Lesson 1: Reflection and Its
Importance
The Role of Light to
Sight
The bottom line is: without light, there would be no
sight. The visual ability of humans and other animals is the
result of the complex interaction of light, eyes and brain.
We are able to see because light from an object can move
through space and reach our eyes. Once light reaches our
eyes, signals are sent to our brain, and our brain deciphers
the information in order to detect the appearance, location
and movement of the objects we are sighting at. The
whole process, as complex as it is, would not be possible if
it were not for the presence of light. Without light, there
would be no sight.
If you were to turn off the room lights
for a moment and then cover all the windows with black
construction paper to prevent any entry of light into the
room, then you would notice that nothing in the room would
be visible. There would be objects present that were capable
of being seen. There would be eyes present which would be
capable of detecting light from those objects. There would
be a brain present which would be capable of deciphering the
information sent to it. But there would be no light! The
room and everything in it would look black. The appearance
of black is merely a sign of the absence of light. When a
room full of objects (or a table, a shirt or a sky) looks
black, then the objects are not generating nor reflecting
light to your eyes. And without light, there would be no
sight.
The objects which we see can be placed
into one of two categories: luminous objects and illuminated
objects. Luminous
objects are objects which generate their own
light. Illuminated
objects are objects which are capable of
reflecting light to our eyes. The sun is an example of a
luminous object, while the moon is an illuminated object.
During the day, the nuclear reactions on the sun's surface
generate sufficient light to illuminate objects on Earth.
The blue skies, the white clouds, the green grass, the
colored leaves of fall, the neighbor's house, and the car
approaching the intersection are all seen as a result of
light from the sun (the luminous object) reflecting off the
illuminated objects and traveling to our eyes. Without the
light from the luminous objects, these illuminated objects
would not be seen. During the evening when the Earth has
rotated to a position where the light from the sun can no
longer reach our part of the Earth (due to its inability to
bend around the spherical shape of the Earth), objects on
Earth appear black (or at least so dark that we could say
they are nearly black). In the absence of a porch light or a
street light, the neighbor's house can no longer be seen;
the grass is no longer green, but rather black; the leaves
on the trees are dark; and were it not for the headlights of
the car, it would not be seen approaching the intersection.
Without luminous objects generating light which propagates
through space to illuminate non-luminous objects, those
non-luminous objects cannot bee seen. Without light, there
would be no sight.
Perhaps you recall the laser beam
demonstration performed in class. With the room lights off,
the laser was turned on and its beam was directed towards a
plane mirror on the side of the room. The presence of the
light beam could not be detected as it traveled towards the
mirror; and the light beam could not be detected after
reflecting off the mirror and traveling towards the wall in
the back of the room. The only locations where the presence
of the light beam could be detected were at the location
where the light beam struck the mirror and at the location
where the light beam struck the wall in the back of the
room. At these two locations, a portion of the light in the
beam was reflecting off the objects (the mirror and the
wall) and traveling towards your eyes. And since the
detection of objects is dependent upon light traveling from
that object to your eye, these were the only two locations
where you could detect the light beam. But in between the
laser and the mirror, the light beam could not be detected.
There was nothing present in the region between the laser
and the mirror which was capable of reflecting the light of
the beam to your eyes.
But then the phenomenal occurred (as it
often does in Physics class). A mister was used to spray
water into the air in the region where the light beam was
moving. Small suspended droplets of water are capable of
reflecting light from the beam to your eye. It was only due
to the presence of the suspended water droplets that the
light path from the laser to the mirror could be detected.
When light from the laser (a luminous object) struck the
suspended water droplets (the illuminated object), the light
could be reflected to your eye. The path of the light beam
could then be seen. With light , there can be sight. But
without light, there would be no sight.
None of us are light-generating objects.
We are not brilliant objects (please take no offense) like
the sun; rather, we are illuminated objects like the moon.
We make our presence visibly known by reflecting light to
the eyes of those who look our way. It is only by reflection
that we, as well as most of the other objects in our
physical world, can be seen. And if reflected light is so
essential to sight, then the very nature of light reflection
is a worthy topic of study among students of physics.
And in this lesson and the several which follow, we will
undertake a study of the way light reflects off objects and
travels to our eyes in order to allow us to view them.
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