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Unformatted text preview: Experiment 7: Lenses and the Human Eye 1 Experiment 7 – Lenses and the Human Eye Introduction Glasses, contact lenses, and your eyes are all examples of lenses. While these objects may look very different from the lenses you will use in Experiment 7, they all have the same function. They focus or spread light. An understanding of these simple lenses will help you to understand more complicated structures with multiple lenses, such as telescopes and microscopes. 1 Physics 1.1 Review of Reflection and Refraction When a parallel beam of light is incident on a plane surface of a material that is transparent (e.g. glass or water) two beams are formed as shown in Figure 1. The following laws describe the reflected and refracted beams with respect to the normal to the surface: 1) The incident, reflected and refracted rays lie in the same plane as the normal to the surface. 2) The angle of reflection is equal to the angle of incidence for all wavelengths and any pair of substances. 3) For monochromatic (only one wavelength present) light and a given pair of substances, a and b, the ratio of sine of the angle of incidence to the sine of the angle of refraction is a constant called the refractive index. Figure 1 Reflection and Refraction at a Plane Surface Experiment 7: Lenses and the Human Eye 2 These basic properties of light (and all electromagnetic radiation) enable the formation of images by refraction from curved surfaces. In the first part of this Experiment, you will study the properties of thin convergent and divergent lenses and their combinations. In the latter part of the Experiment you will make some studies of the human eye and the way its optical performance can be modified by the use of lenses. 1.2 Summary of Thin Lens Theory – Convergent Lenses A convergent lens has two foci, F 1 and F 2 . For a double convex lens with symmetric lens concavity, such as the one in the picture (and the ones you will use in the laboratory) F 1 and F 2 are the same distance from the center of the lens. In general, f 1 ! f ¡ but in this case f 1 = f ¡ and f ! f 1 = f 2 will be referred to as the focal length ( f ). In order to construct the following picture 3 principal rays must be identified. Figure 2 Real Image formation by a Convergent Lens Parallel Ray (1): A ray parallel to the axis on the incident side passes through the focus on the other side. Focal Ray (2): A ray through the focus on the incident side emerges parallel. Center Ray (3): A ray directed towards the center of the lens on the incident side emerges undeflected. These three rules allow geometrical construction of the image formed of any object. We assume the object to be on the left of the lens and denote its distance from the lens by u , which is greater than zero. The distance of the image from the lens is denoted by v . v is taken as positive if the image is one the opposite side of the lens as the object and negative if it is on the same side. One can then prove the so-called lens formula connecting...
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This note was uploaded on 10/22/2009 for the course CHEM 140A taught by Professor Whiteshell during the Spring '04 term at UCSD.
- Spring '04