PHYSICS 2

• Lens


  A transparent device with two curved surfaces, usually made of glass or plastic, that uses refraction to form an image of an object

  
• Mirror


  A curved surface designed to reflect rays to form images
• Image formation


  A system of lenses and/or mirrors forms an image by gathering rays from an object and then causing them to converge or diverge

  
• Real image


  Formed when the optical system causes the rays to converge to a point
• Virtual image


  Formed at the location from which the rays seem to originate
• Spheroidal surfaces


  • When parallel rays of light pass through a lens with convex spheroidal surfaces, or reflect from the surface of a spheroidal concave mirror, they are brought to a focus
  • Light rays passing through the center of a lens do not deviate from their original path
  • Light paths do not depend on the direction in which light is travelling
• Focal length (f)


  The distance of the focal point from the lens (or mirror) that characterizes the optical performance

  
• Types of lenses and mirrors

  • Converging (convex) lenses and converging (concave) mirrors
  • Diverging (concave) lenses and diverging (convex) mirrors

  
• Ray diagrams


  Used to determine the image location, size, orientation and type of image formed by objects when placed at a given location in front of a lens

  
• Lens equation


  Expresses the quantitative relationship between the object distance (do), the image distance (di), and the focal length (f)

  
• Magnification equation


  Relates the ratio of the image distance and object distance to the ratio of the image height (hi) and object height (ho)

  
• Solving a sample problem

  1. Identify known information

  2. Identify unknown quantities to solve for

  3. Use lens equation to determine image distance

  4. Use magnification equation to determine image height

  
• Eye


  The most interesting of all optical instruments, forming images and detecting rich detail and color, often needing correction to reach "normal" or ideal vision
• The cornea and lens of an eye act together to form a real image on the light-sensing retina, which has its densest concentration of receptors in the fovea and a blind spot over the optic nerve

  
• The power of the lens of an eye is adjustable to provide an image on the retina for varying object distances
• Object location


  • Beyond the 2F point (two focal lengths from the lens)
  • Image located between the 2F point and the focal point

  
• The eye is perhaps the most interesting of all optical instruments
• The eye is remarkable in how it forms images and in the richness of detail and color it can detect

  
• Our eyes commonly need some correction, to reach what is called "normal" vision, but should be called ideal rather than normal
• Image formation by our eyes and common vision correction are easy to analyze with the optics discussed in Geometric Optics

  
• Cornea and lens of an eye

  Act together to form a real image on the light-sensing retina

  
• Layers of tissues with varying indices of refraction in the lens are shown here. However, they have been omitted from other pictures for clarity
• Refractive indices are crucial to image formation using lenses
• Refractive indices relevant to the eye


  • Water: 1.33
  • Air: 1.0
  • Cornea: 1.38
  • Aqueous humor: 1.34
  • Lens: 1.41 average (varies throughout the lens, greatest in center)
  • Vitreous humor: 1.34
• The biggest change in the refractive index, and bending of rays, occurs at the cornea rather than the lens
• The cornea provides about two-thirds of the power of the eye, owing to the fact that speed of light changes considerably while traveling from air into cornea
• The lens provides the remaining power needed to produce an image on the retina
• The cornea and lens can be treated as a single thin lens, even though the light rays pass through several layers of material (such as cornea, aqueous humor, several layers in the lens, and vitreous humor), changing direction at each interface
• The image formed is much like the one produced by a single convex lens

  
• This is a case 1 image

  
• Images formed in the eye are inverted but the brain inverts them once more to make them seem upright
• A flat piece of glass, such as in a window, is like a lens with an infinite focal length
• You can often see a reflection when looking at a sheet of glass, particularly if it is darker on the other side
• When you focus a camera, you adjust the distance of the lens from the film

  
• A thin lens has two focal points, one on either side, at equal distances from its center, and should behave the same for light entering from either side
• Vision is blurry when opening eyes under water, but a face mask enables clear vision