Conceptual Physics

Now that the three rays have been drawn separately, we draw them together in Concept 5. They converge at the head of the image. The image

is both inverted (upside down) and smaller than the original object. It is located between the focal point and the center of curvature. The image

is real, being located on the same side of the mirror as the object. Since it is real, if a piece of paper were placed at the point of intersection of

the reflected rays, a small inverted image would be projected onto it.

Image

Located at intersection of rays

Inverted, smaller, real

31.12 - Ray tracing: a second example

We will use ray tracing to determine the nature of the image created by an object that lies within the focal point of a concave mirror. As with the

rays in the prior example, the incident rays all start or pass through a point at the top of the object, the penguin. They also have the same basic

properties as the previous rays: One is parallel to the principal axis, one passes through the focal point, and the third passes through the center

of curvature.

In contrast to the prior example, the reflected rays do not converge. We will need to use virtual rays to locate the image.

Ray 1 starts parallel to the principal axis, reflects, and passes through the focal point. Note that we extend the reflected ray backward through

the mirror surface as a virtual ray (dashed line).

Ray 2 must pass through the focal point before reaching the mirror. We draw it as passing through the focal point before intersecting the

penguin’s head. It then strikes the mirror and reflects parallel to the principal axis.

What will be the nature of the

image?

Ray 1

Incident parallel to axis

Ray 2

Passes through focal point

(^584) Copyright 2007 Kinetic Books Co. Chapter 31