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EA Notes (Scen 101), Tillery
Chapter 7
Light
Introduction
• Light is hard to study because you can't see it , you only see it's effects.
• Newton tried to explain the energy in a light beam as the KE of a
particle stream, and failed.
• About 1800, some experiments proved that light acts like waves.
• About 1900, other experiments proved that light acts like particles.
• Present idea: sometimes like waves, sometimes like particles.
 Waves when Interference (as with sound waves) is important.
 Particles when Mass (momentum) is important.
• "Quantum Theory" lets us use both descriptions.
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Sources of Light
• Luminous Source: actually produces light.
• Light bulbs in ceiling fixtures and projector are luminous.
• Everything else in room merely reflects that light.
• Incandescent Source: produces light by high temperature.
• Fluorescent tubes are not incandescent.
• Standard light bulbs are. So is the SUN.
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• Electromagnetic (Wave) Spectrum: ( Introduced end of last chapter. )
• Propagating Perpendicular E & M Fields shown in Fig.7.2.
• There is a very large frequency range for this EM Radiation.
• Propagation Speed: ENTIRE range has SAME speed. (Later in Chap).
• Production Method: DIFFERENT at low and high ends.
 f  1011 Hz production is by Antenna (last chapter).
 Otherwise by the Quantum method described later.
• Blackbody Radiation:
• The EM radiation from any object at ANY temperature.
• Experiment (ca. 1880): Intensity varies with wavelength as
CORRECTLY shown in Fig.8.2, 5th Ed. (Shown as next slide.)
• As temp. increases: peak gets higher and moves to shorter .
• Fig.7.4, 7th Ed has an unacceptable X-Axis: 0 at far right,  at origin.
• Sun's Radiation:
• The Sun's center is actually at 5700°C. Surface is 5200°C.
• The Sun's spectrum in Fig.7.5 has the correct frequencies for limits, but
the arrow points in the wrong direction.
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Note that BOTH Wavelength & Intensity go properly to ZERO at origin.
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Properties of Light
• Light Ray: { BOARD } A line drawn to represent a thin beam of light.
• Sources emit rays in all directions.
• We use an ARROW to show their direction.
• They’re STRAIGHT LINES, except at material boundary.
Light Interacts with Matter
• When light meets a material boundary, its energy is split into 3 parts:
• Reflected
• Transmitted through material, where some usually is -• Absorbed
• How much energy into each depends on the
material, the surface roughness, and the Ray Angle.
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Diffuse Reflection
• From a Rough surface. (Scattered Light)
• Rays leave object in MANY random directions.
• Most light entering our eye is from diffuse reflections.
Regular Reflection
• From a Smooth surface.
• Rays leave object in ONE predictable direction.
• DEMO
Dra w:
• Incident Ray: The ray coming INTO the surface.
• Normal: Line drawn perpendicular to surface where incident ray hits.
• Angle of Incidence: (i) Between incident ray and normal.
• Reflected Ray: The ray coming FROM the surface.
• Angle of Reflection: (r) Between reflected ray and normal.
• Law of Reflection:
i  r
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Flat Mirror Images (Fig.7.12)
• Our eye tells us the direction from which light entered it. Our brain
thinks objects are behind mirrors. They aren't, ONLY their Image is.
• DEMO Draw a Plane mirror image location.
• Pick a point on the Object a distance P from mirror. Pick Two Rays
from the point that are incident onto mirror.
• The reflected rays extended backward intersect at a distance Q
behind mirror.
• Brain believes the object is at point Q.
• Accurate drawing shows: Q = P and
Line QP is perpendicular to the mirror.
• Construction can be repeated for other points. Image point is
ALWAYS on the NORMAL and Q = P.
• This is called a Virtual Image: because NO light actually comes from
the image point. It just appears to.
• The other possibility is a Real Image: in which light actually comes
from image point (like a projector screen image).
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Refraction
• Refraction: Bending of a ray passing through a material boundary.
(Recall Sound Waves)
• Refraction depends on speed of light difference between materials.
• DEMO
Dra w:
• Incident Ray: at Angle (i) to the normal:
• Refracted Ray: Bent ray on OTHER SIDE of the surface.
• Angle of Refraction: (r) Between refracted ray and normal.
Speed of Light:
• Speed in vacuum (and air) = c  3.00 108 m/s .
• SLOWER in everything else.
• Just as with sound waves:
• Ray refracted toward normal going to slower speed.
• Away from normal going to faster speed.
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Index of Refraction:
• Dimensionless ratio easier to use in tables and formulas than the
speed of light for each material.
c
n
• Index of Refraction:
[no units]
v
• v is always less than c, thus n > 1.00 (Tab.7.1, p.187)
• NOTE: Rounded value for air is 1.00.
• Rule for ray bending at boundary in terms of n:
• Ray is refracted toward normal going to higher index.
• Away from normal going to lower index.
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Total Internal Reflection: (Abbreviated "TIR")
• For light moving from higher to lower index material.
• Rule: Ray bent away from normal going to lower index.
• DEMO
Dra w:
• Critical (Incident) Angle: Refracted ray at 90°.
• For Incident angles greater than Critical, 100.000...% reflection.
• Today's BIG use for TIR: Keeps light energy from leaking out the
walls of FIBER OPTICS. {DRAW Fiber Construction }(see p.214)
• Fiber Optics uses:
• Medicine: First developed for Endoscopes to see inside body.
• Communication: (much use now, tremendous future growth):
• Developed for long distance (digital) telephone.
• Potential for any DIGITAL high capacity communication.
• Earlier TIR uses:
• "Sparkle" in faceted diamond gemstones.
• 90° bends with no light loss using 45° glass prism.
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Visible Light, Color and Dispersion
c
c  f   
f
• Wave Equation (reminder):
• Visible Light:
• Electromagnetic waves of a particular frequency range that can be
detected by the human eye. Physicists usually define light with:
• Wavelength, range: 4  107 to 8  107m.
• Color:
• The colors we observe fall into specific smaller ranges.
( Tab.7.2, p.202 for reference. )
• Most Important: Blue & Violet are shorter than Red & Orange.
• Spectrum: Light spread out according to wavelength.
• Done with triangular "prisms" of glass using dispersion (below).
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Visible Light, Color and Dispersion (Continued)
• Dispersion:
• For most materials (but not for air or vacuum), the index of
refraction varies slightly with wavelength.
• This means refracted angle varies with wavelength.
A CLOSER LOOK: Optics
• p.204 — top 206: A lot of interesting information in this extra section.
Evidence for Waves
• The observations discussed so far COULD be explained by either
particles or waves.
The ones in this section REQUIRE light to be a wave.
Early Reasoning on Nature of Light
( about 1670 )
• Newton advocated a Particle Theory (for reasons discussed next slide.)
• Huygens proposed Longitudinal Waves, (as part of a more complete
theory that used WAVEFRONTS to explain how lenses focus.)
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Diffraction Important info from Ed.5 that was dropped.
• Diffraction: The spreading of a wave as it passes through a hole
(or the edge of an opaque object).
DEMO Draw:
• Consider a Particle beam passing the Edge of a Wall:
• Particles would go straight past it. Sharp shadow.
• Light appears to make sharp shadows, like a particle beam.
This is why Newton tried to use particles in his explanation of light.
• Consider a Water or sound wave passing the Edge of a Wall:
• Energy bends around the edge. Shadow not sharp.
Which is why you can hear around corners.
• Waves always diffract, but detecting it is sometimes difficult.
  d Easy Observation
• Much Spreading:
  d Hard Observation
• Little Spreading:


• for LIGHT  ~ 6  10 7 m and normal objects — Hard.
• for SOUND   ~ 0.3 m or WATER with normal objects — Easy.
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Double Slit Interference
• Experiment (Young, 1801) is proof that light is not particles.
• Start with 2 narrow slits illuminated by the same monochromatic
(one wavelength) source.
• If light were particles, there would be only two bright lines, one
above, one below the midpoint of a screen behind the slits.
• Young saw bright line at midpoint and many lines above and below.
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Double Slit Interference
DEMO
• Here's how waves explain the experiment:
• Light diffracted through slits spreads out and overlaps on screen.
• Consider the two rays reaching the mid-point of the screen.
• The distance from the two slits is EQUAL,
• Thus there are the same number of wavelengths on each ray,
• Thus the two rays are always in phase.
• This is Constructive interference. (A BRIGHT SPOT).
• Now move up from the mid-point.
• When distance from slits differs by HALF wavelength,
• two rays arrive completely out of phase
• This is maximum Destructive interference. (A DIM SPOT).
• Continue moving up:
• When distance differs by a WHOLE wavelength there's Constructive interference again. (ANOTHER BRIGHT SPOT).
• Move further up, dark and bright areas continue to alternate.
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Polarization
• Def: The selection of one vibration direction from many.
• Only TRANSVERSE WAVES have more than one vibration direction.
• The effect is seen with very few materials, the newest being "Polaroid"
polarizing film, developed by that company’s founder for Military Pilots.
• Polarizing sunglasses have this film in them.
• DEMO
Draw or Show:
• Place one piece of this material so light passes through it.
• Rotate the piece.
• Intensity of transmitted light is constant.
• Repeat rotation with a second piece of material.
• Intensity of transmitted light still constant.
• Place both pieces so light passes through them.
• Rotate either piece.
• Intensity of light through second piece varies from bright to zero
and back to bright with 180° rotation.
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Polarization (Continued)
• TRANSVERSE Wave Explanation: (Fig.7.21, p.209)
• Blackbody and Fluorescent light is produced unpolarized, with its
Electric (and Magnetic) Field vibrating in Random directions in the
plane perpendicular to its propagation direction.
• First "Polarizer" transmits only one vibration direction.
• Second "Polarizer" blocks vibrations not parallel to its polarizing
direction.
• Polarization by Reflection:
• Light reflected from a surface at an angle greater than 0° is partly
polarized with the electric field vibration parallel to surface.
• (Fully polarized at one specific angle that depends on index of refraction.)
• Polarizing sunglasses have their polarizing direction vertical, thus
partly block reflected light.
• Fig.7.22, p.209: This drawing tries to show that reflected light is
parallel to the blue "lake," which is horizontal.
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Scattering of Sunlight
• Scattering:
• Absorption and re-radiation of light by particles that are smaller
than the wavelength of light:
• All atmospheric gas molecules.
• Very small dust particles.
(
Particle Volume 2
• Rayleigh Scattering:
Scattered Intensity 
4
• Scattering gets stronger with shorter wavelength (blue - violet).
• DEMO
Dra w:
• Blue sunlight is scattered more than red.
• Blue sky color is all scattered light.
• If there's no atmosphere, the sky around Sun is black.
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Maxwell's Wave Theory (1865)
(See end of Chap 6 Notes)
• Proposed that radiation is a transverse wave of periodic electric and
magnetic fields. Using this theory he:
• Described how EM radiation could be emitted from vibrating
electrons and then travel through empty space. (End of Chap. 6:
Vibrating electrons in an antenna create a varying circular magnetic
field, which in turn creates a varying electric field.)
• Calculated the correct speed of light from two basic physical
constants.
• Provided theoretical basis for Rayleigh's sunlight scattering.
• But it failed to explain some important experimental observations
on Emission & Absorption of light discussed in the next Section.
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Evidence for Particles
Blackbody Radiation
• Re-show the Blackbody Radiation shown in Fig.8.2, 5th Ed.
• Maxwell's Wave Theory didn't explain Intensity going to zero at
zero wavelength. (It said it should continue to increase to infinity.)
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Blackbody Radiation & Planck's Quantum Proposal
• Quantization of Energy (1900)
• Max Planck found a fit to the Experimental Data with a RADICAL idea:
• Assume a radiating blackbody has atomic vibrators whose
energies can have only whole multiples of discrete amounts:
 nL hc 
• Planck's Discrete Energy Levels: Eq  nL hf  
;
nL  1, 2,3, 4,

 

where f is the frequency, h  6.6  1034 J s , (fit Blackbody Curves)
 = wavelength, and c = speed of light.
• Assume atom radiates energy when its level decreases one step:
• Planck's Radiated Quantum:
 hc 
Eq  hf  




• Assume the Intensity at each frequency depends on the
number of atoms ( natoms ) able to emit that energy.
• Since emitted energy can't be infinite,
natoms must go to zero at zero wavelength.
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Photoelectric Effect
• Fig.7.23, p.211: When light hits a clean metal surface, electrons are
sometimes ejected.
• Three experimental observations differed from Maxwell's Wave
Theory predictions:
• Below a threshold light frequency there’s no electron ejection.
• The KE of an ejected electron depends only on the light frequency,
not intensity as predicted by wave theory.
• The Current (Num. of electrons/sec) depends only on the light
intensity, not frequency as predicted by wave theory.
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Einstein's Photoelectric Explanation (Photons)
• Einstein (1905, led to Nobel Prize) extended Planck's emitted
quantum to describe propagating light. He said:
••• Light exists only as bundles or "particles" of energy called
Photons whose energy equals Planck's quantum jump Eq :


hc
Ep  hf 
• Einstein's Photon:

•  Energy hitting metal surface depends on frequency.
• From chemical Ionization: Each material has a minimum energy
(Work Function) for losing an electron.
•  Must be a threshold frequency for Ejection, (Maxwell Missed)
•  Ejected electron's KE = Photon Energy – Work Function.
(Maxwell predicted Intensity)
• Assume (along with Planck): the intensity at each frequency
depends on the number of photons at that frequency.
•  Rate of electron ejection (current) will depend on intensity.
(Maxwell predicted Frequency)
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The Present Theory
• Light is an Extremely small-sized phenomenon compared to the world
that Galileo and Newton knew, and this is what makes it different.
We are forced to accept that:
• Sometimes light acts like waves with a frequency and
sometimes like particles with a KE.
• We switch between the concepts as needed, and

• Quantum Theory Ep  hf

provides the bridge.
• Chapter 8 of the Current Edition of Tillery (the first chapter of the
Chemistry section) explores the Extremely small-sized phenomena of
Electrons and Atoms, again using Quantum Theory.
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