More Sign Conventions
Consider an object distance s which is less than the focal length:
- Ray through the center of the sphere (light blue) is reflected straight back.
- We call this a virtual image, meaning that no light from the object passes through the image point.
- Proof left to student: This situation is described by the same mirror equations as long as we take the convention that images behind the mirror have negative image distances s’. ie:
In this case, s’ < 0, which leads to M > 0, indicating that the image is virtual (s’ɘ) and not inverted (Mɬ).
- Ray parallel to axis (white) passes through focal point f.
- These rays diverge! ie these rays look they are coming from a point behind the mirror.