Part 1: Waves & Wave Properties

Hilo, Hawaii, 1960, 6700 miles from an Earthquake in Chile How did an Earthquake in Chile cause such damage in Hawaii? Was energy involved?

Movement of Energy Waves make it possible for energy to get from one place to another even when the source of the energy is far away.

Wave Definition: A disturbance that transfers energy from place to place. What carries waves? A medium, a medium is the material through which a wave travels. A medium can be a gas, liquid, or solid. What’s the medium for sound?

Not all waves require a medium to travel. Electromagnetic waves do not require a medium Light (an EM wave) from the sun travels through empty space.

What causes waves? Waves are created when a source of energy causes a medium to vibrate. A vibration is a repeated back and forth or up and down motion.

Types of waves: Waves are classified according to how they move.

Transverse wave Waves that move the medium at right angles to the direction in which the waves are traveling is called a transverse wave. Transverse means across.The highest parts are called crests the lowest parts are called troughs.

Compressional Wave or Longitudinal Wave Matter vibrates in the same direction as the wave travels. Check out a “slinky” it can show both kinds of waves

Basic Properties of Waves Amplitude Wavelength Frequency Speed

Frequency The number of complete waves that pass a given point in a certain amount of time. AKA number of vibrations per second. Frequency measured in hertz (Hz).

Wave Period Easy to confuse with frequency The period of a wave is the time for one complete wave cycle to complete It is measured in units of time such as seconds, hours, days, or even years. What is the period for each Earth rotation around the Sun?

Frequency vs Period Frequency is cycles/second (Hertz) Period is seconds (or mins or hours)/cycle Let’s think about this… A woodpecker smacks a tree 2 times/second, the freq is 2 Hz. Each smack must last ½ second, so the period is 0.5 s

Frequency vs Period If the woodpecker smacks a tree 4 times in one second, what is the freq? What would be the period?

Mathematically Of course math is involved. F is used for Freq. & T is used for period

Try this Question The Sears building in Chicago sways back and forth at frequency of about 0.1 Hz. What is its period of Vibration? F = 0.1 Hz Period = 1/f so P = 1/0.1

Try This Question The Sears building in Chicago sways back and forth at frequency of about 0.1 Hz. What is its period of Vibration? Period = 1/f so P = 1/0.1 = 10 seconds So each back and forth or crest to crest movement takes 10 seconds

Speed The speed, wavelength, and frequency of a wave are related to each other by a mathematical formula. Speed = wavelength x frequency Frequency = speed/wavelength Wavelength = speed/frequency

Speed Waves in different mediums travel at different speeds. Sound travels about 340 m/s in air and about 1360 m/s in water.

Wave Speed How do we figure out speed. We can do some easy math The formula for wave speed… In words: Wave speed = wavelength x frequency In symbols: ν = גּ (f)

Wave Speed practice A wave moving through the Earth has a frequency, f, of 200Hz and it has a wavelength of 20 meters. What is the speed of this wave? A wave has an f of 2Hz and a wavelength of 3m, what is the speed of this wave? The speed of a wave is 343 m/s, with a wavelength of 1.30 m, what is the frequency?

Answers V = 200 Hz (1/s) X 20 m = 4000 m/s V = 2 Hz (1/s) X 3 m = 6 m/s F = v/ גּ so f = 343 m/s / 1.3 m = 264 Hz

Speed of Sound,Light & Doppler Effect Part II

The Medium Determines the Speed What is the medium for sound? How fast do sound waves move through its medium? Sound moves 340 m/sec in air but can actually travel 4 times faster in water. They can move 15 to 20 times faster in a solid like steel – could you hear a train coming when it is far away by listening to the track?

Wave Speed and Medium The speed of the wave is determined by the medium

Kinetic Theory - Gases The gas molecules are far apart So the sound wave will move each molecule a lot before they touch another molecule Sound waves move slower in a gas

Kinetic Theory - Liquids The liquid molecules are closer together Sound waves move faster in a liquid

Kinetic Theory - Solids The solid molecules are very close together and bound to each other Sound waves travel very quickly in solids

Speed of Light Light does have a speed limit. The speed of light is 3 X 10 8 m/s This is very fast – miles in 1 second! The speed of light is a constant and is represented by the symbol “c”

The Doppler Effect As a jet flys past, the sound of the jet changes from a high pitch to a low pitch The pitch is determined by the frequency of the sound waves hitting your ear The higher the frequency the higher the pitch

The Doppler Effect If the jet wasn't moving and you could see the sound waves they would look like the picture on to the right.

The Doppler Effect If the jet was moving just below the speed of sound. You would hear the pitch get higher as the jet approaches and then get much lower as it passes.

The Doppler Effect The jet at supersonic speed As it approaches you, you would hear nothing As it passes you will hear a loud boom

Faster than Sound on Land

Wave Interactions, Part III Reflection and Refraction

Two Types of Interactions 1. Reflection (Two types) 2. Refraction

Reflection – Type 1 Reflection at a free boundary. The wave bounces off the boundary and goes off in the opposite direction.

Reflection – Type 2 Reflection at a fixed boundary. Like hitting a wall – the wave goes back in the opposite direction but is turned upside down.

Refraction Refraction is the bending of light caused by the wave moving from one medium to another. Example: the wave moves from water to air

Interference What happens when two waves try to occupy the same space. Two types of Interference... Constructive Interference Destructive Interference

Constructive Interference When one crest overlaps another crest.

Destructive Interference When the crest of one wave meets the trough of another

Interference & Light