Investigation 21D: Build a microscope and a telescope

Essential questionsHow do a microscope and telescope work?
The basic microscope and the telescope are each a compound optical device that uses two lenses. The object is imaged by the first (or objective) lens; this image is used as the object by the second (or eyepiece) lens. By choosing particular types of objective and eyepiece lenses, and by separating the two lenses by a suitable distance, both the microscope and the telescope can produce magnified images.
Caution: Never look directly at the Sun with either a microscope or a telescope!
Part 1: Build a simple microscope

Create a simple microscope using two lenses
  1. Use three lenses (illustrated at right).
  2. Using two lenses at a time, determine which two lenses create a microscope by generating a magnified image of a nearby object.
  3. Construct a microscope on the optics track. Adjust the lens positions to improve the microscope magnification without significant distortion.
  1. What kinds of lenses are used to make a microscope?
  2. Which lens (if any) has a shorter focal length, the objective or eyepiece? Read the text aloud Show Alternative way to mount lenses
Part 2: Model the simple microscope

Model a simple microscope using the interactive simulation
  1. Enter your microscope’s parameters into the compound optics simulation.
  2. Make small modifications to the positions of the lenses to see what range in positions produces a large magnification.
  1. Where is the object for the objective lens located relative to its focal point?
  2. Where is the eyepiece’s object located relative to its focal point?
  3. How is the separation of the lenses related to the focal lengths of the two lenses? Read the text aloud
In this interactive simulation, you can create a compound optical system by combining two lenses. When the focal length of a lens is f > 0, it is a convex lens; when f < 0, it is a concave (or diverging) lens. With two lenses you can construct a microscope or telescope, but each may require a different choice of lenses for the objective and eyepiece. You can enter the position of the object; the telescope images a far-away object, whereas the microscope images a nearby one. Show Magnification of a microscope
Part 3: Build and model a simple telescope

  1. Use the same lenses as in Part 1. Holding two lenses at a time—one at arms length and the other close to your eye—determine which lenses create a telescope that magnifies a distant object.
  2. Construct your telescope on the optical track.
  3. Devise a method to measure its magnification for a particular eye position.
  1. What kinds of lenses are used to make a Galilean telescope? A Keplerian telescope?
  2. Which lens (if any) has a shorter focal length, the objective or the eyepiece? Read the text aloud Show Part 4:  Correcting human vision using eyeglasses

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