chapter25.1-25.3 - Optical Instruments Generally involves...

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3/29/2009 1 Optical Instruments Generally involves the laws of reflection and refraction Geometric optics (Mirror equation, lens’ equation) Sometime the wave nature of light must be used to explain certain phenomena For example, the resolution of optical instruments is limited by diffraction f q p 1 1 1
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3/29/2009 2 Components Opaque, light-tight box Converging lens Produces a real image Object distance is normally beyond 2f ( real, inverted, smaller image ) Film (or CCD sensor) behind the lens Receives the image Focusing: Adjusting the image distance q (lens-to-sensor distance) q depends on the object distance and on the focal length of the lens: Focusing leads to sharp images f q p 1 1 1
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3/29/2009 3 The shutter: a mechanical device that is opened for selected time intervals Dark scenes: long exposure time Fast moving object: short exposure time Exposure time too long Short exposure time Typical shutter speeds: 1/30 s, 1/60 s, 1/250 s But short exposure time may lead to dark photos Adjustable aperture for further light intensity control Light intensity: the rate at which energy (per unit area) received by the sensor
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3/29/2009 4 Light intensity reaching the sensor: D: diameter of the camera Area of the image: For p>>f, q~f. Therefore The brightness of the image depends on The ƒ -number of a camera is the ratio of the focal length of the lens to its diameter: ƒ -number = f/D The ƒ -number is often given as a description of the lens “speed”; A lens with a low f- number is a “fast” lens 2 D I 2 2 2 / so q D I q A f q p 1 1 1 2 2 f D I 2 2 f D I Increasing the ƒ
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