Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chapter 24. Optical Instruments Eyeglasses, microscopes and telescopes aid our senses by using lenses and mirrors to form images we wouldn’t be able to see, or see as well, with our eyes alone. Chapter Goal: To understand some common optical instruments and their limitations. 2/2/09 1Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Lenses in Combination The analysis of multi-lens systems requires only one new rule: The image of the first lens acts as the object for the second lens. 2/2/09 2Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. The Camera • A camera lens forms a real inverted image on a light-sensitive detector in a light-tight box. • A adjustable focus combination lens acts as a single lens with an effective focal length • The effective focal length is changed by varying the spacing between a converging lens and diverging lens. 2/2/09 3Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. 2/2/09 4Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 24.2 Focusing a camera QUESTION: 2/2/09 5Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 24.2 Focusing a camera 2/2/09 6Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. 2/2/09 7 Human eyeCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Focusing and Accommodation • The ciliary muscles to change the curvature of the lens surface. • Tensing the ciliary muscles causes accommodation, which decreases the lens’s radius of curvature and thus decreases its focal length. • The farthest distance at which a relaxed eye can focus is called the eye’s far point (FP). The far point of a normal eye is infinity. • The closest distance at which an eye can focus, using maximum accommodation, is the eye’s near point (NP), normally 25 cm. 2/2/09 8Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. 2/2/09 9Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. 2/2/09 10Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 24.4 Correcting hyperopia QUESTION: 2/2/09 11Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 24.4 Correcting hyperopia 2/2/09 12Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 24.4 Correcting hyperopia 2/2/09 13Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 24.5 Correcting myopia QUESTION: 2/2/09 14Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 24.5 Correcting myopia 2/2/09 15Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. The Microscope • A specimen to be observed is placed on the stage of a microscope, directly beneath the objective, a converging lens with a relatively short focal length. • The objective creates a magnified real image that is further enlarged by the eyepiece. • The lateral magnification of the objective is • Together, the objective and eyepiece produce a total angular magnification 2/2/09 16Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. 2/2/09 17Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. 2/2/09 18Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. The Telescope • A simple telescope contains a large-diameter objective lens which collects parallel rays from a distant object and forms a real, inverted image at distance s' = fobj. • The focal length of a telescope objective is very nearly the length of the telescope tube. • The eyepiece functions as a simple magnifier. • The viewer observes an inverted image. • The angular magnification of a telescope is 2/2/09 19Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. 2/2/09 20 The TelescopeCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. 2/2/09 21 Diffraction effects A lens both focuses and diffracts light.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. The Resolution of Optical Instruments The minimum spot size to which a lens can focus light of wavelength λ is where D is the diameter of the circular aperture of the lens, and f is the focal length. In order to resolve two points, their angular separation must be greater than θmin, where is called the angular resolution of the lens. 2/2/09 22Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Estimate of the angular resolution of the eye. 2/2/09 23 This corresponds to resolving two dimes (1 cm) at a distance of 1/(3e-4) = 3e3 cm = 30 m. The density of rods and cones has evolved to match the diffraction limit.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A moment of appreciation 2/2/09 24 The microscope opened the microscopic universe making possible the study of biology at the cellular level. The telescope opened the celestial universe enabling the discovery of our place in the solar system and in the universe. Let us not forget too the art of the emulsion photographic film permitting the observation of small dim signals.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A. Causes the light rays to focus closer than they would with the first lens acting alone. B. Inverts the image but does not change where the light rays focus. C. Causes the light rays to focus farther away than they would with the first lens acting alone. D. Prevents the light rays from reaching a focus. The second lens in this optical instrument 2/2/09 25Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A. Causes the light rays to focus closer than they would with the first lens acting alone. B. Inverts the image but does not change where the light rays focus. C. Causes the light rays to focus farther away than they would with the first lens acting alone. D. Prevents the light rays from reaching a focus. The second lens in this optical instrument 2/2/09 26Copyright © 2008 Pearson Education, Inc.,
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