http://www.ck12.org Chapter 20. Geometric Optics
FIGURE 20.6
Rear view mirror with warning statement.FIGURE 20.7
A concave mirror, with reflecting surface on the left, and back off the mirror
on the right.This curvature means that the reflection in this bowl-like mirror seems smaller or closer than it really is. TheFigure
below shows how a concave mirror reflection differs from a flat mirror. In the flat mirror, the parallel red lines show
the center of the reflection. The reflected rays are symetrical about each of these. In the concave mirror, the same
red lines are at angles perpendicular to the curved surface. This creates an image that is larger than the original
object.
Curved surfaces always produce these distortions of size depending on how they curve, as seen in theFigure20.9.
What follows is the procedure of how to calculate the image height and distance, based on the height and distance
of the original object.
Focus and Focal Length
A concave mirror has afocal point(orfocus), which is where parallel rays of light shined into it converge. This is
illustrated inFigure20.10, showing rays of light coming in from the left. Because of the curvature, these rays will
all intersect at a common point. The terminology of this is as follows:
The focus: A common point (also called the focal point) where the reflected rays cross (intersect) each other.