Lesson 2: Image Formation in
Plane Mirrors
Why is an Image
Formed?
In Lesson 1 of this unit of the
Physics Classroom, the manner in which light reflected off
objects in order to allow us to see them was discussed. A
major principle in that lesson was expressed as follows:
In order to view an object, you must sight along
a line at that object; and when you do light will come
from that object to your eye along the line
of sight.
This very principle can be extended to the
task of viewing the image of an object in a plane (i.e.,
flat) mirror:
In order to see the image of an object in a
mirror, you must sight at the image; when you sight at
the image, light will come to your eye along that line of
sight.
The image location is
thus located at that position where observers are sighting
when viewing the image of an object. It is the location
behind the mirror where all the light appears to diverge
from. In the diagram below, three individuals are sighting
at the image of an object along three different lines of
sight. Each person sees the image due to the reflection of
light off the mirror in accordance with the law
of reflection. When each line of sight is extended
backwards, each line will intersect at the same point. This
point is the image point of the object.
![diagram]()
![diagram]()
This principle was illustrated in class
using a 5-foot plane mirror and a pair of large cylinders.
One cylinder was placed n front of the mirror and students
from different locations in the room were asked to sight at
its image. The second cylinder was then aligned along the
line of sight and readjusted until it was in line with each
person's line of sight. Regardless of who is viewing the
image and from where they are viewing the image, each sight
line must intersect in the same location. It is possible
that the second cylinder is aligned with one student's line
of sight but not with another student's If this is so, then
the cylinder is not placed at the exact location of the
image. This is depicted in the diagram below.
![diagram]()
In a case such as this, the cylinder position would have
to be adjusted until it is located at the position where all
students in the classroom can see it extending above the
mirror and in line with the image which each students sees
when looking in the mirror. Only, then can we conclude the
cylinder is located at the image position.
Since there is only one image for an
object placed in front of a plane mirror, it is reasonable
that every sight line would intersect in a single location.
This location of intersection is known as the image
location. And as mentioned in an earlier lesson, the
distance between the object and the mirror is equal to the
distance from the image to the mirror. In fact, the image is
located directly across the mirror from the object and and
an equal distance from the mirror.
![diagram]()
Of course, it is possible that certain
individuals in the room will be unable to view the image of
an object in a plane mirror. Because of the person's
position relative to the image position and to the
extremities of the mirror, the person is unable to detect a
ray of light reflecting to their eye as they sight at the
image location. This does not mean that there is no image.
Indeed, any object positioned in front of a plane mirror has
an image regardless of whether there are people positioned
in an appropriate location to view it. In the diagram below,
there is an image of an object located on the other side of
the mirror. However, Ray Zuvlite is unable to view the image
due to his position in the room. Ray is certainly able to
sight in the direction of the image location; however, there
light is unable to reflect off the mirror in accordance with
the law of reflection and travel to his eye along his line
of sight. Since light from the object does not make it to
his eye, Ray is unable to see the image of the object in the
mirror.
![diagram]()
Of course, this problem could be remedied
if the mirror were wider, if the object were moved to the
left or closer to the mirror, and/or if Ray moved to the
right. Repositioning the object, the mirror, and/or the
person could result in a ray of light from the object
reflecting off the mirror and traveling to Ray's eye. The
diagram below depict this remedy.
![diagram]()
So why is an image formed by a plane
mirror? An image is formed because light emanates from an
object in a variety of directions. Some of this light (which
we represent by rays) reaches the mirror and reflects off
the mirror according to the law of reflection. Each one of
these rays of light can be extended backwards behind the
mirror where they will all intersect at a point (the image
point). Any person who is positioned along the line of one
of these reflected rays can sight along the line and view
the image - a representation of the object.
![diagram]()
This principle of image formation was
applied in the Plane Mirror Lab. A pin was positioned in an
upright position in front of a plane mirror. Students
sighted along lines at the image of the pin. A line was
drawn along the line of sight. The procedure was repeated
for other positions and other lines of sight. Each sight
line was then traced backwards beyond the mirror; they all
intersected at the image location. The image was thus formed
at the single point on the opposite side of the mirror from
where all the light rays appear to have diverged. Validation
of the accuracy of your sighting and ray tracing was
accomplished by measuring angles of incidence and angles of
reflection on the diagram. These should be equal for each
individual sight line; that is, angle A should equal angle
B; angle C should equal angle D; and angle E should equal
angle F. Finally, the object distance was compared to the
image distance; these should also be equal.
![diagram]()
![diagram]()
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