You will be given a definition of a task first, then some input of the task.
Given a scientific passage and an answer, generate a question for the given answer.

Passage: - For lenses, the distance from the center of the lens to the focus is . Focal lengths are positive for converging lens and negative for diverging lens. The distance from the center of the lens to the object in question is , where distances to the left of the lens are positive in sign. The distance from the center of the lens to the image is . This number is positive for real images (formed to the right of the lens), and negative for virtual images (formed to the left of the lens). Answer: focal lengths
Output:
What lengths are positive for converging lens and negative for diverging lens?