Wave Behaviour Lesson 4 Jamie Nielsen

Boundary Behavior The behavior of a wave when it reaches the end of its medium is called the wave’s BOUNDARY BEHAVIOR. When one medium ends and another begins, that is called a boundary.

Fixed End One type of boundary that a wave may encounter is that it may be attached to a fixed end. In this case, the end of the medium will not be able to move. What is going to happen if a wave pulse goes down this string and encounters the fixed end?

Fixed End Here the incident pulse is an upward pulse. The reflected pulse is upside-down. It is inverted. The reflected pulse has the same speed, wavelength, and amplitude as the incident pulse.

Free End Another boundary type is when a wave’s medium is attached to a stationary object as a free end. In this situation, the end of the medium is allowed to slide up and down. What would happen in this case?

Free End Here the reflected pulse is not inverted. It is identical to the incident pulse, except it is moving in the opposite direction. The speed, wavelength, and amplitude are the same as the incident pulse.

Change in Medium Our third boundary condition is when the medium of a wave changes. Think of a thin rope attached to a thick rope. The point where the two ropes are attached is the boundary. At this point, a wave pulse will transfer from one medium to another. What will happen here?

Change in Medium In this situation part of the wave is reflected, and part of the wave is transmitted. Part of the wave energy is transferred to the more dense medium, and part is reflected. The transmitted pulse is upright, while the reflected pulse is inverted.

Change in Medium The speed and wavelength of the reflected wave remain the same, but the amplitude decreases. The speed, wavelength, and amplitude of the transmitted pulse are all smaller than in the incident pulse.

All we have left to discover is how waves interact with each other. Wave Interaction All we have left to discover is how waves interact with each other. When two waves meet while traveling along the same medium it is called INTERFERENCE.

Constructive Interference Let’s consider two waves moving towards each other, both having a positive upward amplitude. What will happen when they meet?

Constructive Interference They will ADD together to produce a greater amplitude. This is known as CONSTRUCTIVE INTERFERENCE.

Destructive Interference Now let’s consider the opposite, two waves moving towards each other, one having a positive (upward) and one a negative (downward) amplitude. What will happen when they meet?

Destructive Interference This time when they add together they will produce a smaller amplitude. This is know as DESTRUCTIVE INTERFERENCE.

Noise-canceling headphones utilize destructive interference

Reflection Reflection occurs when a wave strikes an object and bounces off of it. All types of wavesincluding sound, water, and light wavescan be reflected

Reflection When a wave bounces off a barrier or surface it is said to be reflected. How much of the wave that is reflected depends on the impedance between the two mediums. Impedance describes the difference in densities of the two mediums, the greater the difference in mediums, the greater the impedance. As the impedance increases, more of the wave's energy is reflected instead of being transmitted into the next medium.

Law of Reflection The angle formed by the incident beam and the normal is the angle of incidence. The angle formed by the reflected beam and the normal is the angle of refection. According to the law of reflection, the angle of incidence is equal to the angle of refection. All reflected waves obey this law.

Types of Reflection

Refraction When a wave passes from one medium to anothersuch as when a light wave passes from air to waterit changes speed. If the wave is traveling at an angle when it passes from one medium to another, it changes direction, or bends, as it changes speed. Refraction is the bending of a wave caused by a change in its speed as it moves from one medium to another.

Sound refraction Sound wave will refract if there is a difference in air temperature. The different densities cause the waves to bend.

Diffraction Diffraction occurs when an object causes a wave to change direction and bend around it. Diffraction and refraction both cause waves to bend. The difference is that refraction occurs when waves pass through an object, while diffraction occurs when waves pass around an object.

Diffraction Diffraction is the bending of waves around obstacles or the edges of an opening in the same medium. It can be explained using Huygens’s principle. The amount of diffraction depends on the nature of waves and their wavelength. Sound waves diffract much more than light waves. Low-frequency (high-wavelength) sound waves diffract more than high-frequency (low-wavelength) light waves.

Diffraction in Speakers Small-diameter speakers, called tweeters, are used to produce high-frequency sound. The small diameter helps to promote a wider dispersion of the sound. The amount of bending is depends on the ratio: λ/w, λ is the wavelength and w is the width of the opening.