Another use for marbles
The image above shows a glass sphere, known as a marble. It can be used as a toy, but can also be a very interesting object to study Optical Physics, because it is transparent, so that light can pass through it.
Imagine you have a lot of marbles that are equal to that one, and you want to use them as spherical lens. Then, you put a random object in front of you, at a short distance , and look at it through the marbles. What is the minimum number of marbles you need in order to see the object without any inversion?
Notes: consider that you are immersed in the air (of course). Also, consider that the glass has a greater refractive index than the air.