Note that different sign conventions are used in the literature. For example, a common convention takes the radius for the second interface as positive if the surface is concave. This is opposite to the convention used above. Thin and Thick Lenses In many practical cases, a lens is so thin...
When you put a convex and then concave lens in front of a light source, the light will be parallel but narrower than when it came in such as in a laser beam expander/compressor. Using a pen laser and a convex and concave lens, is it possible to focus the beam by putting a convex ...
However, we also need to consider the sign convention for the object distance. According to the sign convention, distances in front of the mirror are positive, and distances behind the mirror are negative. Therefore, the closer position is at 18 cm in front of the mirror (positive do), ...