1: Reflection and its Importance
The Role of Light to
The Line of Sight
The Law of Reflection
Specular vs. Diffuse
Lesson 2: Image Formation in
Why is an Image Formed?
What is an Image?
What Portion of a Mirror is
Right Angle Mirrors
Other Multiple Mirror
Lesson 3: Concave
The Anatomy of a Curved
Reflection and Image
Two Rules of Reflection
The Mirror Equation
Lesson 4 : Convex
Reflection and Image
The Mirror Equation
Lesson 2: Image Formation in
The line of sight principle
suggests that in order to view an image of an object in a
mirror, a person must sight along a line at the image of
the object. When sighting along such a line, light from
the object reflects off the mirror according to the
law of reflection and travels
to the person's eye. This process was discussed and
explained earlier in this
lesson. One useful tool which is frequently use to
depict this idea is known as a ray diagram. A
ray diagram is a
diagram which traces the path which light takes in order
for a person to view a point on the image of an object.
On the diagram, rays (lines with arrows) are drawn for
the incident ray and the reflected ray. Complex objects
such as people are often represented by stick figures or
arrows; in such cases it is customary to draw rays for
the extreme positions on such objects.
section of Lesson 2 details and illustrates the procedure
for drawing ray diagrams. Let's begin with the task of
drawing a ray diagram to show how Suzie will be able to
see the image of the green object arrow in the
diagram below. For simplicity sake, we will suppose that
Suzie is viewing the image with her left eye closed.
Thus, we will focus on how light travels from the two
extremities of the object arrow (the left and right side)
to the mirror and finally to Suzie's right eye as she
sights at the image. The four steps of the process for
drawing a ray diagram are listed and applied below.
- Draw the image of the object.
the principle that the object distance is equal to the
image distance to determine the exact location of the
object. Pick one extreme on the object and
carefully measure the distance from this extreme
point to the mirror. Mark off the same distance on
the opposite side of the mirror and mark the image of
this extreme point. Repeat this process for all
extremes on the object until you have determined the
complete location and shape of the image. Note that
all distance measurements should be made by measuring
along a segment which is perpendicular to the
- Pick one extreme on the image of the object and
draw the reflected ray which will travel to the eye as
it sights at this point.
the line of sight principle:
the eye must sight along a line at the image of the
object in order to see the image of the object. It is
customary to draw a bold line for the reflected ray
(from the mirror and to the eye) and a dashed line as
an extension of this reflected ray; the dashed line
extends behind the mirror to the location of the image
point. The reflected ray should have an arrowhead upon
it to indicate the direction that the light is
traveling. The arrowhead should be pointing towards
the eye since the light is traveling from the mirror
to the eye, thus enabling the eye to see the
- Draw the incident ray for light traveling from the
corresponding extreme on the object to the mirror.
incident ray will undergo the law
of reflection at the mirror's surface. But rather
than measuring angles, you can merely draw the
incident ray from the extreme of the object to
the point of incidence on the mirror's surface. Since
you drew the reflected ray in step 2, the point of
incidence has already been determined; the point of
incidence is merely the point where the line of sight
intersects the mirror's surface. Thus draw the
incident ray from the extreme point to the
point of incidence. Once more, be sure to draw an
arrowhead upon the ray to indicate its direction of
travel. The arrowhead should be pointing towards the
mirror since light travels from the object to the
- Repeat steps 2 and 3 for all other extremities on
completing steps 2 and 3, you have only shown how
light travels from a single extreme on the
object to the mirror and finally to the eye. You will
also have to show how light travels from the other
extremes on the object to the eye. This is
merely a matter of repeating steps 2 and 3 for each
individual extreme. Once repeated for each extreme,
your ray diagram is complete.
The best way to
learn to draw ray diagrams involve trying it yourself.
It's easy. Merely duplicate the two set-ups below onto a
blank sheet of paper, grab a ruler/straight-edge, and
begin. If necessary, refer to the four-step procedure
listed above. When finished, compare your diagram with
the completed diagrams at the bottom
of this page.
Ray diagrams can be particularly useful
for determining and explaining why only a portion of the
image of an object can be seen from a given location. The
ray diagram below shows the lines of sight used by the
eye in order to see a portion of the image in the mirror.
Since the mirror is not long enough, the eye can only
view the topmost portion of the image. The lowest point
on the image which the eye can see is that point in line
with the line of sight which intersects the very bottom
of the mirror. As the eye tries to view even lower points
on the image, there is not sufficient mirror present to
reflect light from the lower points on the object to the
eye. The portion of the object which cannot be seen in
the mirror is shaded green in the diagram below.
Similarly, ray diagrams are useful
tools for determining and explaining what objects might
be viewed when sighting into a mirror from a given
location. For example, suppose that six students - Abe,
Bev, Cis, Don, Eva, and Flo sit in front of a
plane mirror and attempt to see each other in the mirror.
And suppose the exercise involves answering the following
questions: Who can Abe see? Who can Bev see? Who can Cis
see? Who can Don see? Who can Eva see? And who can Flo
The task begins by locating the images
of the given students. Then, Abe is isolated from the
rest of the students and lines of sight are drawn to see
who Abe can see. The leftward-most student who Abe can
see is the student who's image is to the right of the
line of sight which intersects the left edge of the
mirror. This would be Eva. The rightward-most student who
Abe can see is the student who's image is to the left of
the line of sight which intersects the right edge of the
mirror. This would be Flo. Abe could see any student
positioned between Eva and Flo by looking at any other
positions along the mirror. However in this case, there
are no other students between Eva and Flo; thus, Eva and
Flo are the only students whom Abe can see? The diagram
below illustrates this using lines of sight for Abe.
Of course the same process can be
repeated for the other students by observing their lines
of sight. Perhaps you will want to try to determine who
Bev, Cis, Don, Eva, and Flo can see? Then check your
answers by looking in the pop-up menu below.
Six students are arranged in front of a mirror. Their
positions are shown below. The image of each student is
also drawn on the diagram. Make the appropriate line of
sight constructions to determine to determine which
students each individual student can see.
Here are completed diagrams for
the two examples given above.
Back to Diagram.