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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.

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Presentation on theme: "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."— Presentation transcript:

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

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