Quick Review from Grade 11 Waves Quick Review from Grade 11

What is a wave (in physics)? Wave: a disturbance that transfers energy Whether ocean waves, waves on a string, earthquake waves, or sound waves in air, they all have a vibration as their source. Waves move large distances across the surface of a lake or the sea, yet the water itself does not travel those distances. Only the ENERGY!

Mechanical Waves Mechanical waves require a medium in order to transport their energy from one location to another. Example: Sound waves

Electromagnetic Waves capable of transmitting its energy through a vacuum (i.e., empty space). produced by the vibration of charged particles. Example: Light waves** Light travels through space (no medium) from the sun to Earth

How are waves produced? Waves are produced by a wave pulse (a force). A wave pulse is a disturbance

Wave Terminology (Transverse Waves)

Key Terms for Waves Frequency - the number of complete waves produced per second. Symbol = f Units = Hertz (Hz) 1 Hz = 1/s Formula: where N is number of cycles, t is time

Key Terms for Waves Wavelength - the shortest distance between two repeated points in a wave. Symbol = λ (lamda) Unit = metre (m)

Key Terms for Waves Amplitude - the greatest displacement of the wave away from its undisturbed position. (height of a hump) Symbol: a

Wave speed (v) = frequency (f) x wavelength () The Wave Equation The wave equation relates the speed of the wave to its frequency and wavelength: Waves travel at constant velocities so you can also use the v = d/t equation. Wave speed (v) = frequency (f) x wavelength () in m/s in Hz in m

Wave Speed Example 1 A spring vibrates at 2.8 Hz. This produces a wave with wavelength 36cm. Calculate the speed of the wave. Answer: v = fλ v = (2.8)(0.36) = 1.008 m/s = 1.0m/s

Wave Interference when 2 waves pass through the same region of space at the same time

Types of Interference Constructive Interference (in phase) When 2 crests arrive at the same time (or 2 troughs) OR at any point where the waves “add” together to make a larger wave (temporary)

Constructive Interference

Constructive Interference

Constructive Interference

Types of Interference Destructive Interference (out of phase) When 1 crest and 1 trough arrive at the same time and cancel each other out if they are identical in size (temporarily). Partially destructive waves may not cancel each other but dampen the wave temporarily.

Destructive Interference

Constructive Destructive Partially Destructive

In Phase/Out of Phase Example If I drop two rocks in water and they produce waves with the same frequency, there are some places where the waves arrive in phase. This means that two crests arrive at the same time. The two crests move the water upwards to make a larger wave. In other places, the waves will arrive out of phase. When a crest arrives from one rock, a trough arrives from the other rock. This time, the effect of the waves cancel each other out. The water does not move at all.

Destructive Interference

Partially Destructive Interference

http://www.youtube.com/watch?v=Xqo6sEt1cUE Cool example of interference.

Quick Quiz 1. Several positions along the medium are labeled with a letter. Categorize each labeled position along the medium as being a position where either constructive or destructive interference occurs.

Answers Constructive Interference: G, J, M and N Destructive Interference: H, I, K, L, and O

Principle of Superposition In the region where the waves overlap, the result and displacement is the algebraic sum of their separate displacements. NOTE: A crest is positive and a trough is negative! NOTE: Waves do not change shape or size when they meet. They can pass through each other. However, when they overlap, a different shaped wave emerges temporarily!

Example of Superposition If one wave has an amplitude of +3cm and the other wave has an amplitude of -6cm, the resultant displacement is … -3cm. http://www.acs.psu.edu/drussell/Demos/superposition/superposition.html

Example 2 of Superposition If one wave has an amplitude of +3cm and the other wave has an amplitude of +6cm, the resultant displacement is … +9cm. See page 355 for diagrams!

Superposition of Waves