1. Chapter 7: Light
Figure 7.8
Good APCs: 2, 3, 5, 6, 9, 10, 14, 18, 19, 20, 22, 23, 25, 28, 29, 30, 31, 42, 45, 46, and 48

2. Electromagnetic Wave Electromagnetic Radiation
Both “fields” vary together
Transverse Waves.
Fig 7.2
Regenerating co-oscillation of electric and magnetic fields
Transverse waves
Pulse of electric and magnetic fields that are locked together
Moving through space (including a vacuum – which sound cannot do)
As they exchange energy back and forth, regenerating each other in an endless cycle until something absorbs it
Electric and magnetic fields vary together
Mutually perpendicular to each other and the direction of the wave

3. Electromagnetic (EM) Spectrum
Fig 7.3
Range of Frequencies (or Wavelengths)
Visible Light only a small portion.
Electromagnetic (EM) Spectrum
Many aspects of their behavior depend upon their frequency
Frequency and wavelength are related
Most concerned with the visible portion of the spectrum in this lesson

4. Sources of Light Matter constantly emits and absorbs radiation
Emission mechanism:
Accelerated, oscillating “charges” produce EM Waves
Different accelerations lead to different frequencies
Key Terms: Luminous and Incandescent.
Matter constantly emits and absorbs radiation
Accelerations and oscillations of charges within matter
Different accelerations lead to different frequencies
Greater the accelerations, the greater the frequency
Luminous
Producing light
The Sun versus the non-luminous Moon
Incandescent
Glowing with visible light from high temperatures (accelerating electrons)
Examples: flames, incandescent light bulbs

5. Blackbody Radiation “Blackbody” Increasing Temperature results in?...
Fig 7.5
Fig 7.4
“Blackbody”
Idealized material
Perfect emitter ; perfect absorber
Increasing Temperature results in?...
Spectrum of Sun light.
Blackbody”: Idealized material that is a perfect emitter and a perfect absorber of Electromagnetic radiation
Increasing Temperature
Amount of radiation increases (T to the fourth power ; I = σT4 )
Peak radiation emitted progressively shifts towards higher and higher frequencies (hence, shorter and shorter wavelengths)
(λ is inversely proportional to Temperature)
Spectrum of Sun light
Mostly Visible and Near IR Radiation

6. Properties of Light: Two “Models”
“Light Ray” Model
Particle-like view
Photons travel in straight lines
Best explains
Mirrors
Prisms
Lenses
“Wave” Model
Traces motions of wave fronts
Best explains
Interference
Diffraction
Polarization

7. Light interacts with matter
Interaction begins at the surface and depends upon?
Possible Interactions?
Transparent vs Opaque?
Fig 7.6
Interaction begins at surface and depends on:
Smoothness of surface
Nature of the material
Angle of incidence
Possible interactions
Absorption
Reflection (and Scattering)
Refraction
Transmission
Transparent versus Opaque

8. Reflection
Angles measured with respect to the “surface normal”
Line perpendicular to the surface
Law of Reflection
Fig 7.11

9. Diffuse Reflection Fig 7.10 Fig 7.7 To “diffuse” means to “spread”
Most common visibility mechanism ; Each point reflects light in all directions
Bundles of light from object are seen by the eye
Colors result from selective wavelength reflection or absorption

10. Why is the sky blue at noon? (A Closer Look)
APC: Sky appears “blue” when the sun is high in the sky because…
… blue light is scattered more than the other colors
APC: We see a blue sky because…
…scattering of light by air molecules and dust is more efficient when the wavelength is shorter

11. Another Behavior of Light
Anyone ever try this? This figure came from “Essentials of Meteorology” by Ahrens
REFRACTION AT WORK ; Try it with a pencil in a glass of water

12. Refraction
Fig 7.15
Light crossing a boundary surface and changing direction (bending)
Reason?
Differing densities.
Animation on the Course Web site
Light crossing a boundary surface and changing direction
2 Transparent materials : transparent means light can travel through it.
Reason: change in light propagation speed
Moving to a more dense medium with a slower propagation speed
Light bends toward surface normal
Moving to a less dense medium with a faster propagation speed
Light bends away from the normal
All revolves around the density of the medium that it propagates through

13. Magnitude (Amount) of Refraction
Depends upon?:
Angle which light strikes the surface
Ratio of the speeds of light in the two materials
#2 helps define the Index of Refraction of a substance
Table 7.1
Depends upon:
Angle which light strikes the surface
Ratio of the speeds of light in the two transparent materials (differing densities of the two mediums)
Index of Refraction (Equation 7.2 and Table 7.1)
n is unitless!
c , the speed of light in a vacuum, is a constant 3 x 108 m/s

14. Example 7.1

15. Mirages “Mirage” is due to Refraction! Similar to Fig 7.17
Several examples of mirages (Images from Essentials of Meteorology by Ahrens)
Like Figs ; Great caption ; Wet Highway
Refraction is the cause:
Colder air has a higher index of refraction (Table 7.1) than warmer air
Like sound, light travels faster in warmer air
The air near the road’s surface is hotter on a clear, calm day
Light rays traveling towards you in this hotter air are refracted upward as they enter the cooler air
Your brain interprets this “refracted” light as “reflected” light
Light traveling from cars and trees traveling downward are also refracted upward.
You “See” them in the “water”
The quivering or shimmering (“heat” you “see” from the rising asphalt or astro turf)
Essentially due to changing refraction due to heating and convection.
Wet Highway?

16. Dispersion and Colors White light Dispersion (Spreading)
Wavelength and frequency are related
White light
Mixture of colors in “sunlight” (visible portion of the EM Spectrum
Separated with a prism
Dispersion (Spreading) Index of refraction varies with wavelength
Different wavelengths refract at different angles
Each has it own index of refraction, n
Shorter wavelengths refracted the most
Wavelength/frequency related via Equation 7.3 ( c = λf )
Continuous spectrum versus Line Spectrum (Chapter 8)
APC #28 A glass prism separates the colors of sunlight in a spectrum because:
each wavelength has its own index of refraction
APC #46: A glass prism …
shorter wavelengths are refracted the most

17. Dispersion and Colors
Range of Wavelengths and Frequencies of the colors of the visible light
The higher the frequency, the shorter the wavelength (Violet and Blue)
Also, the more the energy (page 202)
The lower the frequency, the longer the wavelength (Red and Orange)
Also, the less the energy (p 202)
ROYGBV but I think of it as:
shorter wavelength (λ) longer wavelength (λ)
VBGYOR
higher frequency (f) lower frequency (f)

18. Rainbows (A Closer Look)
Box Fig 7.7
Fig 1.15
Model back in Chapter 1 ; Each of us see our own “unique” individual rainbow
Refraction - Reflection - Refraction occurs within each individual rain drop

19. Optics The use of lenses to form images
BOX FIG 7.1
Concave lenses
Diverging
Convex lenses
Converging
The use of lenses to form images
Concave lenses
Diverging lenses
Convex lenses
Converging lenses
Most commonly used lens
Magnifiers, cameras, eyeglasses, telescopes, …

20. The Human Eye Uses a convex lens to change focal length
BOX FIG 7.4
Uses a convex lens to change focal length
Nearsightedness (myopia) corrected with a:
Concave Lens
Human Eye:
Uses a convex lens with muscularly controlled curvature to change focal distance
Corrective lenses (glasses, contacts): Correction used to move images onto retina
Nearsightedness (myopia) - images form in front of retina
Farsightedness (hyperopia) - images form behind retina
Farsightedness (hyperopia) corrected with a:
Convex Lens.

21. The nature of light Wave-like behavior
Diffraction & Interference
Fig 7.19
Huygen’s theory (light is a longitudinal wave)
Young’s modification: Light is a transverse wave
Diffraction
Bending of waves around objects or through a narrow slit
Causing light to spread and to produce light and dark fringes
Shadows do not have sharp edges
Interference
Young’s two slit experiment
Interference pattern - series of bright and dark zones
Explanation - constructive and destructive interference
Relative phase difference between two light waves produces light and dark zones (spots)
A result of light’s wave-like nature

22. Wave-like behavior - Polarization
Alignment of electromagnetic fields
Unpolarized vs. Polarized light
(Figs 7.20 and 7.21)
Alignment of electromagnetic fields
Unpolarized light: mixture of randomly oriented fields
Transverse waves vibrating in all directions
Polarized light: electric fields oscillating on one direction
Waves vibrating in one direction only (plane-polarized light: only one plane)
Normal or at right angles (perpendicular)
Two filters: passage depends on alignment
All transverse waves get absorbed
NOTE: Reflected light is partially polarized (in a horizontal direction)
Refracted light is not polarized
This is why polaroid sunglasses can help eliminate glare and help you differentiate between a mirage of water and a puddle of water on a road.
How can you tell if a pair of sunglasses are polarized?
One pair: look at a clear sky at a 90 degree angle from the sun
If the sky appears dark as you turn: “polarized”
Two pairs: put on one pair, hold the other pair at a 90 degree angle
If the sky turns dark: “polarized”

23. The nature of light: Particle-like behavior
“Quanta” and Photons
Quantization of energy: Precursor to Chapter 8!
Energy comes in discrete quanta (amount)
Vibrating molecules have energy in certain amounts (in multiples of energy)
Used by Max Planck to explain blackbody radiation observations
Einstein expanded on Max Planck’s quanta concept: quantum concept to light (Ch 8 p 228)
Particles of light: Photons
When a photon is absorbed, it gives up all of its energy
Thus, light is a form of energy
This helps explain the photoelectric effect
Considering light to be photons with discrete quanta of energy versus a wave of continuous energy
Present-Day theory: page 212
Dual nature of light: sometimes acts as a wave and sometimes acts as a particle

24. Example 7.3 Good to practice these for your upcoming lab experiment
Sometimes you must convert the UV wavelength of 3.00 X 10-7 m to Hz using the “Wave Equation” of c=λf
Rearrange to f = c/λ where c = 3.0 x 108 m/s
Then find E using E = hf ; E = h(c/λ)

25. Photoelectric Effect Fig 7.23
Ejection of electrons from metal surfaces by photon impact
Movement of electrons as a result of energy acquired from light (absorbed photon)
Minimum photon energy (frequency) needed to overcome electron binding PE
Additional photon energy goes into KE of ejected electron
Intensity of light related to number of photons, not energy
Absorbed light will increase the absorbing object’s temperature because the energy level is increased
Electrons moving faster
Photoelectric Cell (Solar Cells)
Energy of light (radiation) converted or transformed into an electric current

26. Next: Chapter 8 Atoms and the Periodic Properties