The Line of Sight
In the first section of Lesson 1, it was stated, "without light, there would be no sight."
Everything that can be seen is seen only when light from that object travels to
our eyes. Whether it be a luminous object (that generates light of its own) or
an illuminated object (that reflects the light that is incident upon it), you
can only view the object when light from that object travels to your eye. As
you look at Mary in class, you are able to see Mary because she is illuminated
with light and that light reflects off of her and travels to your eye. In the
process of viewing Mary, you are directing your sight along a line in the
direction of Mary. If you wish to view the top of Mary's head, then you direct
your sight along a line towards the top of her head. If you wish to view Mary's
feet, then you direct your sight along a line towards Mary's feet. And if you
wish to view the image of Mary in a mirror, then you must direct your sight
along a line towards the location of Mary's image. This directing of our sight
in a specific direction is sometimes referred to as the line of sight.
It is a rather simple principle:
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.
A luminous object emits light in a variety of directions; and
an illuminated object reflects light in a variety of directions. Although this
light diverges from the object in a variety of directions, your eye only sees
the very small diverging cone of rays that is coming towards it. If your eye
were located at a different location, then you would see a different cone of
rays. Regardless of the eye location, you will still need to sight along a line
in a specific direction in order to view the object.
While simple, this concept of the line of sight is also
profound! This very principle of the line of sight will assist us in
understanding the formation of images in both this unit (reflection) and the next
unit (refraction).
Using the Line
of Sight
A common Physics lab involves the determination of the image
location of a pencil (or some object) as formed by a plane mirror. In the
process of determining the image location, the manner in which light from the
object travels to your eye is investigated. First, the method of parallax is
used to locate the image of the object. Two pencils are inserted into rubber
stoppers; one stoppered pencil serves as the object and the other serves to
assist the student in locating the image. The object
pencil is placed in front of a plane mirror. Then the
student sights at the image of the object pencil in the
mirror. As a student sights along a line (the line of sight) at the image of
the pencil, the second pencil is placed behind the mirror along the same line
of sight; this is called the image pencil. When
placed along the line of sight, the portion of the image pencil that extends
above the mirror will be aligned with the image that is seen in the mirror.
Then the eye location is repositioned to the other side of the object pencil
and the process is repeated. The precise image location of the object is the
location where all lines of sight intersect regardless of where the eye is
located. Two important ideas are gleaned from such a lab: one pertains to how
light travels from the object to the eye and one pertains to the location of
the image of an object.
As you sight at the image of an object in the mirror (whether
it be a stoppered pencil or any object), light travels along your line of sight
towards your eye. The object is being illuminated by light in the room; a
countless number of rays of light are reflecting off the object in a variety of
directions. When viewing the image of the object in a plane mirror, one of
these rays of light originates at the object location and first travels along a
line towards the mirror (as represented by the blue ray in the diagram below).
This ray of light is known as the incident ray - the
light ray approaching the mirror. The incident ray intersects the mirror at the
same location where your line of sight intersects the mirror. The light ray
then reflects off the mirror and travels to your eye (as represented by the red
ray in the diagram below); this ray of light is known as the reflected ray.
So the manner in which light travels to your eye as you view
the image of an object in a mirror can be summarized as follows.
To
view the image of an object in a mirror, you must sight along a line at the
image. One of the many rays of light from the object will approach the mirror
and reflect along your line of sight to your eye.
The second important idea that can be gleaned from this
stoppered pencil lab pertains to the location of the image. Observe in the
diagram above that the image is positioned directly across the mirror along a
line that runs perpendicular to the mirror. The distance from the mirror to the
object (known as the object distance) is equal
to the distance from the mirror to the image (known as the image distance). For all plane mirrors, this equality holds true:
Object distance = Image distance
Check Your Understanding
The following diagrams depict some ideas about how light
might travel from an object location to an eye location when the image of the
object is viewed in a mirror. Comment on the incorrectness of the following
diagrams. Discuss what makes them incorrect.
|
1.
This diagram depicts
light moving in curved lines. The light by which we view objects on Earth
moves in straight lines. |
|
2.
This diagram depicts the eye looking at a location that does
not correspond to the image location. The eye must sight along a line at the
image location. |
|
3.
This diagram depicts
light passing through the mirror on the way to the mirror and on the way to
the eye. Light always reflects off the mirror; and never passes through it. |
|
4.
This diagram depicts the
eye looking at a location that does not correspond to the image location. The
eye must sight along a line at the image location. |
