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
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.