The main effect of lenses is that they map object points to image points.
Let say we have two object points, for example the Top T0 and the bottom B0 of a vase. A two element compound lens works like this :
The object points T0 an B0 are mapped to image points T1 and B1 by lens element number 1. The lens elements are numbered starting from object side.
The images T1 and B1 then become the new “object points” of lens element 2 and map T1 and B1 to “images of image” of T0 and T0, called T2 and B2″.
A lens with 10 lens elements accordingly generates sequences
T0->T1->T2->… -> T10
The image produced by a lens can be in front or behind the lens.
It can be smaller or bigger than the object.
Of course we want images of trees to be smaller than trees.
Image of microscopic details we of course want to be larger than the details.
Light that we can see (“visible light”, “VIS”) is a small part of a spectrum of a thing called “electromagnetic radiation”, distinguished by something we call “wavelength”.
As the wavelength varies in the visible spectrum, the light appearantly changes color from violet to red.
There are no actual boundaries between one range of wavelengths and another. So numbers associated with a certain range are only approximate.
If we explore the spectrum from long wavelengths to shorter wavelengths, we meet :
“radio waves” : regular broadcast wavelengths are for example 500 meters long, but even longer radio waves exist