Diverging Lens

6 Key excerpts on "Diverging Lens"

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  eBook - ePub

  A User's Guide to the View Camera

  Third Edition

  • Jim Stone(Author)
  • 2015(Publication Date)
  • Routledge

    (Publisher)

  This piece and the same diameter disc cut from a larger sphere such as a basketball would both be spheric sections, but the latter would have a flatter surface, due to its greater radius of curvature. A lens that can focus an image is called a convergent or positive lens; to do so it must be thicker in the center than at the edges. The reason a lens can focus an image is because the glass refracts, or bends, light passing through it. The amount of refraction depends on the angle the light makes with the glass surface and also on the refractive index, a measure of the light-bending power of a specific kind of glass. The three shapes of convergent lenses are, from left to right, biconvex, plano-convex, and positive meniscus. ■ The divergent lens elements, used only in combination with positive elements, are negative meniscus, biconcave, and plano-concave. ■ A simple, convergent lens distorts the image it produces in a number of predictable ways. These distortions, known as aberrations, may be separated into seven broad categories (described on pp. 116 – 119), and have been well known to lens designers since the early nineteenth century. Lens aberrations can be corrected, or at least diminished, by adding other simple lenses into the light path. For example, a divergent (or negative) lens, which is thinnest in its center, is used only in combination with a convergent lens to correct an aberration. A set of simple lenses assembled along a common optical axis to focus an image forms a compound lens. The simple lenses are then referred to as elements or components and may be cemented together into groups. Any air space between elements and groups is as important to the design of a compound lens as the elements themselves; a fixed position of the elements and spaces is assured by a rigid lens barrel, its opaque outer shell. In a compound lens, a single element isolated by air spaces is also considered a group

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  eBook - ePub

  Science in Nursing and Health Care

  • Tony Farine, Mark A. Foss(Authors)
  • 2013(Publication Date)
  • Routledge

    (Publisher)

  focal length , and the point of convergence is the focal point or principal focus. Note that in the case of a convex lens, the focal point is behind the lens. If a piece of paper is held at this point, parallel rays of light from a distant object pass through the lens and are brought to a focus on the paper. This is a simple experiment that most children who have owned a magnifying glass have performed on a sunny day. When the magnifying glass is held facing the sun and the paper is held at the focal point, a yellow dot comes into focus on the paper. This is actually an image of the sun. Such an image is described as being real, since it can be projected on to a surface and viewed. In addition, the image in this case is also diminished (smaller than the object) and inverted (upside-down). This simple experiment is illustrated in Figure 12.9 . Convex lenses are also used in the correction of certain vision defects. This is explained in Practice point 12.1 .

  Figure 12.9

  Using a convex lens to produce an image of the sun.

  Practice point 12.1 Convex lenses and the eye

  Figure 12.10 is a cross-sectional diagram of the eye. The eye is a hollow spherical structure, the outer part of which is formed from three coats. The outermost is the tough sclera; interior to this is the vascular choroid. The innermost layer is the light-sensitive retina. At the front of the eye, the sclera becomes transparent, and this is known as the cornea. A thin transparent membrane called the conjunctiva covers the cornea, and this also forms the inner surface of the eyelids.

  When we look at the eye from the front, we can see through the cornea to a pigmented doughnut-shaped muscle that is part of the choroid.

    What is this pigmented muscle called?

  This is the iris. It is the iris to which we refer when we describe the colour of someone’s eyes. The hole at the centre of the iris is called the pupil. The pupil becomes constricted (decreased in size) with the contraction of muscle cells in the iris, which are arranged in a circular manner. In contrast, the contraction of muscle cells that radiate out from the pupil cause it to dilate (enlarge). By this means, the amount of light entering the eye under conditions of varying light intensity can be regulated.

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  eBook - ePub

  Optics For Dummies

  • Galen C. Duree(Author)
  • 2011(Publication Date)
  • For Dummies

    (Publisher)

  mirrors that bow out toward the object, diverge incident parallel rays such that they seem to originate from a virtual focal point behind the mirror. For each ray incident on the surface of the mirror surface, you apply the reflection equation, and the reflected rays direct away from a common point, as shown in Figure 5-5. The focal point is virtual because the light actually reflects and travels away from the common point (not through it).

  Concave lenses

  (lenses whose surfaces bow in toward the center of the lens) diverge incident parallel rays such that the rays seem to come from a point on the incident side of the lens, as shown in Figure 5-6. With Snell’s law, you can determine the new propagation direction. The rays appear to diverge away from the virtual focal point; you know it’s virtual because the light actually refracts (bends) and doesn’t travel through the point.

  Figure 5-5: Virtual focal point created by a convex mirror.

  Figure 5-6: Virtual focal point produced by a concave lens.

  Passage contains an image

  Chapter 6 Imaging with Mirrors: Bouncing Many Rays Around In This Chapter

  Reflecting on flat, convex, and concave mirrors

  Examining the characteristics of mirror-produced images

  C

  hapter 4 shows you that you can change where light goes by using reflection, where light bounces off a surface. But you can further control the reflection of light with very shiny surfaces called mirrors.

  Mirrors

  are special light reflectors because they can make images of objects appear at locations other than where the actual object is located. Unlike regular objects, which reflect the incident light in all directions, mirrors reflect light but keep the relative orientation of the incoming rays. This characteristic is why you can see an image in a mirror but not in this page.

  In Chapter 4, I show the basic idea for all reflected light using a single ray — that the angle of incidence (the orientation of the incoming light ray) is the same size as the angle of reflection (the orientation of the outgoing, or reflected, ray). But to see the objects around you, you need more than one ray, and using the law of reflection for many rays is a tedious task, at best. Fortunately, you can use geometry to simplify this process so that you have a single, easy-to-use equation.

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  eBook - ePub

  Understanding Forensic Digital Imaging

  • Herbert L. Blitzer, Karen Stein-Ferguson, Jeffrey Huang(Authors)
  • 2010(Publication Date)
  • Academic Press

    (Publisher)

  Move it infinitely far away and it becomes a dot, located at what is known as a vanishing point. Visually, people see this effect and expect it. Most lenses that capture images over a wide angle, so-called fish-eye lenses, will appear to be distorting the image. In reality they are making the items that are far away appropriately smaller than those that are close. The photographer must be aware of these issues and take a set of photographs that help the viewer see the scene as it was, see the details that are important to understanding what happened, and not exaggerate or mislead the viewers. INTRODUCTION TO LENSES We have all worked with a magnifying glass at one time or another, and know that these are sometimes called “burning glasses.” The latter designation gets to the heart of the matter. The burning glass, which of course is a lens, takes the beam of light from the sun and causes it to converge onto a small spot. In this way the energy from all the photons in the beam is concentrated in a single, small location, and is able to heat that spot to the point where it starts to burn. There are two key functions involved: (1) collect the light rays, and (2) redirect them in a predictable way. Camera lenses perform the same two basic functions in cameras. They gather light rays from the scene, or object, and redirect them to create an image of the object (without starting fires). A lens comprising a single piece of glass is a simple lens. There are two fundamental types of simple lenses commonly used in cameras, shown in Figure 3.4. One type is convex toward the center and is called a convex or positive lens. The other type is concave toward the center and is called a concave or negative lens. Convex lenses cause incoming light rays to bend toward each other, and under the right conditions, converge to a point. At that point the rays will cross each other and then diverge beyond that distance

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  eBook - ePub

  The Manual of Photography

  • Elizabeth Allen, Sophie Triantaphillidou(Authors)
  • 2012(Publication Date)
  • Routledge

    (Publisher)

  Figure 6.6 .

  Cardinal planes

  A simple thin lens is normally unsuitable as a photographic lens due to aberrations. A practical lens consists of a number of separated elements or groups of elements, the physical length of which is a significant fraction of its focal length. This is a compound, thick or complex lens. The term equivalent focal length (EFL) is often used to denote the composite focal length of such a system. For two thin lenses of focal lengths f1 and f2 , the EFL, f, is given by:

  Figure 6.6    Image formation by a positive lens (a, b) and a negative lens (c, d). (a) For a distant subject. F is the rear principal focal plane. (b) For a near subject. Focusing extension E = (v − f); I is an inverted image. (c) For a distant subject ‘O:F’ is the front principal focal plane. ‘I’ is virtual, upright image. (d) For a near subject.

  where d is their axial separation.

  Thin lens formulae can be used with thick lenses if conjugate measurements are made from two specific planes perpendicular to the optical axis. These are the first and second principal planes that are planes of unit transverse magnification. For thin lenses in air the two planes are coincident. For thick lenses in air the principal planes are separated and coincident with two other planes, the first and second nodal planes (N1 and N2 ) (Figure 6.7 ). An oblique ray incident at N1 emerges undeviated on a parallel path from N2 . The terms principal and nodal are used interchangeably but when the surrounding optical medium is different for object and image spaces, such as an underwater lens with water in contact with its front element, then the two pairs of planes are not coincident. Focal length is measured from the rear nodal point. For a symmetrical lens, the nodal planes are located approximately one-third of the way in from the front and rear surfaces. Depending on lens design and configuration they may or may not be crossed over or even located in front of or behind the lens. The location of nodal planes can be used to classify major types of lenses (see Figure 6.8

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  eBook - ePub

  Fundamentals of Light Microscopy and Electronic Imaging

  • Douglas B. Murphy, Michael W. Davidson(Authors)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  (See the Note for definitions of real and virtual images.) Positive lenses magnify when held in front of the eye. A negative lens causes parallel incident rays to diverge; negative lenses are thinner in the middle than at the periphery, and have at least one concave surface. Negative lenses do not form a real image, and when held in front of the eye, they reduce or demagnify. The geometry of positive and negative simple lenses is shown in Figure 4.4. Figure 4.4 Examples of positive and negative lenses. For any given lens, there are two principal planes, one each for the front and back surface of the lens. For the special case of a thin biconvex lens, the two principal planes are coincident in the middle of the lens. Microscope objectives contain multiple lens elements, some of which may be united with transparent sealing compound to make a complex thick lens. The principal planes of thick lenses are physically separated, but their locations can be determined by ray tracing. Most lens elements used in microscope optics are ground and polished with spherical curvatures. Note: Real and Virtual Images Images can be defined as regions where rays, or the extensions of rays, become convergent as the result of refraction by a lens or reflection by a mirror. If the rays intersect and physically reunite, the image is said to be real. A real image can be seen on a viewing screen or recorded on a piece of film when a screen or film is placed in the image plane. If rays diverge, but the imaginary extensions of the rays become convergent and intersect, the image is said to be virtual. The plane occupied by a virtual image cannot be observed on a viewing screen or recorded on film. To be perceived, a real image must be formed on the retina of the eye. In the case of viewing an image in a microscope (Fig. 1.3), a real image is formed on the retina but is perceived as a virtual image located some 25 cm in front of the eye

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