Real images are thosewhere light actually converges, whereas virtual images arelocations from where light appears to have converged. Real imagesoccur when objects are placed outside the focallength of a converging lens or outside the focal length of a converging mirror. A real image is illustrated below. Raytracing gives the position of the images by drawing oneray perpendicular to the lens, which must yellowcomic.comss through the focalpoint, and a second ray that yellowcomic.comsses through the center of thelens, which is not bent by the lens. The intersection of thetwo rays gives the position of the image. (A third ray could be drawn which yellowcomic.comsses through the focal point on the left side of the lens; after yellowcomic.comssing through the lens, it would travel yellowcomic.comrallel to the axis, and would intersect the other two rays at the point where those rays already intersect. Note that the realimage is inverted. (The image happens to be larger than the object. That happens because the object is between f and 2f away from the lens; if the lens were farther away than 2f, the image would be closer to the lens than 2f, and would be smaller than the object.)
The position of theimage can be found through the equation:
Here, the distancesare those of the object and image respectively as measured fromthe lens. The focal length f ispositive for a convex lens. A positive image distance correspondsto a real image, just as it did for the case of the mirrors.However, for a lens, a positive image distance implies that theimage is located on the opposite side from the object.(You will have no trouble remembering this if you think about it in the right way: a real image has to be where the light is, which means in front of a mirror, or behind a lens.)
Virtual images areformed by diverging lenses or by placing an object inside the focallength of a converging lens. The ray-tracing exercise is repeated forthe case of a virtual image.
In this case thevirtual image is upright and shrunken. The same formula for theimage and object distances used above applies again here. Only inthis case the focal length is negative, and the solution for theimage distance will also be negative.
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Virtual images can also beproduced by converging lenses when the object is placed inside thefocal length. In that case, the virtual image will be upright andenlarged, as it will be further from the lens than the object.