Ray
Diagrams for Convave Mirrors - Case D
Object Located Between the
Focal Point and the Mirror
A GIF Animation
Ray diagrams are useful diagrams for determining the location
of an image as produced by a concave mirror. To determine the
location of an image using a ray diagram, at least two sets of
incident and reflected rays must be constructed for strategic
positions on the object. The image of an object is the location where
light rays from that object intersect upon reflecting from a mirror.
By constructing at least two sets of incident and reflected rays,
this image location can easily be found.
If the object is represented by an arrow, then it is common
practice to pick the two extreme positions (the top and the
bottom of the arrow) as starting points for the incident and
reflected rays. Ray construction will result in the determination of
the image locations for these two extreme positions on the object.
The complete image is merely an arrow connecting these two image
locations. This task is further simplified if the object is
positioned as an arrow standing upon the principal axis of the
mirror. If this is the case, then the image will be standing
upon the principal axis of the mirror (and either inverted or
upright).
Of all the rays which emanate from the top of the object arrow
and are incident to the mirror, there are two rays whose behavior at
the mirror surface can be easily predicted. These are the two
incident rays which are used in the ray construction. One of the rays
moves parallel to the principal axis and reflects through the focal
point. The second ray passes through the focal point on the way to
the mirror and reflects parallel to the principal axis. All concave
and convex mirror ray diagrams can be constructed from knowledge of
the behavior of these two rays.
In the animation above, a right-side-up object is located
above the principal axis between the focal point (F) and the mirror.
The ray diagram shows that the image of this object is located as a
right-side up image positioned behind the mirror. In fact, it
can be generalized that anytime the object is located between the
focal point (F) and the mirror, the image will be located behind the
mirror. In such cases, the image will be upright (not inverted) and
larger in size than the object. Such images are characterized as a
virtual images because they are not formed by the actual
convergence of reflected light rays at the image location. Virtual
images are always formed on the opposite side of the mirror as the
object.
For more information on the ray nature of light, visit
The
Physics Classroom. Specific information is available there on the
following topics:
This page was created by
Tom
Henderson of
Glenbrook South
High School.
Comments and suggestions can be sent by e-mail to
Tom
Henderson.
A hearty thanks is due to lab assistants Bryce Mautner and Amit Patel
for
their assistance with the graphics and GIF animation.
This page last updated on 8/11/98.