Formulate rules for constructing images in a lens. Which image is called valid? imaginary?

To construct an image of a point, you can choose any two of three standard rays: a) a ray parallel to the main optical axis, which, after refraction, passes through the main focus; b) a beam that coincides with the side optical axis and passes without refraction through the optical center of the lens; c) a ray passing through the main focus, going behind the lens parallel to the main optical axis. The real image is obtained at the intersection of the rays of light, and the imaginary one – at the intersection of the continuation of these rays.

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