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Chapter 7.1-7.2

CHEM 1A Chapter Notes - Chapter 7.1-7.2: Photon

Course Code
Arnold Yuan

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Lecture 33: All Aglow
7.1-7.2 pg. 276-284
7.1 Quantum Mechanics: The Theory That Explains the Behavior of the Absolutely Small
electrons are extremely small
light disturbs the electron – changes the electrons position  inability to observe electrons
- knowledge of electron behavior has limits
-quantum-mechanical model – model of how electrons exist in atoms and how
electrons determine properties of elements
7.2 The Nature of Light
wave-particle duality of light
light similar to electrons
electromagnetic radiation – energy embodied in oscillating electric and magnetic fields
electric field – space where electrically charged particle experiences a force
wave of oscillating, perpendicular electric and magnetic fields
3.0 x 108 m/s speed
amplitude – vertical height of crest
- determines light’s intensity – greater amplitude of electric and magnetic waves
wavelength (λ) – distance between adjacent crests
energy of wave depends on wavelength and amplitude
frequency (v) – number of crests that pass through stationary point in given period of
time (s-1)
- related to speed of light
v = c/λ
visible light – wavelength determines color
white light – spectrum of wavelengths
400-750 nm
longer is red, shorter is violet
substance color – some wavelengths absorbed, some wavelengths reflected
electromagnetic spectrum – all wavelengths of electromagnetic radiation
short wavelength, high-frequency (high energy) and long-wavelength, low-frequency
(low energy)
gamma (γ) ray – highest energy, biologically damanging
X-rays – pass through many substances that block visible light, can be damaging
UV radiation – can damage biological molecules
Visible light (violet to red) – causes molecules in eyes to change shape, send signal to
IR radiation – heat (warm objects emit infrared)
Microwaves – radar and ovens – efficiently absorbed by water
Radio wavesAM and FM, telephone, television, communication
Interference – cancel each other out, build each other up, depending on alignment
Constructive interference – align with overlapping crests
Destructive interference – crest and trough overlap
Diffraction – when wave encounters obstacle or slit comparable in size to its wavelength,
it diffracts/bends around it
2 slits + interference  interference pattern (2 new waves interfere  bright and dark line
photoelectric effect – observation that metals emit electrons when light shines on them
(classical model) dislodge electron if enough energy is transferred to electron, also lag
time if a dim light is used (need to transfer sufficient energy)
experimental results: threshold frequency, no lag time with low intensity light as long as
above threshold
light energy in packets: E = hv, h = 6.626 * 10-34 J s
-photon, quantum
E = hc/λ
Emission of electrons depends on whether or not photon has enough energy
hv = Φ, the binding energy of the electron
as frequency increases past threshold, excess energy is transferred to ejecred electron as
kinetic energy
KE of ejected electron = hv - Φ
Lecture 34: Absorbing
7.2 pg. 281-284
see Lecture 33