In which medium would light have the longest wavelength?

Color implies visible light, so to answer your question: red light has the longest wavelength (~620 - 740 nm). However, there also exists light with even longer wavelengths that is not visible to the human eye, from infrared light to radio waves (with wavelengths that are meters long!)

The  grandmother cell  is a hypothetical  neuronthat represents a complex but specific concept or object.[1]  it activates when a person "sees, hears, or otherwise sensibly discriminates"[2]  a specific entity, such as his or her grandmother. the term was in use at least as early as 1966 amongst staff and students in the department of experimental psychology, university of cambridge, england. a similar concept, that of the  gnostic neuron, was proposed two years later