Reflection of Waves Page 18
Waves in a coiled spring or a rope are reflected from a fixed end. Ocean waves crashing onto a rocky cliff are reflected away from the cliff. Sound waves reflected from a concrete wall are heard as echoes. Radio waves from a communications satellite are reflected to the focal point of a parabolic antenna on Earth. Laws govern the way in which waves, both visible and invisible, are reflected.
When waves run into a straight barrier, as illustrated, they are reflected back along their original path.
If a wave hits a straight barrier obliquely, the wavefront is also reflected at an angle to the barrier. The angles formed by the incident wave front and the barrier and by the reflected wave front and the barrier are equal. These angles are called the angle of and the angle of , respectively.
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In neither case does the reflection produce any change in the wavelength or in the speed of the wave.
Sometimes we refer to wave rays instead of to wavefronts, in describing the behaviour of waves. Wave rays are simply straight lines perpendicular to wavefronts indicating the direction of transmission.
When describing the reflection of waves, using wave rays, the angles of incidence and reflection are measured relative to a straight line perpendicular to the barrier, called the normal. This line is constructed at the point where the incident wave ray strikes the reflecting surface. The angle of incidence (θi) equals the angle of reflection (θr). This is one of the laws of reflection.
θi = θr
What about when a wave strikes a curved parabolic surface? In this case straight waves are reflected by a parabolic reflector to one point, called the focal point. This could have been predicted by means of the laws of reflection and wave rays.
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Diffraction of Waves
Many city waterfronts are protected by breakwaters-concrete obstacles that reflect the wave energy away from the shoreline, protecting the beach and harbour. In the breakwater there are openings to permit boats to enter. Some of the energy being transmitted by the waves passes through these openings to the region behind the breakwater. The bending of the waves that occurs as they pass through such small openings is called diffraction.
Water waves are diffracted when they pass by the edge of a barrier or through a small opening in a barrier. How much they are diffracted depends mainly on their wavelength. Shorter wavelengths are diffracted slightly. Longer wavelengths are diffracted to a greater extent.
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As a general rule, diffraction is increased when the size of an opening is decreased. If the wavelength remains constant, diffraction becomes more apparent when the opening is decreased to approximately the same dimension as the wavelength.
Questions
Go to the following webpage to answer these questions:
Before you begin, make the follwing changes to the setup of the Ripple Tank:
- Setup: Plane Wave
- Leave 1 Plane Source, 1 Freq the same
- Mouse=Edit Walls
- Colour Scheme 3
- Turn Simulation Speed to the left
For each of the following, create a wall then click Clear Walls before moving on to the next step.
1. Create a Flat Wall Parallel to the Waves.
Wave Heading
Wall
When a straight wave strikes a barrier so that its wavefront is parallel to the barrier, in what direction is the wave reflected?
2. Create a Flat Wall at an angle to the waves.
Wave Heading
When an incident wavestrikes a barrier at an angle (that is, obliquely), how do the angles between the barrier and the incident and reflected waves compare?
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3. Create a wall shaped like a parabola.
Wave Heading
Wall
How are straight waves reflected by a parabolic reflector?
4. Create Two flat walls with an opening between them.
Wave Heading
Walls
What kind of wavelengths are diffracted more long ones or short ones? (Try changing the source frequency to adjust the wavelength.)
5. Try it with a large opening and a small opening.
What is the relationship between the size of the opening and the amount of diffraction?