Behavior of Waves
Student Extras
Visit The Physics Classroom's Flickr Galleries and enjoy a visual tour of the topic of wave behavior.
University of Salford Tutorials: Diffraction
Learn more about wave diffraction using this animated tutorial.
Teacher's Guide
Need ideas? Need help? Explore The Physics Front's treasure box of catalogued resources on How Waves Move and Interact.
PhET Simulation: Wave Interference
This PhET simulation provides a virtual environment for demonstrating a wealth of wave properties and behaviors - including the reflection and diffraction of waves.
Phun with PhETUse this simulation from PhET to demonstrate the change in wavelength and direction for a light wave bending at a boundary.
Reflection, Refraction, and Diffraction
Previously 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 into 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?
The
study of waves in two dimensions is often done using a
ripple tank. A ripple tank is a large glass-bottomed tank of
water that is used to study the behavior of water waves. A
light typically shines upon the water from above and
illuminates a white sheet of paper placed directly below the
tank. A portion of light is absorbed by the water as it
passes through the tank. A crest of water will absorb more
light than a trough. So the bright spots represent wave
troughs and the dark spots represent wave crests. As the
water waves move through the ripple tank, the dark and
bright spots move as well. As the waves encounter obstacles
in their path, their behavior can be observed by watching
the movement of the dark and bright spots on the sheet of
paper. Ripple tank demonstrations are commonly done in a
Physics class in order to discuss the principles underlying
the reflection, refraction, and diffraction of waves.
If a linear object attached to an
oscillator bobs back and forth 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 dark lines
stretching across the paper and the troughs are the bright
lines. 
These
waves will travel through the water until they encounter an
obstacle - such as the wall of the tank or an object placed
within the water. The diagram at the right depicts a series
of straight waves approaching a long barrier extending at an
angle across the tank of water. The direction that these
wavefronts (straight-line crests) are traveling through the
water is represented by the blue arrow. The blue arrow is
called a ray and is
drawn perpendicular to the wavefronts. Upon reaching the
barrier placed within the water, these waves bounce off the
water and head in a different direction. The diagram below
shows the reflected wavefronts and the reflected ray.
Regardless of the angle at which the wavefronts approach the
barrier, one general law of reflection holds true: the waves
will always reflect in such a way that the angle at which
they approach the barrier equals the angle at which they
reflect off the barrier. This is known as the
law of reflection. This
law will be discussed in more detail in Unit
13 of The Physics Classroom.
The
discussion above pertains to the reflection of waves off of
straight surfaces. But what if the surface is curved,
perhaps in the shape of a parabola? What generalizations can
be made for the reflection of water waves off parabolic
surfaces? Suppose that a rubber tube having the shape of a
parabola is placed within the water. The diagram at the
right depicts such a parabolic barrier in the ripple tank.
Several wavefronts are approaching the barrier; the ray is
drawn for these wavefronts. Upon reflection off the
parabolic barrier, the water waves will change direction and
head towards a point. This is depicted in the diagram below.
It is as though all the energy being carried by the water
waves is converged at a single point - the point is known as
the focal point. After passing through the focal point, the
waves spread out through the water. Reflection of waves off
of curved surfaces will be discussed in more detail in
Unit 13 of The Physics
Classroom.
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 the 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 travel. So if the medium (and its properties) is
changed, the speed of the waves is changed. The most
significant property of water that would affect the speed
of waves traveling on its surface 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,
the
wavelength decreases, and the direction changes.
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 is placed in the bottom of the tank, 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 wavelength (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 
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 is
smaller than the obstacle, no noticeable diffraction
occurs.
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 songbirds. 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 Tutorial.
Reflection, refraction and diffraction are all boundary behaviors of waves associated with the bending of the path of a 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 that 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.