In optics, the image formed by the convergence of actual light rays is called a real image;On the contrary, it is called a virtual image.When explaining the difference between real images and virtual images, experienced physics teachers often mention a method of distinction: "Real images are inverted, while virtual images are upright." The terms "upright" and "inverted" are, of course, relative to the original image.

The three types of virtual images formed by plane 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 pinholes, are all inverted without exception.Of course, concave mirrors and convex lenses can also form virtual images, and the two types of virtual images they form are also upright.

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 of external objects on the retina must be a real image.According to the above empirical rule, the object image on the retina seems to be inverted.But we usually see any object, it is clearly upright?This problem of conflict with the "law of experience" actually involves the adjustment of the cerebral cortex and the influence of life experience.

When the distance between the object and the convex lens is greater than the focal length of the lens, the object forms an inverted image. As the object approaches the lens from a greater distance, the image gradually becomes larger, and the distance from the image to the lens also gradually increases;When the distance between the object and the lens is less than the focal length, the object forms an enlarged image, which is not the convergence point of the actual refracted light rays, but the intersection point of their reverse extension lines. It cannot be received by a light screen and is a virtual image.It can be compared with the virtual image formed by a plane mirror (which cannot be received by a light screen, but can only be seen with the eyes).

When the distance between the object and the lens is greater than the focal length, the object forms an inverted image, which is formed by the light rays from the candle that are converged by the convex lens. This is the convergence point of the actual light rays, which can be received by a light screen and is a real image.When the distance between the object and the lens is less than the focal length, the object is an upright virtual image.