Properties of Waves

The Ripple Tank

• To study waves, we will use a ripple tank. A ripple tank works by having a wave generating probe produce a continuous wave in the water. The ripple tank has a glass bottom that allows light to be projected down through the water and the glass onto a screen below the table

Rectilinear Propagation

Rectilinear Propagation for Straight Waves

• Straight waves are produced by periodically dipping a long straight edge probe into the water.

• For straight waves, the path of two points show that the direction of propagation of the advancing straight wave is perpendicular to the wave front.

Rectilinear Propagation for Circular Waves

• Circular waves are created by periodically dipping a pointed probe into the water.

• The direction of propagation of the two segments of the advancing circular wave lie along radial lines away from the center of disturbance. (See diagram on chalkboard).

Reflection

• Some examples of reflection are echoes from walls, reflection of light off a mirror, and the reflection of water waves from the edge of a pool.

When does reflection occur?

• A wave is turned back, or reflected, when it encounters a barrier that is the boundary of the medium in which the wave is traveling.

Reflection of Straight Waves

• Case 1: The wave fronts are parallel to the boundary.

• In this case, both the direction of propagation of the incident waves and the reflected waves will lie perpendicular to the surface of the barrier. See diagram on board.

Straight Waves continued

• Case 2: The barrier is turned at an angle to the incident waves

The Law of Reflection

• The Law of Reflection states that when a wave disturbance is reflected at the boundary of a transmitting medium, the angle of incidence (Θi) will equal the angle of reflection (Θr).

Partial and Complete Reflection

• Reflection may be partial or complete depending on the nature of the reflecting boundary. If there is no boundary mechanism for extracting energy from the wave, all of the energy incident with the wave is reflected back.

Free end termination

• A wave crest is reflected back as a wave crest and a trough as a trough during free end termination reflection. This occurs when the reflecting medium is less dense than the medium in which the waves are traveling.

Free end termination diagram

Longitudinal Waves

• Longitudinal waves reflect from a less dense material with a change in phase. That is, if sound traveling in a solid reflects from air back into the solid, a compression will reflect as a rarefaction and a rarefaction as a compression.

Fixed end termination reflection

• Reflection from a fixed end occurs with a 180 degree phase shift. That is, a crest will reflect back as a trough and a trough as a crest. This occurs when the reflecting medium is more dense than the medium in which the wave is traveling.

Fixed end reflection diagram

Longitudinal Waves

• Longitudinal waves reflect from a more dense material without a change in phase. That is, if sound traveling in air reflects from a solid back into the air, a compression will reflect as a compression and a rarefaction as a rarefaction.

Refraction

• The properties of the medium through which a certain wave disturbance travels determines the propagation speed of that disturbance.

• Traveling waves passing from one medium into another experience a change of speed at the interface (boundary) of the two materials.

Waves traveling from deep to shallow water

Waves traveling from deep to shallow water at an angle

Diffraction

• Diffraction is the spreading of a wave disturbance beyond the edge of a barrier.

Fixed Aperture

• When waves of different wavelengths encounter a fixed aperture (opening) size, the extent to which diffraction occurs varies. See diagram on board.

• Conclusion: For a fixed aperture, as wavelength increases, diffraction also increases.

Fixed Wavelength

• When waves of a fixed wavelength encounter different aperture sizes, the extent to which the waves diffract varies. See diagram on board.

• Conclusion: For waves of a fixed wavelength, as aperture size increases, diffraction decreases.

Superposition Principle

• When two or more waves travel simultaneously through the same medium, (1) Each wave proceeds independently as though no other waves were present and (2) the resultant displacement of any particle at a given time is the vector sum of the displacement that the individual waves acting alone would give.

Interference

• The term interference is used to describe the effects produced by two or more waves that superpose while passing through a given region.

Constructive Interference

• When two waves are added together and the resulting displacement for a given particle is greater than it had been in either of the waves being added, the waves are said to interfere constructively.

Constructive Interference

Destructive Interference

• When two waves are added together and the resulting displacement for a given particle is in between the waves being added, the waves are said to interfere destructively.

Destructive Interference