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Lesson 3: Behavior of WavesReflection, Refraction, and DiffractionPreviously in Lesson 3, the behavior of waves traveling along a rope from a more dense medium to a less dense medium (and vice versa) was discussed. The wave doesn't just stop when it reaches the end of the medium. Rather, a wave will undergo certain behaviors when it encounters the end of the medium; specifically, there will be some reflection off the boundary and some transmission ino the new medium. But what if the wave is traveling in a two-dimensional medium such as a water wave traveling through ocean water? Or what if the wave is traveling in a three-dimensional medium such as a sound wave or a light wave traveling through air? What types of behaviors can be expected of such two- and three-dimensional waves?
If a linear object attached to an
oscillator bobs up and down within the water, it becomes a
source of straight waves. These straight waves
have alternating crests and troughs. As viewed on the sheet
of paper below the tank, the crests are the bright lines
stretching across the paper and the troughs are the dark
lines. 
Reflection involves a
change in direction of waves when they bounce off a barrier;
refraction of waves
involves a change in the direction of waves as they pass
from one medium to another. Refraction, or bending of the
path of the waves, is accompanied by a change in speed and
wavelength of the waves. In Lesson
2, it was mentioned that the speed of a wave is
dependent upon the properties of the medium through which
the waves pass. So if the medium (and its properties) are
changed, the speed of the waves are changed. The most
significant property of water which would effect the speed
of waves traveling through it is the depth of the water.
Water waves travel fastest when the medium is the deepest.
Thus, if water waves are passing from deep water into
shallow water, they will slow down. And as mentioned in
the previous section of Lesson
3, this decrease in speed will also be accompanied by a
decrease in wavelength. So as water waves are transmitted
from deep water into shallow water, the speed decreases,
This boundary behavior of water waves can be observed in a ripple tank if the tank is partitioned into a deep and a shallow section. If a pane of glass placed in the bottom of the water, one part of the tank will be deep and the other part of the tank will be shallow. Waves traveling from the deep end to the shallow end can be seen to refract (i.e., bend), decrease wavlength (the wavefronts get closer together), and slow down (they take a longer time to travel the same distance). When traveling from deep water to shallow water, the waves are seen to bend in such a manner that they seem to be traveling more perpendicular to the surface. If traveling from shallow water to deep water, the waves bend in the opposite direction. The refraction of light waves will be discussed in more detail in a later unit of The Physics Classroom.
Reflection involves a
change in direction of waves when they bounce off a barrier;
refraction of waves Diffraction of water waves is observed in a harbor as waves bend around small boats and are found to disturb the water behind them. The same waves however are unable to diffract around larger boats since their wavelength is smaller than the boat. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners, allowing us to hear others who are speaking to us from adjacent rooms. Many forest-dwelling birds take advantage of the diffractive ability of long-wavelength sound waves. Owls for instance are able to communicate across long distances due to the fact that their long-wavelength hoots are able to diffract around forest trees and carry farther than the short-wavelength tweets of song birds. Diffraction is observed of light waves but only when the waves encounter obstacles with extremely small wavelengths (such as particles suspended in our atmosphere). Diffraction of sound waves and of light waves will be discussed in a later unit of The Physics Classroom.
Reflection, refraction and diffraction are all boundary behaviors of waves associated with the bending of the path of the wave. The bending of the path is an observable behavior when the medium is a two- or three-dimensional medium. Reflection occurs when there is a bouncing off of a barrier. Reflection of waves off straight barriers follows the law of reflection. Reflection of waves off parabolic barriers results in the convergence of the waves at a focal point. Refraction is the change in direction of waves which occurs when waves travel from one medium to another; refraction is always accompanied by a wavelength and speed change. Diffraction is the bending of waves around obstacles and openings; the amount of diffraction increases with increasing wavelength. |
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© Tom Henderson, 1996-2001
Lesson
1: The Nature of a Wave
the
wavelength decreases, and the direction changes.
involves
a change in the direction of waves as they pass from one
medium to another; and
diffraction involves a
change in direction of waves as they pass through an opening
or around a barrier in their path. Water waves have the
ability to travel around corners, around obstacles and
through openings. This ability is most obvious for water
waves with longer wavelengths. Diffraction can be
demonstrated by placing small barriers and obstacles in a
ripple tank and observing the path of the water waves as
they encounter the obstacles. The waves are seen to pass
around the barrier into the regions behind it; subsequently
the water behind the barrier is disturbed. The amount of
diffraction (the sharpness of the bending) increases with
increasing wavelength and decreases with decreasing
wavelength. In fact, when the wavelength of the waves are
smaller than the obstacle, no noticeable diffraction
occurs.