8B - 14_Images_11

8B - 14_Images_11 - Real vs Virtual Images Real image The...

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Real vs Virtual Images Real image - The light is converging so it can make an image on a screen. The light reflected from a real image on a screen acts like a source. Light emerges from a real image as from a real object. Virtual image - The light appears to emerge from a place but has not passed through that place. The light is diverging and does not make an image when shined on a screen. Nevertheless, a virtual image can be seen by the eye because the lens of the eye converges the rays onto the retina which creates an image in the brain.
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Demo - various optical elements
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Plane Mirrors For mirrors, by convention (that is, to get our equations to work out), distances in front of a mirror are taken as positive . Distances behind the mirror are negative . For a plane (flat) mirror, the object is in front , so s is positive . The image is behind the mirror, so s’ is negative . Thus, for a plane mirror we can write | s’ | = s or s = - s’ for plane (flat) mirrors
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Plane Mirror - Extended Source Every point on the source gives a point on the virtual image. The image is virtual because it cannot be seen on a screen, and no light actually emanates from it. The lens of the eye creates a real image on the retina.
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Demos - plane mirrors Light bulb -virtual image (Light reflected from glass creates a virtual image of lighted bulb. Appears to arise from unlit bulb.) Glass reflects some light Hand held and right-angle mirrors
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Demo - Phantom light-bulb Light is reflected from the glass pane (a plane mirror). Shielded bulb appears to be at position A (a virtual image).
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Use of Plane Mirrors - Rear View Rear view mirror can be flipped to reduce glare of headlights behind you. Reflects from two wedge - shaped plane surfaces: Mirror (strongly) and Glass (weakly). From “Principles of Physics”, Serway and Jewett, 4th Ed., p. 870. During the day, eyes are adapted to bright light. The mirror is adjusted so the bright light (B) reflected from the mirror surface reaches your eyes. At night, eyes are dark adapted; prefer to have less light reflected. Adjust so that less light (dark, D) reflected from the glass surface reaches your eyes. Mirror Glass
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Spherical Mirrors For the plane mirror the center of curvature C (the center of the sphere of which the mirror's surface is part) was infinitely far. Radius of curvature, r, is . For the concave mirror r is finite, and in front. (con - CAVE = in, as in b) For a convex mirror the surface is “flex ed out” as in (c). Curving the surface in this way (1) moves the center of curvature C to behind the mirror and (2) increases the field of view. It also (3) moves the image of the object closer to the mirror and (4) shrinks it.
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Focal Lengths of Spherical Mirrors The focal length, f, of a concave mirror is in front of the mirror and is thus a positive quantity. For a convex mirror, f is behind the mirror and is thus a negative quantity. For mirrors of both types, the focal length f is related to the radius of curvature r of the mirror by This result for f is valid for rays whose distance from the central axis is << radius of curvature; other rays give a fuzzy focus. To have
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