The image of a circular object through a thin lens
This article is translated from a Chinese article on my Zhihu account. The original article was posted at 2019-07-12 15:40 +0800.
The center of the circular luminous object is , and its radius is . The center of the thin lens is , and its focal length is . The object is in the same plane as , and is perpendicular to the thin lens, and .
Set up Cartesian coordinate with being the origin and being the -axis. Then, . The luminous object is described by the parametric equations Pick point on the object. According to the formula for imaging of thin lenses the point is transformed to . Therefore, we can have the parametric equations of the image: Cancel , and we have which means that the image is a conic section with the focus being , the directrix being line , and the eccentricity being .
Alternatively, let , and we have and we may have the same conclusion through this equation.
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