Telescopes

Telescopes Refracting Telescope: 2 lenses at opposite ends of a long tube. The objective lens is closest to the object, & the eyepiece is closest to the eye. See figure    The Magnification is

Astronomical telescopes need to gather as much light as possible, which means that the objective must be as large as possible. So, usually, mirrors are used instead of lenses, because they can be made much larger & with more precision. Figure 33-38. A concave mirror can be used as the objective of an astronomical telescope. Arrangement (a) is called the Newtonian focus, and (b) the Cassegrainian focus. Other arrangements are also possible. (c) The 200-inch (mirror diameter) Hale telescope on Palomar Mountain in California. (d) The 10-meter Keck telescope on Mauna Kea, Hawaii. The Keck combines thirty-six 1.8-meter six-sided mirrors into the equivalent of a very large single reflector, 10m in diameter.

A terrestrial telescope, used for viewing objects on Earth, should produce an upright image. Here are two models, a Galilean type and a spyglass: Figure 33-39. Terrestrial telescopes that produce an upright image: (a) Galilean; (b) spyglass, or erector type.

(Valid for fo , fe << ℓ) Compound Microscope A Compound Microscope also has an objective & an eyepiece; it is different from a telescope because the object is placed very close to the eyepiece. The Magnification is Figure 33-40. Compound microscope: (a) ray diagram, (b) photograph (illumination comes from the lower right, then up through the slide holding the object). (Valid for fo , fe << ℓ)

Microscope. A compound microscope consists of a 10X eyepiece & a 50X objective 17.0 cm apart. Calculate (a) The overall magnification. (b) The focal length of each lens. (c) The position of the object when the final image is in focus with the eye relaxed. Assume a normal eye, so N = 25 cm. Solution: a. The overall magnification is 500X. b. The eyepiece focal length is N/Me = 2.5 cm. Then, solving equation 33-8 for do gives do = 0.29 cm. Finally, the thin lens equation gives fo = 0.28 cm. c. See (b). do = 0.29 cm.

Aberrations of Lenses and Mirrors Spherical Aberration: Rays far from the lens axis do not focus at the focal point. Figure 33-41. Spherical aberration (exaggerated). Circle of least confusion is at C. Solutions: Compound-lens systems; Use only the central part of the lens.

Distortion: Caused by variation in magnification with distance from the lens. The figures show Barrel & Pincushion distortion: Figure 33-42. Distortion: lenses may image a square grid of perpendicular lines to produce (a) barrel distortion or (b) pincushion distortion.

Chromatic Aberration: Light of different wavelengths has different indices of refraction so that it focuses at different points. Figure 33-43. Chromatic aberration. Different colors are focused at different points.

Solution: Use a chromatic doublet, made of lenses of 2 different materials. Figure 33-44. Achromatic doublet.

Summary Lens uses refraction to form real or virtual image. Converging lens: rays converge at focal point. Diverging lens: rays appear to diverge from focal point. Power is given in Diopters (m-1):

Thin lens equation: Magnification:

Camera focuses image on film or electronic sensor; lens can be moved and size of opening adjusted (f-stop). Human eye also makes adjustments, by changing shape of lens and size of pupil. Nearsighted eye is corrected by diverging lens. Farsighted eye is corrected by converging lens.

Magnification of simple magnifier: Telescope: objective lens or mirror plus eyepiece lens. Magnification: