The above figure takes the luminous object placed outside of the focal length as an example. The parallel light emitted from the top of the light source is refracted by the lens. The refracted light will pass through the intersection point F. The light emitted from the top of the light source and passing through the center of the lens is not bent. Converge to a point, the vertical distance of this point from the central axis of the lens is denoted as A; in fact, all the rays emitted from the top will converge to point C after being refracted by the lens. The light path in the middle of the picture. The same is true for the light rays emitted from other places after being refracted, and they each converge at the corresponding concentration point, that is, all the light energy passing through the lens is concentrated and overlapped. At this time, the imaging plane is placed at the A position, and a clear image will be obtained.
At this time, if the imaging plane is placed at position B, it will be found that the three light rays used in the above example fall on three different positions, and the energy is dispersed, so the image is blurred. Other differences are the same as the position of A. This is the case of a single lens. Observing with a telescope is a relatively complicated combined lens case, but the basic principle is the same.
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