Unit 9 Section 4 Notes Wave Interactions

Reflection Reflection: the bouncing back of a wave as it meets a surface or boundary. Example: When light hits a shiny bumper it reflects back off of it.

Reflection When a boundary is free to move, the reflected wave is exactly like the original wave.

Reflection When a boundary is fixed (doesn’t move), the reflected wave is like the original, but turned upside down and travels in the opposite direction.

Diffraction Diffraction: the bending of a wave as it passes an edge or opening.

Diffraction Example: A person standing in a hallway is able to hear voices coming from a classroom because the sound waves bend as they pass the opening of the door.

Refraction Refraction: the bending of waves as they pass from 1 medium to another

Refraction Example: A spoon in a glass of water looks bent because of refraction. The light waves bend as they pass from the air to the water to your eyes. Each time the waves enter a new medium, they bend because of a change in wave speed. The waves reflected from the bottom of the spoon reach your eyes from a different angle than the waves at the top of the spoon.

Interference Interference: the combination of 2 or more waves that exist in the same place at the same time and result in a single wave

Interference Constructive Interference: any interference in which waves combine so that the resulting wave is larger than the original wave. Diagram of constructive interference:

Interference Destructive Interference: any interference in which waves combine so the resulting wave is smaller than the largest of the original wave. Diagram of destructive interference:

Interference of Light Waves Why do bubbles have a swirling rainbow effect? Some waves bounce off the outside of the bubble and travel directly to your eye, while other waves travel into the thin shell of the bubble and bounce off the inner side of the bubble’s shell, then back through the shell, air, and your eye. At times, the 2 sets of waves are out of phase with each other. They interfere constructively at some frequencies (colors) and destructively at other frequencies (colors).

Interference of Sound Waves When compressions from 2 sound sources arrive at your ear at the same time, constructive interference occurs and the sound is louder. A short time later, a compression from one source and a rarefaction from the other arrive together, destructive interference occurs and a softer sound is heard. You hear a pattern of alternating loud and soft sounds, called beats.

Standing Waves Standing Waves: a wave caused by interference that appears not to move along the medium and that shows some regions of no vibration (nodes) and other regions of maximum vibration (antinodes)

Standing Waves Nodes: Points that have no vibration They are located at points where crests of original waves meet the troughs of reflected waves Antinodes: Point that have maximum vibrations They are formed where crests of original waves line up with crests of reflected waves and constructive interference occurs

Standing Waves Diagram

Key Ideas What happens when a wave meets an object? What happens when a wave passes into another medium? When a wave meets a surface, the wave bounces back. That is called REFLECTION. When a wave passes the edge of an object or passes through an opening, the wave bends. That is called DIFFRACTION. A wave also bends when it passes from one medium to another at an angle. That is called REFRACTION.

Reflection, Diffraction & Reflection Interactive http://www.acoustics.salford.ac.uk/schools/teacher/lesson3/flash/whiteboardcomplete.swf