Download U2: Day 4 Notes

AP Unit II Day 4
Objectives:
 Discuss the duality of light
 Learn how energy and light are related
 Learn how to apply the Bohr model to solve for energy, wavelength, and frequency
of light
Quiz
History of the Atom Review
Model – explains things not understood in more familiar terms
 Ex: “hoodie”
 ”plant cell”
 “star”
 “platypus” - egg-laying, venomous, duck-billed, beaver-tailed, otter-footed
mammal
Duality of Light
What is light?
o A form of electromagnetic radiation
o Radiation carries energy through space
o Diffracts like waves
o Comes in packets – photoelectric effect – Einstein – particle
o Use whichever one is “handy”
o Theory: light is composed of photons that have both particle and wave
properties
o Light as a WAVE:
 Has a wavelength, λ
 Has a frequency, ν
 Has a speed, c
 Electromagnetic spectrum is a display of the various types of electromagnetic
radiation
 Visible radiation wavelengths 400 nm(violet)-750 nm(red)
o BUT, some phenomena can not be explained using a wave model-Light as a
PARTICLE:
 Blackbody radiation is the emission of light from hot objects
 Ex: light bulb, heating element
 Certain threshold necessary
 Planck investigated this and proposed that energy can only be
absorbed or released from atoms in certain amounts, called
quanta
 A discrete quantity of energy
 “fixed amount”
 A quantum is the smallest amount of energy that can be
emitted or absorbed as electromagnetic radiation: E = hν


 what else is quantized? Piano, stairs, letter grades
Photoelectric effect is the emission of electrons from metal surfaces on which
light shines
 Provides evidence for the particle nature of light AND
quantization
 light striking metal causes the metal to lose e minimum frequency required – aka: certain threshold of
E needed
 light behaves as tiny energy packets – photons
 therefore is particle-like AND wave-like
 E=hν
 Electrons are only ejected IF the photons have sufficient
energy
Emission spectra are the emissions of light from electronically excited gas
atoms
Light Waves
o Amplitude
o Trough
o Wavelength
o Node
o Frequency
o String
o Crest
Energy and Light
o c=hν
o Electromagnetic radiation – oscillation of the electromagnetic field
o c= 2.998*108 m/s
o h= 6.626*10-34 J sec
KNOW the electromagnetic spectrum – relative orders (overhead)

Radio micro IR ROYGBIV UV x-ray gamma 

Inverse and direct proportionalities of it


c= 
E= h
 to  inverse
E to  directly

E = hc/ E to  inverse

1/2= E2/E1 inverse

1/2= E1/E2 direct
Problems

6.21: Molybdenum metal must absorb radiation with a minimum frequency of
1.09*1015 s-1 before it can emit an electron from its surface via the photoelectric
effect. What is the minimum energy needed to produce this effect?
What wavelength radiation will provide a photon of this energy?
If Mo is irradiated with a light of wavelength of 120 nm, what is the maximum
possible kinetic energy of the emitted electrons?

6.17: Some sunburns occur with exposure to UV light of about a 325 nm wavelength.
What is the energy of a photon of this wavelength?
What is the energy of a mole of these photons?
How many photons are in a 1.00 mJ burst of this radiation?