This means its path is closer to perpendicular, or "normal," to the surface. When light passes from a region of lower n, such as air, through a surface into a region of higher n, such as glass, the light changes direction. These numbers mean that the speed of light is 1.33 times slower in water and 2.42 times slower in diamond than in a vacuum. Water = 1.33 (at 68 degrees Fahrenheit or 20 degrees Celsius).Air = 1.0003 (at standard temperature and pressure).The degree to which a material causes light to slow down is called that material's refractive index, denoted as "n." According to, approximate values of n for common materials are: However, when light encounters a transparent material, it slows down. In a vacuum, the speed of light, denoted as "c," is constant. Normally, light travels in a straight line, and changes direction and speed when it passes from one transparent medium to another, such as from air into glass. (Image credit: Crok Photography Shutterstock ) Refraction This phenomenon is often corrected by using a combination of lenses, or in the case of large telescopes, by using parabolic mirrors, which are shaped like rounded cones that focus all the light from a source to a single point.Ī "bent" spoon in a glass of water is an example of refraction. This results in what is called spherical aberration. Light striking near the edge of the mirror does not focus at the exact same spot as light striking nearer to the center. The shape of a spherical mirror affects the image that is reflected. Also, the image appears much brighter than it does to the naked eye because the mirror gathers and concentrates the light. The eyepiece lens then focuses the light. The secondary mirror diverts the light out through a hole in the side of the tube. However, before they reach the focal point, they strike a secondary, flat mirror that is tilted at a 45-degree angle. The rays are then reflected back up through the telescope tube toward the focal point. In a Newtonian reflecting telescope, light rays from distant objects, which are essentially parallel (because they come from so far away), strike the concave main mirror at the same angle. Newton used a concave spherical mirror to make his reflecting telescope, a design that is still popular with amateur astronomers due to its simplicity, low cost and high degree of image quality. The radius of curvature of a mirror determines its magnification factor and its focal length. This generally produces a magnifying effect, such as that seen in a makeup mirror. If the surface is concave, or curved inward, a group of light rays from a distant source is reflected back toward a single location known as the focal point. These mirrors are often used for outside rearview mirrors on cars and for keeping large areas under surveillance in stores. If the mirror is convex, or curved outward, it will reflect a wider area, in which images appear smaller and farther away than those from a flat mirror. The most common curved surface used in optical devices is a spherical mirror. However, if the surface of the mirror is curved, the angles of reflection are different at different points on the surface. This happens because the light rays change direction when they go from one transparent material (air) into another (water). For example, when you look at a spoon in a glass of water, the submerged part of the spoon appears to be in a different place than expected. The third law governs how light rays behave when they pass between two different media, such as air and water. The second states that when a light ray encounters a smooth, shiny (or conducting) surface, such as a mirror, the ray bounces off that surface. The first law states that light rays move through similar transparent media in straight lines. Geometric optics treats light as continuous rays (as opposed to waves or particles) that move through transparent media according to three laws. (The other class is physical optics.) Geometric optics Geometric optics is one of two broad classes of optics, the field that "deals with the propagation of light through transparent media," according to Richard Fitzpatrick, a professor of physics at the University of Texas at Austin, in lecture notes for a course in Electromagnetism and Optics. The reflection of light rays is one of the major aspects of geometric optics the other is refraction, or the bending of light rays.
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