Behavior of Waves

Key Words

-Antinode -Constructive Interference -Destructive Interference -Diffraction -Interference -Node -Reflection -Refraction -Standing Wave

Objectives

-Describe how reflection, refraction, diffraction, and interference affect waves -State a rule that explains refraction of a wave as it passes from one medium to another -Identify factors that affect the amount of refraction, diffraction, interference -Distinguish between constructive and destructive interference and explain how standing waves form

What is meant by a reflection in a wave?

A reflection occurs when a wave hits something and bounces back upon itself at the same speed.

Why does a standing wave happen only at particular frequencies?

A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord.

Antinode

An antinode is a point where a crest or trough occurs midway between two nodes.

Standing Wave

At certain frequencies, interference between a wave and its reflection can produce a standing wave. A standing wave is a wave that appears to stay in one place--it does not seem to move through the medium.

Constructive Interference

Constructive interference occurs when two or more waves combine to produce a wave with a larger displacement. The crests of waves 1 and 2 combine to make a higher crest in wave 3. At the point where two troughs meet, wave 3 has a lower trough.

Destructive Interference

Destructive interference occurs when two or more waves combine to produce a wave with a smaller displacements. Two waves with the same frequency could meet, but this time the crest of wave 1 meets the trough of wve 2. The resulting wave 3 has a crest at this point, but it is lwoer than the crest of wave 1. Destructive interference produces a wave with a reduced amplitude.

Diffraction

Diffraction is the bending of a wave as it moves around an obstacle or passes through a narrow opening. The pattern produce is very similar to the circular ripples you see when a pebble is tossed into a pond. Diffraction also occurs when waves bend around an obstacle. A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle. If the wavelength is small compared to the opening or obstacle, the wave bends very little. The larger the wavelength is compared to the size to the opening or obstacle, the more the wave diffracts.

Describe how a wave can change direction.

If a wave hits an obstacle, part of the wave may continue at the same speed, and part of the wave may slow down. This causes the wave to bend in a certain direction.

Interference

Interference occurs when two or more waves overlap and combine together. Two types of interference are constructive interference and destructive interference. The displacement of waves combine to increase amplitude in constructive interference and to decrease amplitude in destructive interference.

Node

Only certain points on the wave, called nodes, are stationary. A node is a point on a standing wave that has no displacement from the rest position. At the nodes, there is complete destructive interference between the incoming and reflected waves.

Reflection

Reflection is a property common to all waves. Reflection occurs when a wave hits an obstacle and bounces back at the same speed. Note that the obstacle will not be another wave, since waves can pass through each other without incident. The angle at which the wave hits the obstacle is called the angle of incidence. The wave will reflect back at the exact same angle, called the angle of reflection. When reflection occurs, the speed of the wave does not change, and the wave essentially flips over on itself. Reflected waves add to the original waves to form patterns. When waves hit a parabola, they reflect at various angles, making the waves focus at the same point in the center of the parabola. Reflection does not change the speed or frequency of a wave, but the wave can be flipped upside down. If reflection occurs at a fixe boundary, then the reflected wave will be upside down compared to the original wave.

Refraction

Refraction occurs when a wave enters a new medium at an angle and either speeds up or slows down. For example, when light passes through a glass of water, the light will slow down as it passes from the air into the water. Light travels slower in the denser water, and the light waves bend, or refract. Refraction causes waves to change direction. For example, if an ocean wave hits shallow water at the shore, the wave begins to bunch up. If the wave hits the shallow water at an angle, the part of the wave that hits first will slow down and then drop behind. This makes the wave swing around and change direction. When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side.

What is the angle of incidence equal to?

The angle of reflection.

What happens when a wave hits a parabola?

The wave focuses at a point.

What is meant by refraction in a wave?

This is when a wave changes speed and either bunches up or spreads out. The wave may or may not change direction.