Wave Behavior
Student Expectation
The student is expected to investigate behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler effect.
Key Concepts
Unlike matter, two or more waves can occupy the same place at the same time. This is called the superposition of waves.
As a wave strikes a boundary, it makes an abrupt change of direction, which depends upon the angle the wave strikes. The speed of the wave is not changed. This is called reflection.
As a wave crosses from one medium to another, the apparent speed of the wave changes. This causes the wave to bend towards the denser medium if it is moving at a non-perpendicular angle to the boundary. This wave behavior is known as refraction.
Diffraction is when a wave spreads or bends as it moves around the edge of an obstacle.
When two waves meet, there is a net effect known as interference. If the two waves are causing a displacement in the same direction, the result is constructive interference. If the two waves are causing a displacement in opposite directions, the result is destructive interference.
All objects have one or more frequencies, or resonant frequencies, where the amplitude of oscillation is very large.
Whenever the source of a wave is moving in relationship to the observer, there is an apparent shift in the wave’s frequency. This phenomenon is known as the Doppler effect.
WAVE BEHAVIOR
Reflection
Reflection is the ability of a wave to bounce back, but not be absorbed. When a wave strikes a boundary, it makes an abrupt change of direction, which depends upon the angle the wave strikes, but the speed of the wave is not changed. One characteristic of waves is the Law of Reflection: the angle at which the wave approaches a flat, reflective surface is equal to the angle at which the wave leaves the surface.
Hitting a fixed barrier, a wave will reverse and reflect inverted; while hitting a movable end, the wave will not invert.
Examples of reflected waves are light waves reflecting off a telescope mirror or sonar produced sound waves that reflect off objects and identify their shape. NASA used the reflection of laser lights to map the surface of the Moon.
Refraction
The speed of a wave changes when a wave crosses from one medium to another. This causes the wave to bend toward the denser medium if it is moving at a non-perpendicular angle to the boundary. This is known as wave refraction. This phenomenon causes objects in water to appear to be bent when light passes from air through water.
The combination of the refraction of sunlight as it enters a water droplet and then it is reflected off the back of the droplet and refracted back out the drop causes the sunlight to split into its various wavelengths and produce a rainbow or spectrum of color. The principle of refraction is used, for example, in making lenses for cameras, eyeglasses, and refracting telescopes.
Interference
When two waves meet in a medium, there is a net effect on the particles in the medium, known as interference.
Constructive Interference: If the two waves are causing a displacement in the same direction, constructive interference occurs. That is an increase in particle motion. An example of constructive interference is the loudness produced when speakers overlap sound waves.
Destructive Interference: If the two waves are causing a displacement in the opposite direction, the result is a decrease in particle motion, which is destructive interference. The superposition of waves means two or more waves can occupy the same place at the same time. An example of destructive interference are the “dead” spots in an auditorium where sound waves cancel out each other.
Diffraction
Diffraction is when a wave spreads or bends as it moves around the edge of an obstacle. You can hear if someone is talking outside of a classroom because the sound wave from their speech goes inside through the window or the door by diffraction. Diffraction gratings contain a large number of parallel, closely spaced slits or grooves. When light waves pass through these slits they interfere to produce the diffraction pattern. Where crest meets crest we have constructive interference and where crest meets trough we have destructive interference. Ocean waves diffract around jetties and other obstacles. Sound waves can diffract around objects, which is why one can still hear someone calling even when hiding behind a tree. Closely spaced tracks on a CD or DVD act as a diffraction grating to form the familiar rainbow pattern seen when looking at a disk or the hologram on credit cards. Diffraction gratings are also used in classroom spectroscopes to produce a spectrum when viewing light emitted from a particular source.
Interference and diffraction are special motions for waves. Both the electromagnetic waves and mechanical waves have interference and diffraction behaviors.
Resonance
All objects have one or more frequencies, or resonant frequencies, at which they have a natural vibration, where the amplitude of oscillation is very large. The resonant frequency is an intrinsic property of an object to oscillate with greater amplitude at some frequencies than at others. Different objects own different resonant frequencies. Resonant systems can be used to generate vibrations of a specific frequency (e.g. musical instruments), or pick out specific frequencies from a complex vibration containing many frequencies (e.g. filters).
Doppler Effect
Whenever the source of a wave is moving in relationship with the observer, there is an apparent shift in the waves frequency. This phenomenon is known as the Doppler Effect. When you go to an airport, you can hear the sound from the planes flying overhead. If you stand in the same position, you may notice that the sound from the plane changes. This is the Doppler Effect, which is due to the motion of wave origin. A moving sound wave such as a train whistle, or light wave from as a distance stare takes slightly less time to reach an observer than the previous wave. The waves “bunch together” producing a higher pitched sound or higher frequency light. When the sound or light source moves away quickly the wave fronts are stretched out giving them a lower frequency. The Doppler Effect is used in astronomy to calculate the distance to stars by measuring the amount of a red shift (moving away) or blue shift (moving closer) of that starlight.