For example, a plane rectangular coordinate system with the optical center as the origin is established.

Then push down the analytical formula of the outgoing light after the light with y=kx+b is incident.

From this, it can be concluded whether it is a straight line or a straight line (only considering the real image). Then you can think about what a square will look like if it is imaged with a convex lens, thus explaining the perspective principle.

Or you can learn about the principle of telescope and microscope, and then think about how the image will change when the distance between objective and eyepiece changes, so you can write a lot.