1. Light as a wave

2. Outline Waves transfer energy Types of waves Polarization
Chemistry
Physics
Applications
Creating electromagnetic waves
Challenges when you model light as a wave

3. Waves transfer energy
A disturbance in one part of the substance travels via collision or elasticity to other parts of the substance. After the pulse of energy passes, the parts of the substance return to their original position due to some sort of restoring force

4. Types of waves: longitudinal
Longitudinal waves: motion of particles is parallel to direction of energy
Pulse through a slinky
Sound

5. Types of waves: transverse
Transverse waves: motion of particles is perpendicular to direction of energy
Ripple on a pond
Wave on a string

6. Light: transverse or longitudinal?
How could we tell the difference?
Why would it make a difference?

7. Polarization analogy What type of waves does a picket fence stop?
Longitudinal waves
Transverse waves
Both
Neither

8. Polarizing light
If you heat and stretch it, the long chains of hydrocarbons that make up the plastic line up (~picket fence) The sheets are dyed with iodine to make them darker (and more effective at blocking light).
Polyvinyl acetate (PVA) is the main ingredient in slime.

9. Polarizing Light Demo 3: Demo 1: Demo 2:
Overlap several pieces of cheap cellophane tape in a fun pattern. Hold one polarizing filter behind the tape and one in front. View a bright light source through the filters and tape. Rotate the filters.
Demo 1:
Hold a polarizing filter about arm’s length. Look at a television, digital watch, computer screen, or calculator through a polarized filter. Rotate the filter.
Demo 2:
Look a bright light reflecting off a table through a polarizing filter. Rotate the filter.

10. Why polarized lenses?
If you have never paid much attention to the effect of polarized lenses, consider the following.

11. Polarization by reflection
When light reflects off a non-metallic surface, the reflected beam is (mostly) polarized in the plane parallel to the surface. To block glare from a lake, in what direction should chains of PVA be oriented?

12. Applications Sunglasses Stress analysis in plastics 3D movies
look at yourself in a mirror with 3-D polarized glasses: with your left eye you cannot see your right eye, and vice versa
Photography

13. Applications Catching prey Looking fine (or fierce)
Telling your hive mates where to find food

14. A problem with light as a wave
All the waves we know require a medium through which they transfer the energy.
We have seen that light’s behavior can be described if we think of light as a wave, but…
Through what medium does it travel?
How does energy from the sun travel through space?
If waves transfer energy through colliding particles, there must be colliding particles to collide.
Water molecules
String
Air

15. Early answers Hypothesis 2:
Light wave is new type of vibration
Does NOT involve physical particles vibrating around equilibrium positions
Does NOT have “restoring forces” acting upon it.
Hypothesis 1: Mechanical vibration that travels through a completely transparent, massless, and elastic medium. For fun, let’s call it luminiferous ether.

16. Hypothesis 1: Ether
It’s wrong. More about this later.

17. Hypothesis 2: EM waves Changing electric field  magnetic field
Changing magnetic field  electric field
Experiment with this applet:
For more details, check Doc Schuster

18. Propagation of waves In this example,
The electric field E varies on the z-axis
The magnetic field B varies on the x-axis
The direction of energy transfer is perpendicular to both the electric and magnetic field; here, down the y-axis

19. How fast does light go? <1638
~ infinite

20. How fast does light go? 1638
Using water clocks and candles, Galileo concluded that it at least 10 times faster than sound

21. How fast does light go? 1675
Measuring speed of transit of Jupiter’s moons, Ole Roemer concluded it was 200,000 km/sec.

22. How fast does light go? 1728
Measuring stellar aberration (apparent change in position of stars), James Bradley concluded it was 301,000 km/s.
How interesting is this story? SO INTERESTING!

23. How fast does light go? 1849
measuring when light is visible through teeth of rapidly rotating disk, Hippolyte Louis Fizeau concluded it was 313,300 km/s
How interesting is this story? SO INTERESTING! Here’s a spazzier version. Here’s a marshmallow version.

24. How fast does light go? 1862
Leon Foucault measured angle of light reflected off rapidly spinning mirror and refined the measurement to 299,796 km/s
Is this story also interesting? YES! It’s science.

25. How fast does light go?
Today: Speed of light, c, is defined as ≡ 299,792,458 m/s
Meters are defined in terms of the distance light travels in one second
A second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom

26. Speed of electromagnetic waves
𝐵 ∙𝑑𝑙= 𝜇 0 𝐽 ∙𝑑𝑆+ 𝜇 0 𝜀 0 𝑑 𝑑𝑡 𝐸 ∙𝑑𝑆
You may not have quite enough math yet to recognize that the term in front of the time-dependent electric field is the inverse of the square of the velocity of the wave.
With exciting manipulation of Maxwell’s equations, we find 𝑣= 1  0  0
𝑣= 1 (8.85 𝑥 10 −12 𝐶 2 𝑁 𝑚 2 )(4𝜋 𝑥 10 −7 𝑁 𝐴 2 ) or 𝑣=3.00 𝑥 𝑚 𝑠

27. Light is an electromagnetic wave!
𝑣=𝑓

28. Applications Communicating at speed of light Radar
Fun fact: You can calculate the speed of light in a microwave!
Find the frequency (required by law to be listed on the microwave)
Sprinkle a light layer of cheese or marshmallows on a microwave safe plate
Cook at low power without rotating the plate.
Measure the distance between bands of melted cheese (wavelength)
Calculate speed: 𝑣=𝑓
Communicating at speed of light
Sight
Radio
Phone
Radar
Global positioning system
Microwaves

29. For more information
Doc Physics re: Maxwell’s Equations: Physics Classroom re: polarization: 1/Polarization Olympus microscopy re: polarization