We have learned that refraction occurs as
light passes across the boundary between two media. Refraction is merely one of several
possible boundary behaviors by which a
light wave could behave when it encounters a new medium or an obstacle in its
path. The transmission of light across a boundary between two media is
accompanied by a change in both the speed and wavelength of the wave. The light
wave not only changes directions at the boundary, it also speeds up or slows
down and transforms into a wave with a larger or a shorter wavelength. The only
time that a wave can be transmitted across a boundary, change its speed, and
still not refract is when the light wave approaches the boundary in a direction
that is perpendicular to it. As long as the light wave changes speed and
approaches the boundary at an angle, refraction is observed.
But why does light refract? What is the cause of such behavior? And why is there this one exception to the
refraction of light? An analogy of marching soldiers is often used to address
this question. In fact, it is not uncommon that the analogy be illustrated in a
Physics class with a student demonstration. A group of students forms a
straight line (shoulder to shoulder) and connect
themselves to their nearest neighbor using
meter sticks. A strip of masking tape divides the room into two media. In one of the media (on one side of the tape), students walk at a
normal pace. In the other media (or on the other side of the tape), students
walk very slowly using baby steps. The group of students walk forward together
in a straight line towards the diagonal strip of masking tape. The students
maintain the line as they approach the masking tape. When an individual student
reaches the tape, that student abruptly changes the pace of her/his walk. The
group of students continues walking until all students in the line have entered
into the second medium. The diagram below represents the line of students
approaching the boundary (the masking tape) between the two media. On the
diagram, an arrow is used to show the general direction of travel for the group
of students in both media. Observe that the direction of the students changes at
the "boundary."
The fundamental feature of the students' motion that leads to
this change in direction is the change in speed. Upon reaching the masking
tape, each individual student abruptly changes speed. Because the students
approach the masking tape at an angle, each individual student reaches the tape
at a different time. The student who reaches the tape first, slows down while
the rest of the line of students marches ahead. This occurs for every student in the line of students. Once a student
reaches the boundary, that student slows down while his/her nearestneighbor marches ahead at the original pace. The
result is that the direction that the line of students is heading is altered at
the boundary. The change in speed of the line of students causes a change in
direction.
Will this refractive behavior always occur? No! There are two
conditions that are required in order to observe the change in direction of the
path of the students:
· The
students must change speed when crossing the boundary.
· The
students must approach the boundary at an angle; refraction will not occur when
they approach the boundary head-on (i.e., heading perpendicular to it).
These are both reasonable enough conditions if you consider
the previous paragraph. If the students do not change speed, then there is no cause factor.
Recall that it was the change in speed of the students that caused the change
in direction. The second condition is also reasonable. If the students approach
the masking tape in a direction that is perpendicular to it, then each student
will reach the tape at the exact same time. Recall that the line of student
changed their direction because they had reached the masking tape at different
times. The first student reached the tape, slowed down, and observed the rest
of the students marching ahead at the original speed. The change in direction
of the line of students only occurs at the boundary when the students change
speed and approach at an angle.
The Marching Soldiers analogy provides an excellent analogy
to understanding the cause of light refraction. The line of students
approaching the masking tape are analogous to a wavefront of
light. The masking tape is analogous to a boundary between two media. The
change in speed that occurred for the line of students would also occur for a
wave of light. And like the marching students, a light wave will not undergo
refraction if it approaches the boundary in a direction that is perpendicular
to it.
The same two conditions that are necessary for bending the
path of the line of students are also necessary for bending the direction of a
light ray. Light refracts at a boundary because of a change in speed. There is
a distinct cause-effect relationship. The change in speed is the cause and the
change in direction (refraction) is the effect.
Laser light shown passing through a rectangular
block of lucite. It
enters the block along the normal line with an angle of incidence of 0°. There
is no bending upon entering, nor upon leaving the block. When light travels
along the normal to the boundary, it changes its speed but not its direction.