FIGURE 34.45 shows a concave mirror, a mirror in which the edges curve toward the light source. Rays parallel to the optical axis reflect from the surface of the mirror so as to pass through a single point on the optical axis. This is the focal point of the mirror. The focal length is the distance from the mirror surface to the focal point. A concave mirror is analogous to a converging lens, but it has only one focal point.

FIGURE 34.45 The focal point and focal length of a concave mirror.

Let’s begin by considering the case where the object’s distance $s$ from the mirror is greater than the focal length $(s>f)$, as shown in FIGURE 34.46 . We see that the image is real (and inverted) because rays from the object point P converge at the image point $\text{P}\prime$. Although an infinite number of rays from P all meet at $\text{P}\prime$, each ray obeying the law of reflection, you can see that three “special rays” are enough to determine the position and size of the image:

• A ray parallel to the axis reflects through the focal point.

• A ray through the focal point reflects parallel to the axis.

• A ray striking the center of the mirror reflects at an equal angle on the opposite side of the axis.

FIGURE 34.46 A real image formed by a concave mirror.

These three rays also locate the image if $s<f$, but in that case the image is virtual and behind the mirror. Once again, virtual images have a negative image distance $s\prime$.