Using the formula 1/f = 1/object distance + 1/image distance, in the

[avocadogirl]

Using the formula 1/f = 1/object distance + 1/image distance,
in the instance where you have two plano-convex lenses glued together on their flat sides, what does it mean if your image distance from the first lens is negative? is the image in front of the second lens or behind the second lens?

This is not a homework problem!

 

[Andy Resnick]

since we can consider the cemented doublet as a single lens, having d_i <0 means it is a virtual image (the doublet has negative power).