physics lens 問題...

1. A man can see an object when light rays from it enter the eyes. We don't know whether the light rays have undergone reflection or refraction, anyway. No doubt, light rays appear to come from the virtual image. That's why our sense tells us apparently an image forms at the position which in fact it does not. If the image cannot be photographed, you cannot see yourself in a mirror neither.

2. This is a method to determine the focal length of a convex lens using a lens-mirror combination. It 's better to be explained by diagrams. Anyway, please see below my answer.

First, you need to know that light rays from a point on the focal plane of a convex lens, after refracted by the lens, become parallel. That's why some textbooks suggest that the image is formed at infinity.

Second, you need to know that, conversely, parallel light rays fallen on a convex lens converge at a point of the focal plane. That's why the image of a very distant object is formed on the focal plane.

Third, you need to know parallel incident light rays fallen on a plane mirror result in parallel reflected rays, by the laws of reflection.

Now, consider the set-up. Light rays from a point of the object placed on the focal plane of the lens become parallel after travelling through the lens. The parallel light rays fall on the mirror, resulting in parallel reflected rays. The parallel reflected rays travel through lens and converge to a point of the focal plane. In other words, when an object is placed on the focal plane of the lens, the image is formed on the focal plane also. No equation is needed to understand the things above. You can also see that the image can be formed on the focal plane regardless of the distance between the lens and the mirror.

Hope all these can help you.