Optical Instruments Or Seeing is Believing

Review of Converging Lens Images Object Distance from Lens Real or Virtual Magnified or Reduced Erect or Inverted Less than 1 focal length Virtual Magnified Erect Between 1 and 2 focal lengths Real Magnified slightly Inverted Greater than 2 focal lengths Reduced Questions: make a chart that shows the three types of images a converging lens can form.

The Camera Final image is reduced, inverted and real Uses one converging lens Focusing is accomplished by changing the distance between the lens and sensor Small distance for far away shots Large distances for close-ups Spherical aberration of lens reduced by using diaphragm (f stop) Chromatic aberration fixed by achromatic lens Questions: What type of lens is used in a camera? What type of image is produced? When focusing on a far away object do you want the maximum or minimum distance between the lens and sensor? How does a camera control spherical and chromatic aberration?

Microscopes and Telescopes Both devices use two converging lenses called the eyepiece and objective. Telescopes objective lens is largest Microscopes eyepiece is larger Image from objective is real, reduced and inverted and is located within 1 focal length of the eyepiece Image from eyepiece is virtual, magnified and still inverted when compared to the original object. Variations for photography Questions: Draw the lens arrangement of a telescope. Where is the image from the objective located relative to the eyepiece lens? Describe the final image from the telescope. How is a microscopes lens arrangement different? What changes must be made for photography for these instruments?

The Human Eye Cornea, vitreous humor, and crystalline lens both act as converging lens producing image that is real, reduced and inverted. Iris regulates light coming into eye Muscles squeeze and stretch lens and cornea to focus. Brain learns to turn inverted image over @ 1-2 weeks. Retina consists of rods (black and white) and cones (colors). Questions: What structures form a lens in the human eye? What type of lens does it form? How does the eye focus? Describe the image the eye forms. Why don’t we see everything upside down? What causes color blindness?

Eye Defects Near Sightedness Can see near but not far Parallel rays from far objects focus in front of retina (eye is too darn long) Need diverging lens in center and no lens on outside Gets worse with age Far sightedness Can see far but not near Divergent rays from close objects are focusing behind retina (eyes too darn short) Need converging lens on outside and no lens for inside rays Gets better with age Questions: What can a near sighted person see and not see? Are their eyes too darn long or short? Draw the type of lens they need. What is an astigmatism? What shape is the lens that is needed to correct this defect? Astigmatism: different curvatures in different directions, need barrel shaped lens to correct