Sign conventions for front and back of lens in multi-element systems

[MichaelT]

Working on some optics problems and I am confused about the front and back of the lens. The sign conventions are key to finding the correct image location. Now, is the front always where the object is located?

The problem I am working on involves a divergent lens and a concave mirror. The object is in front of the lens. An image forms in front of the lens. Then the concave mirror forms an image between the lens and the mirror, which would be on the opposite side of the lens.

Would the object now be considered as being in back of the lens, or would it be the front? The other side of the lens was previously the front. Does this change, or is one side considered the front for the whole problem?

Thanks!

 

[rl.bhat]

Instead of saying front and back of lens, you can say , the face which receives the ray and the face which refract the ray. Measure the distances from the center of the lens.