Originally posted by ajack AFIAK there should not be any refraction at that interface since both sides are the same medium... right?
Originally posted by dcshooter a grouping of two or more elements with different, offsetting refractive indices (usually one crown glass and one flint glass) combined to produce an element that bends the light in desired ways without introducing undesired aberrations.
That's exactly it. In more simple terms (just in case), a change in refractive index (determined by the nature and density of the material, be it glass, plastic, water, etc) brings a change in the path of light. Lens design involves taking rays of light and bringing them to a desired spot on the sensor. To achieve this, you play with the number of elements, their position, curvatures and materials. Talking about "groups" simply means that, for your design, you require some elements to touch.
Originally posted by ajack Interesting, I didn't realize there were different types of glass used but I guess it makes sense.
There are so many!!! Literally hundreds of choices. Mainly their differences regard refractive indexes, but their light transmission also differs (not all glass transmit all colours in the same way), as well as their absorption , and so forth.
Lens design is a science and an art, and selecting the right glass types is a large part of that.