Unit 9 – Introduction to Waves
Unit Big Idea: Waves transfer energy and interact in predictable ways.
Lesson 1 – Waves
Essential Question: What are waves?
By the end of this lesson, you should be able to distinguish between types of waves based on medium and
direction of motion.
- wave – disturbance that transfers energy form one place to another
- examples: water, light, sound
- transfers energy only – not matter
- crest – highest point of the wave
- trough – lowest point of the wave
- medium – material through which a wave travels
- once the wave has travelled through it, disturbing it, the material returns to its original place
- most waves need a medium to pass through
- light waves can travel through space & don’t need a medium
- rarefaction - spreading out of the wave
- compression – pushing together of the wave
- longitudinal wave – particles move back and forth in the same direction that the wave travels, or
parallel to the wave
- ex: sound waves
- transverse wave – particles move perpendicular to the direction the wave travel – up & down
- ex: fans doing the “wave” in the stands at a ball game
- surface waves (Rayleigh waves) – combination of transverse and longitudinal waves
- ex: ripples on a pond
- Earthquakes have all three types of waves
- P waves are longitudinal waves that travel the fastest
- S waves are transverse waves that travel slower but do more damage
- mechanical wave – waves that must have a medium to travel through
- ex: water waves travel through water; earthquake waves travel through the earth
- sound waves travel through more than one medium – air, water, & solid objects like a wall
- waves travel at different speeds in different mediums
- if there is no medium for a mechanical wave to travel through, it will not go anywhere
- electromagnetic (EM) wave - disturbance in electric and magnetic fields
- they are transverse waves and don’t need a medium to travel through
- ex: light travels through space & space is a vacuum with no medium
- all EM waves travel through space at the same speed – the speed of light = 300,000,000 m/sec
- examples from longest wavelength to shortest wavelength: radio, microwave, infrared, visible
light, ultraviolet, x-ray, gamma ray
Lesson 2 – Properties of Waves
Essential Question: How can we describe a wave?
By the end of this lesson, you should be able to identify characteristics of a wave and describe wave
behavior.
- wave – disturbance that transfers energy from one place to another but does not transfer matter
- move up and down or back and forth
- amplitude – measure of how far the particles in a medium move away from their normal rest position
- how high up the crest above normal or height of wave
- wavelength – distance from any point on a wave to an identical point on the next wave
- how long the wave is
- from one crest to the next crest; from one trough to the next trough
- measures the length of the cycle or repetition
- wave period (or just period) – time it takes for one cycle or wavelength
- how long it takes one full wave to go past a given point
- frequency – how many cycles occur in a certain amount of time – usually 1 second (s)
- hertz (Hz) is the unit of frequency
- 1 Hz = 1 cycle / second
Amplitude or Frequency
- for mechanical waves, amplitude is related to the amount of energy the wave carries
- for two similar waves, waves with greater amplitude have more energy
- greater frequency can also mean greater energy in a given amount of time
- the more times a wave hits a spot in the same time frame the more energy is transferred to that spot
- ex: 9 waves hitting the side of a boat in 2 minutes carries more energy than 3 waves hitting the
side of a boat in 2 minutes
- for electromagnetic (EM) waves, energy is more strongly related to frequency
- very high frequency EM waves, like x-rays and gamma rays, carry enough energy to damage the human
body – they can also travel much further through different mediums
- lower frequency EM waves, like visible light, can be absorbed safely in the human body
- a medium, like rock, may not transfer all of a wave’s energy
- a medium may warm up, shift, or charge in other ways and use up some of the wave’s energy
- as a wave travels through more of a medium, it loses more of its energy
- many times, high frequency waves lose energy faster than low frequency waves
- when you stand very far away from a musical instrument, you may only hear the low frequency
waves
- waves spread out in more than one dimension
- wavefronts - crests of waves that are drawn as shapes like circles or spheres
- as the wave moves from the source, it spreads out over a greater area and loses energy
- the total amount of energy for the wave is the same but because it’s spread out over a much larger
are, it appears less at any one point
- sound waves expand in three dimension
- ripples on the surface of water expand in two dimensions
- waves travel at different speeds in different media
- sound waves travel at about 340 m/s in air at room temperature
- sound waves travel at 1,500 m/s in water
- sound waves travel even faster in solids than in water
- wave speed – speed waves travel at; it depends on the properties of the medium it’s traveling through
- waves travel fastest in solids and slowest through gases
- particle interaction (collisions) happens faster in solids than in liquids or gases because the
particles are closer together in solids than in liquids or gases
- wave speed also depends on the density of the medium
- waves travel slower through more dense solids than less dense solids because the
particles are more closely packed together and resist moving – it is the same as for
liquids
- in gases, wave speed depends on temperature and density
- waves travel faster in hot air than in cool air since the particles collide with each other
more in hot air and transfer the wave energy
- speed of sound in air at 20 °C is 340 m/s; speed of sound in air at 0 °C is 330 m/s
- electromagnetic (EM) waves don’t need a medium, they can travel through a vacuum
- all EM waves travel through empty space at the same speed
- speed of light – speed all EM waves move through empty space – 300,000,000 m/s
- EM waves travel more slowly through solids and liquids than through a vacuum
-
calculate the wave speed from its frequency and wavelength
the crest of a wave moves a distance of one wavelength in one cycle
the time for the cycle happens in one period
wavelength becomes distance and wave period becomes time
- frequency is the inverse of the wave period:
wave speed = frequency x wavelength