Thick Lenses

3 Key excerpts on "Thick Lenses"

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  eBook - ePub

  Light and Optics

  Principles and Practices

  • Abdul Al-Azzawi(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  10 Lenses 10.1 INTRODUCTION

  A typical lens is made of glass or plastic. Lenses are classified in two categories: thin lenses and Thick Lenses. Each category has its applications and equations for calculations. Lenses have two refraction surfaces; each surface is a segment of either a sphere or a plane. Lenses are commonly used to form images by the refraction of light. Light refraction in lenses is one application of the theory of the second law of light: the law of refraction. Lenses are used in building optic/optical fibre devices and instruments, such as cameras, microscopes, slide projectors, and fibre optical switches. The graphical method for locating images formed by mirrors will be used in this chapter as well. The experimental work in this chapter will enable students to practise tracing image formation by a lens or lens combination.

  10.2 TYPES OF LENSES The following sections introduce the types of lenses which are used in building optic and optical fibre devices and instruments.

  10.2.1 CONVERGING AND DIVERGING LENSES

  Figure 10.1 and Figure 10.2 show some common shapes of lenses. Typical spherical lenses have two surfaces defined by two spheres. The surfaces can be convex, concave, or planar. Lenses are divided into two types: (a) converging and (b) diverging. Converging lenses have positive focal lengths and are thickest at the middle. Common shapes of converging lenses are (1) biconvex, (2) convex–concave, and (3) plano-convex. Diverging lenses have negative focal lengths and are thickest at the edges. Common shapes of diverging lenses are (1) biconcave, (2) convex–concave, and (3) plano-concave.

  FIGURE 10.1 Converging and diverging lenses.

  FIGURE 10.2 Various shapes of lenses.

  Figure 10.3 illustrates the geometries of two common types of lenses: (a) a biconvex lens and (b) a biconcave lens. The type and thickness of the lens depends on the radius of curvatures R1 and R2

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  Optical Principles and Technology for Engineers

  • James Stewart(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  Lenses 4 A lens is made of a material, such as glass, that has a refractive index greater than one. The surfaces are ground and polished until they have the desired shape. Usually the surfaces are spherical in shape because it is very costly to grind aspherical surfaces of high quality, but there are exceptions. Spectacle lenses may have cylindrical shapes superimposed on spherical surfaces to correct for visual astigmatism. Sometimes in the case of lenses to be produced in high quantity, such as certain camera lenses, plastic lenses may be used because the cost of preparing a mold with aspheric surfaces is spread over a large number of lenses. For those engineers who are inclined to compute the properties of an optical system and who are equipped with a suitable computer program or a great deal of patience, the prescriptions of many commercially available lenses are avail­ able from the manufacturers, either published in lens catalogs or distributed on computer diskettes, or available for specific lenses by request. A complete prescription includes the radii of curvature of all surfaces, the thickness of all elements, the interelement spacing, and the glass types of all elements, from which the refractive indices and dispersions (or change of refractive index with wavelength) can be obtained. 45 46 Chapter 4 Figure 4.1 Converging lens. Parallel rays from a distant object point converge to a real image point. 4.1. THIN LENSES In the interests of simplicity, lenses are sometimes assumed to have negligible thickness. The thin lens approximation is used in preliminary optical design. The focal length of a thin lens in air is given by the equation (4.1) The refractive index of the material (relative to air) is n and the radii of curvature of the two surfaces are Rl and R 2. I shall use the convention that radiation propagates from left to right and the sign of a radius of curvature is positive if the center of curvature is to the right of the surface.

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  Physics for Scientists and Engineers with Modern Physics

  • Raymond Serway, John Jewett(Authors)
  • 2018(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 35.4 Images Formed by Thin Lenses 943 (Fig. 35.27c), the image is always virtual and upright, regardless of where the object is placed, like the image in a door peephole. These geometric constructions are reasonably accurate only if the distance between the rays and the principal axis is much less than the radii of the lens surfaces. Refraction occurs only at the surfaces of the lens. The uniform material inside the lens simply propagates the light, but does not affect the direction in which it travels. A certain lens design takes advantage of this behavior to produce the Fresnel lens, a powerful lens without great thickness. Because only the surface curvature is important in the refracting qualities of the lens, material in the middle of a Fresnel lens is removed as shown in the cross sections of lenses in Figure 35.28. Because the edges of the curved segments cause some distortion, Fresnel lenses are generally used only in situations in which image quality is less important than reduction of weight. A classroom overhead projector often uses a Fresnel lens; the circular edges between segments of the lens can be seen by looking closely at the light projected onto a screen. Q UICK QUIZ 35.6 What is the focal length of a pane of window glass? (a) zero (b) infinity (c) the thickness of the glass (d) impossible to determine Figure 35.28 A side view of the construction of a Fresnel lens. (a) The thick lens refracts a light ray as shown. (b) Lens material in the bulk of the lens is cut away, leaving only the material close to the curved surface.

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