In optics, the image formed by the convergence of actual light is called the real image; otherwise, it is called the virtual image. Experienced physics teachers often mention such a method of distinguishing when telling the difference between real image and virtual image: “The real image is upside down, while the virtual image is upright.” The so-called “upright” and “upside down”, Of course it is relative to the original image.
The three kinds of virtual images formed by flat mirrors, convex mirrors, and concave lenses are all upright; the real images formed by concave mirrors and convex lenses, as well as the real images formed by aperture imaging, are all upside-down. Of course, the concave mirror and the convex lens can also be virtual images, and the two virtual images formed by them are also in an upright state.
So, is the image formed by human eyes a real image or a virtual image? We know that the structure of the human eye is equivalent to a convex lens, so the image formed by external objects on the retina is the real image. According to the above rules of experience, the image on the retina seems to be upside down. But any objects we usually see are clearly upright? This conflict with the “law of experience” actually involves the adjustment of the cerebral cortex and the impact of life experience.
When the distance between the object and the convex lens is greater than the focal length of the lens, the object becomes an inverted image. When the object approaches the lens from a far distance, the image gradually becomes larger, and the distance between the image and the lens gradually becomes larger; when the distance between the object and the lens When it is smaller than the focal length, the object becomes a magnified image. This image is not the convergence point of the actual refracted light, but the intersection of their reverse extension lines, which cannot be received by the light screen. It is a virtual image. It can be compared with the virtual image formed by the flat mirror (cannot be received by the light screen, only visible with the eyes).
When the distance between the object and the lens is greater than the focal length, the object becomes an upside-down image. This image is formed by the light from a candle projecting to the convex lens through the convex lens. When the distance between the object and the lens is less than the focal length, the object becomes an erect virtual image.