Summary
Highlights
This section introduces the concept of ray diagrams for concave mirrors, outlining the learning goals: drawing such diagrams and describing the images they produce. It highlights that image descriptions follow rules from plane mirror diagrams and explains different possible object locations relative to the mirror's center of curvature (C) and focal point (F).
For objects located beyond C, at C, or between C and F, the same set of rules apply. The first rule states that an incident ray parallel to the principal axis reflects through the focus (F). The second rule is the inverse: an incident ray passing through F will reflect parallel to the principal axis. The intersection of these reflected rays determines the top of the image.
An example demonstrates drawing the ray diagram for an object placed beyond C. The resulting image is smaller, inverted, located between C and F, and is a 'real' image because it forms on the same side of the mirror as the object.
Different rules apply when the object is precisely at the focal point (F). The first rule remains the same: an incident ray parallel to the principal axis reflects through F. However, the second rule changes: an incident ray that appears to come from the center of curvature (C) will reflect back along the same line.
When the object is at F, the reflected rays are parallel and never cross, meaning no image is formed. This is a consistent outcome whenever an object is placed at the focal point of a concave mirror.
For an object placed between the focal point (F) and the mirror (V), the first rule is again an incident ray parallel to the principal axis reflects through F. The second rule is an incident ray that appears to come from C reflects back along its original path. In this case, the reflected rays diverge.
Since the reflected rays diverge, they don't cross on the real side of the mirror. However, extending these reflected rays backward behind the mirror reveals an intersection point. This creates a larger, upright, and 'virtual' image behind the mirror. Dashed lines are used to indicate that light does not actually pass through the mirror.