Refraction and Sight
In Unit 13 of The Physics Classroom Tutorial, it was emphasized that we are able to see because light from an object
can travel to our eyes. Every object that can be seen is seen only because
light from that object travels to our eyes. 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.
The
Broken Pencil
As light travels through a given medium, it travels in a
straight line. However, when light passes from one medium into a second medium,
the light path bends. Refraction takes
place. The refraction occurs only at the boundary. Once the light has crossed
the boundary between the two media, it continues to travel in a straight line.
Only now, the direction of that line is different than it was in the former
medium. If when sighting at an object, light from that object changes media on
the way to your eye, a visual distortion is likely to occur. This visual
distortion is witnessed if you look at a pencil submerged in a glass
half-filled with water. As you sight through the side of the glass at the
portion of the pencil located above the water's surface, light travels directly
from the pencil to your eye. Since this light does not change medium, it will
not refract. (Actually, there is a change of medium from air to glass and back
into air. Because the glass is so thin and because the light starts and
finished in air, the refraction into and out of the glass causes little
deviation in the light's original direction.) As you sight at the portion of
the pencil that was submerged in the water, light travels from water to air (or
from water to glass to air). This light ray changes medium and subsequently
undergoes refraction. As a result, the image of the pencil appears to be
broken. Furthermore, the portion of the pencil that is submerged in water
appears to be wider than the portion of the pencil that is not submerged. These
visual distortions are explained by the refraction of light.
In this case, the light rays that undergo a deviation from their
original path are those that travel from the submerged portion of the pencil,
through the water, across the boundary, into the air, and ultimately to the
eye. At the boundary, this ray refracts. The eye-brain interaction cannot
account for the refraction of light. As was emphasized in Unit 13, the brain
judges the image location to be the location where light rays appear to
originate from. This image location is the location where
either reflected or refracted rays intersect. The eye and brain assume that
light travels in a straight line and then extends all incoming rays of light
backwards until they intersect. Light rays from the submerged portion of the
pencil will intersect in a different location than light rays from the portion
of the pencil that extends above the surface of the water. For this reason, the
submerged portion of the pencil appears to be in a different location than the portion
of the pencil that extends above the water. The diagram at the right shows a
God's-eye view of the light path from the submerged portion of the pencil to
each of your two eyes. Only the left and right extremities (edges) of the
pencil are considered. The blue lines depict the path of light to your right
eye and the red lines depict the path of light to your left eye. Observe that
the light path has bent at the boundary. Dashed lines represent the extensions
of the lines of sight backwards into the water. Observe that these extension
lines intersect at a given point; the point represents the image of the left
and the right edge of the pencil. Finally, observe that the image of the pencil
is wider than the actual pencil. A ray model of light that considers the
refraction of light at boundaries adequately explains the broken pencil
observations.
Flickr Physics Photo
A straw is placed with a diagonal orientation
within a half-filled beaker of water. At the surface of the water, the straw
appears to be misaligned or broken; the portion of the straw above the water is
shifted relative to the image viewed under the water. The bending of the path
of light as it passes from the water to air causes the observed distortion of
the image of the straw.
The broken pencil phenomenon occurs during your everyday
spearfishing outing. Fortunately for the fish, light refracts as it travels
from the fish in the water to the eyes of the hunter. The refraction occurs at
the water-air boundary. Due to this bending of the path of light, a fish
appears to be at a location where it isn't. A visual distortion occurs.
Subsequently, the hunter launches the spear at the location where the fish is
thought to be and misses the fish. Of course, the fish are never concerned about
such hunters; they know that light refracts at the boundary and that the
location where the hunter is sighting is not the same location as the actual
fish. How did the fish get so smart and learn all this? They live in schools.
Now any fish that has done his/her physics homework knows
that the amount of refraction that occurs is dependent upon the angle at which
the light approaches the boundary. We will investigate this aspect of
refraction in great detail in Lesson 2. For now, it is
sufficient to say that as the hunter with the spear sights more perpendicular
to the water, the amount of refraction decreases. The most successful hunters
are those who sight perpendicular to the water. And the smartest fish are those
who head for the deep when they spot hunters who sight in this direction.
Since refraction of light occurs when it crosses the
boundary, visual distortions often occur. These distortions occur when light
changes medium as it travels from the object to our eyes.
Flickr Physics Photo
That frosty mug full of A&W root beer seems
to drain so quickly. What seems like a lot of root beer is actually a lot of
glass. The deception is uncovered when the root beer mug is submerged in water.
When looking at the mug above the water level, there appears to be a lot of
root beer. But when looking at the mug below the water level, one observes the
truth; the water surrounding the glass minimizes the effect of refraction and
reveals the large quantity of glass. We've been deceived!
