter where the screen is placed. A virtual image can be seen by looking through the lens towards the object. Real images appear inverted, while virtual images are right side up with respect to the object. If the incoming light rays are parallel to the

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## This note was uploaded on 09/23/2008 for the course PHYS 0095 taught by Professor Budick during the Spring '07 term at NYU.

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Lenses - Lenses Performed: 4/12/07, Due: 4/19/07 Theory:...

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