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Optical Instruments Lecture 27 Physics II Chapter 24 Ray Optics Optical Instruments Course website: http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII Lecture Capture: http://echo360.uml.edu/danylov201415/physics2spring.html 95.144 Danylov Lecture 27 Department of Physics and Applied Physics Lenses in Combination 95.144 Danylov Lecture 27 Department of Physics and Applied Physics Combination of two converging lenses f1 f1 f2 f2 Object 1 h 3 f f f1 2 f1 2 2 Eyepiece h' Objective lens Virtual image seen through the eyepiece .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. .It is a ray‐tracing diagram of a simple astronomical telescope. 95.144 Danylov Lecture 27 Department of Physics and Applied Physics Telescopes Distant objects are difficult to see because light from them has spread out by the time it reaches your eyes. Your eyes are too small to gather much light. A telescope forms enlarged images of distant objects. A refracting telescope consists of two convex lenses, one at each end of a tube. The larger lens is called the objective. The objective gathers the light coming from an object and focuses the rays to form a real image. The lens close to your eye is called the eyepiece. The eyepiece magnifies the image so you can see it clearly. The image seen through the refracting telescope is upside down. Microscopes To look at small, nearby objects, you would use a microscope. A microscope is an optical instrument that forms enlarged images of tiny objects. A microscope uses a combination of lenses to produce and magnify an image. For example, the microscope shown in the figure uses two convex lenses to magnify an object, or specimen. The specimen is placed near the objective. The objective forms a real, enlarged image of the specimen. Then the eyepiece enlarges the image even more. 95.144 Danylov Lecture 27 Department of Physics and Applied Physics ConcepTest Combination of Lenses Is it possible to use this optical A. Yes configuration of lenses for a camera ? B. No C. I don’t care Actually, only real images are of any value for a camera, since an array of camera detectors (CCDs) responds on actual light presence. There is no light in the region of a virtual image. This system can be used in a telescope because this virtual image becomes a real image in our eye for our retina (light detector) The Camera A camera “takes a picture” by using a lens to form a real, inverted image on a light- sensitive detector in a light-tight box. In many cameras, the lens automatically moves closer to or away from the film until the image is focused. Changing the size of the aperture controls how much light hits the film. 95.144 Danylov Lecture 27 Department of Physics and Applied Physics The Camera We can model a combination lens as a single lens with an effective focal length (usually called simply “the focal length”). A zoom lens changes the effective focal length by varying the spacing between the converging lens and the diverging lens. 95.144 Danylov Lecture 27 Department of Physics and Applied Physics ConcepTest Combination of Lenses The parallel light rays will be A. Closer to focused at a point _______ the second lens than would light B. The same distance from focused by the second lens acting alone. C. Farther from Zoom of a camera When cameras focus on objects that are more than 10 focal lengths away (roughly s > 20 cm for a typical digital camera), the object is essentially “at infinity” (s>>f) and s' f . If we keep increasing S, S’ becomes closer to f 1 1 1 Then, the lateral magnification of the image is ≫ ≪1 1) the image on the detector is much smaller than the object itself and can fit a small area of a camera detector (CCD). 2) More important, the size of the image is directly proportional to the focal length of the lens. ′ 95.144 Danylov Lecture 27 Department of Physics and Applied Physics Optical Zoom of a camera 2) More important, the size of the image is directly proportional to the focal length of the lens. ′ ≫ ′ ≫ You magnify image (increase h’) to see it better. So, a max possible magnification of a camera is ′ 90 ′~ = 20 ′ 4.5 95.144 Danylov Lecture 27 Department of Physics and Applied Physics f-number The amount of light passing through The aperture sets the the lens is controlled by an adjustable effective diameter aperture (diaphragm), shown in the D of the lens. picture. ~ aperture ~′ ~ ′~ That is how D/f comes as ~ a parameter of a camera The light‐gathering ability f‐number of a lens is specified by its f‐number A smaller f‐number means a larger aperture, while a larger f‐number means a smaller aperture. 95.144 Danylov Lecture 27 Department of Physics and Applied Physics Focal length and f-number information is stamped on a camera lens. Focal length and f-number information is stamped on a camera lens. This lens is labeled 4.590.0 mm 1:3.56.8. The first numbers are the range of focal lengths. They span a factor of 20, so this is a 20 zoom lens. The second numbers show that the f-number ranges from f/3.5 (for the f = 4.5 mm focal length) to f/6.8 (for the f = 90.0 mm focal length). 95.144 Danylov Lecture 27 Department of Physics and Applied Physics Manual Mode If your camera is in an AUTO mode, it doesn’t display shutter speed or f-number If your camera is in a MANUAL mode, it displays shutter speed or f-number For example, the display 1/125 F5.6 means that your camera is going to achieve the correct exposure by adjusting the diameter of the lens aperture to give f/D=5.6 and by opening the shutter for 1/125 s. 10 If f=10 mm, the diameter of the lens aperture will be 1.8 / 5.6 95.144 Danylov Lecture 27 Department of Physics and Applied Physics Depth of Field One important thing to remember here, the size of the aperture has a direct impact on the depth of field, which is the area of the image that appears sharp. A large f-number such as f/32, (which means a smaller aperture) will bring all foreground and background objects in focus, while a small f-number such as f/1.4 will isolate the foreground from the background by making the foreground objects sharp and the background blurry. f/2.8 f/8.0 If we had used a much smaller aperture such as f/32 in this shot, the background would be as visible as WALL‐E (WALL‐E, short for Waste Allocation Load Lifter Earth‐class, is the last robot left on Earth) 95.144 Danylov Lecture 27 Department of Physics and Applied Physics Thank you See you on the final exam 95.144 Danylov Lecture 27 Department of Physics and Applied Physics.
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