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Chapter 5
Electronic Structure
Light Through a Prism
• White light (sunlight) is a blend of all colors (ROY G BIV)
combined together.
• The wavelength (λ) and frequency (υ) for each color are unique to
that color.
• As light passes through a prism…
- the different wavelengths of the colors are separated.
- individual colors can be detected by the eye.
- a rainbow appears.
The Electromagnetic Spectrum
• All substances (radioactive or not) emit electromagnetic radiation.
• Only part of the spectrum that human eyes can detect is visible
light: (ROY G BIV)
• All other radiations have wavelengths that are either too long or
too short for our eyes to detect.
• The Electromagnetic Spectrum (EM spectrum) is made of
the different forms of electromagnetic waves and is based on
the frequency and wavelength of the wave. EM waves do
not require a medium to travel…..
Cosmic
rays
Microwaves
too short to see
too long to see
7.4 x 10-6 m
longest
4.2 x 10-6 m
shortest
Increasing danger
•The following list [A to G] names the forms of
the EM spectrum in order of energy,
frequency and wavelength.
[A…..largest wavelength,
lowest frequency & lowest energy]
A . radio waves (includes AM, FM,
TV, radar, shortwaves) used for
communication
B. microwaves -- used in cooking; can
penetrate air, glass, paper and plastic
C. infrared (heating lamps) felt as warmth;
used to keep food warm
D. visible (only portion that humans
can see; smallest part of the spectrum)
ROY G. BIV…violet has the greatest
energy, frequency, and smallest
wavelength of all the colors
E. ultraviolet (causes sunburn)
F.x-rays (medical use) will pass
through the human body with
the exception of some bones
and tissues
G. gamma rays (treatment of
cancer ) can kill living cells
[G…..smallest wavelength, highest
frequency & highest energy]
Wavelength vs. Frequency
• Wavelength (λ): distance b/n crests of a wave.
long wavelength
crest
Wavelength
Wavelength
trough
short wavelength
• Frequency (ν): # of wavelengths that pass a certain point in a
given amount of time.
- units are Hertz (Hz)
• These 2 waves are traveling at = speeds… which wave will have
more crests cross the ‘finish line’ in a matter of one min.?
high frequency = short wavelength
*** higher frequency = higher energy ***
FINISH
low frequency = long wavelength
Wave Calculations
• all waves on the EM spectrum travel at the speed of light (c).
• wavelength (λ) and frequency (υ) are inversely related.
c = λ(ν)
νλ
λν
• c = speed of light = 3.00 x 108 m/s.
• to solve for λ …
• to solve for υ …
c
λ= ν
c
ν=
λ
• Practice…
Calculate the υ of a wave that has a wavelength of 5.00 x 10-6 m.
ν= c
λ
3.00 x 108
ν =
5.00 x 10-6
ν = 6.00 x 1013 Hz
What is the λ of radiation with a frequency of 1.50 x 1013 Hz?
λ= c
ν
3.00 x 108
λ=
1.50 x 1013
λ=
2.00 x 10-5 m
Does this radiation have a shorter or longer λ than red light?
longer…red light ~ 7.4 x 10-6 m
• Max Planck explained that light is emitted in packets (quanta)
called ‘photons’ which are distinct bundles of energy.
• Planck also assumed that energy of a photon is directly
proportional to the frequency of the light.
E = h(ν)
h
h
E
ν =
h
• E = energy (in Joules)
• υ = frequency
• h = Planck’s constant (6.626 x 10-34 J/Hz)
• Practice…
• Calculate the energy of radiation with a υ of 5.00 x 1015 Hz.
E = h(ν)
E = 6.626 x 10-34 (5.00 x 1015)
E = 3.31 x 10-18 J
Obj. 5…Atomic Spectra
• all elements will emit light when excited (i.e. by electricity).
• atoms absorb energy and then emit an = amount of energy in the
form of light.
- atoms emit a characteristic color
- Ne = orange - red
- Na = bright yellow
• if we pass this light through a prism (separate the λ) we get an
atomic emission spectrum.
set-up
ex. of wavelengths emitted
Obj. 5 cont…
• emission spectra are unique to particular elements.
• only show certain lines of the continuous spectrum (white light).
• have helped us gather a lot of info. about our universe!
• atomic absorption spectra shows colors missing from the
continuous spectrum (missing λ were absorbed by the element).
continuous
absorption
emission
Obj. 6-7…How Atoms Absorb and
Emit Energy
• e- are found on certain energy levels (orbitals) around the atom.
- there is a maximum of seven energy levels in an atom.
- e- on the energy level closest to the nucleus have the lowest
energy. The 7th energy level has the highest energy.
- an e- requires one ‘quanta’ of energy to jump to the next
energy level.
• e- at their lowest energy level are considered to be at the
ground state (most stable).
• if e- absorb a quantum or more of energy (from electricity), they
can jump to higher energy levels (excited state).
• e- must lose energy in order to fall from the excited state back to
the ground state.
- this energy is emitted in the form of visible light!
Obj. 6-7 cont…
• ‘Heinsburg’s uncertainty principle’ states that it is impossible to
know exactly where an e- is, b/c it is constantly in motion.