1. Waves, Light, and the EM Spectrum
What is a wave?
What are the main properties of waves?
What two things do all waves transport?

2. Waves are a type of disturbance that can propagate or travel.
Waves carry information and energy.
Properties of a wave
wavelength ()
crest
amplitude (A)
trough
velocity (v)
Period (T): time between crest (or trough) passages
Frequency (f): rate of passage of crests (or troughs), f
wave speed = wavelength/period = wavelength * frequency 1 T
(units: Hertz or cycles/sec)
(v = f)

3. Types of Waves Longitudinal Waves Transverse Waves
Waves of compression
Disturbance propagates along direction of travel
Sound waves
Transverse Waves
Disturbance is perpendicular to the direction of travel
Water waves, light
How does the medium that supports a wave move?
(Shockwave Demo) (Web Link)

4. Sound Waves
Caused by alternating “compressions” and “rarefactions” in some medium, usually air.
Frequency of waves same as frequency of source
Heard as pitch or tone
Human ear ~ 20 to 20,000 hertz
Loudness determined by amplitude of waves
Requires a medium to propagate (Demo)

5. Resonance
Any elastic object will vibrate at its own set of frequencies when disturbed
Called natural frequencies
Determined by elasticity and shape
Bells, violin strings, idling cars
Resonance
Dramatic increase in amplitude when frequency of forced vibrations matches natural frequency of object
Pumping a swing
Resonance occurs when compressions
and rarefactions are timed to the
natural frequency of the tuning fork.
(Tacoma Narrows Bridge Video)
(Web Link)

6. Interference Constructive interference Destructive interference
When the peaks of two waves coincide
Amplitude increases
Ex. Sonic Boom
Destructive interference
When the peak of one wave “fills in” the trough of another
Waves cancel
Ex. Noise Cancellation

7. Waves bend when they pass through material of different densities.
Refraction
Waves bend when they pass through material of different densities.
air
water
swimming pool
prism
glass
air
air

8. All radiation (including visible light) travels as electromagnetic waves.
That is, waves of electric and magnetic fields travelling together.
What are some examples of objects with magnetic fields:
What are some examples of objects with electric fields:

10. Electric Force - opposites attract, likes repel
Oscillating charges radiate
All objects have temperatures greater than absolute zero - random thermal motion
All objects radiate!
Why don’t we see the radiation coming from many ordinary objects?
Dog whistle analogy

11. Electromagnetic Radiation
(How we get information about the cosmos)
What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them?

12. Electromagnetic Radiation
(How we get information about the cosmos)
What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them?
Light (see)
Infrared (heat)
Ultraviolet (sunburn)
Microwaves (cooking, communication)
AM radio (communication)
FM radio (communication)
TV signals (communication)
Cell phone signals (communication)
X-rays (medial applications)

13. Bigger  means smaller f
The speed of all electromagnetic waves is equal to the speed of light.
c = 186,000 miles / s
or c = 300,000 km / s
light takes 8 minutes
Sun
Earth
c =
f
Bigger  means smaller f
A changing electric field creates or induces a magnetic field and vice-versa!
(Shockwave Demo) (Web Link)

14. Constancy of the Speed of Light
What physical principle requires that the speed of EM radiation be constant in vacuum?

15. Constancy of the Speed of Light
What physical principle requires that the speed of EM radiation be constant in vacuum?
Conservation of energy
If light were to slow down, the changing electric field would generate a weaker magnetic field and so on...
Light wave would die out
If light were sped up, then energy of light would increase indefinitely

16. The Electromagnetic Spectrum
1 nm = m , 1 Angstrom = m
c =
f

17. The Doppler Effect
How does the pitch or tone of a sound wave change when the source of the sound is moving towards or away from you?
What about when you are moving towards or away from the source?
Does this effect occur for all types of waves or just for sound waves?

18. The frequency or wavelength of a wave depends on the relative motion of the source and the observer. (Shockwave Demo) (Web Link)
For visible light, the frequency (or wavelength) determines its color.