Download AS III Electromagnetic Radiation (EMR)

3-1
AS III
Electromagnetic Radiation
(EMR)
3-2
Modern Structure of Atom
concerned mostly w/ location
&
energy
of e–’s
experimental basis:
Physics Background Info
spectroscopy =
• Wave Nature of EMR
interaction between matter &
• Particulate Nature of EMR
Ref
7: 1
Prob
FUP: 7: 1, 2
E of C: 7: 1 - 16
EMR (“light ....”)
NB.
EMR has wave & particle properties
Adv Rdg 7: 2, 3
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Background on Wave Physics
HT Fig. 3.1
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Traveling Wave Variables
After 1 period (at t = 1T)
wave has gone through 1 cycle,
has traveled 1 wavelength (λ)
def. of frequency:
general,
for 1 cycle
# of cycles
,
time
1
= T
ν =
velocity of wave:
distance
time
λ
= T
= λν
v =
m
check units: “ s ”= “m” “s–1”
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Important Characteristics of Waves
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HT Fig. 3.2
1) Diffraction & Interference
see HT Fig. 3.2 – 3.5; also SB Fig. 7.4 – 7.5
1-1 Diffraction
• at “slit”, “edge” new wave is generated
in all directions
• slit should be of same size as wavelength
• edge should be sharp
HT Fig. 3.3
1-2 Interference
trains of waves interact:
• “in-phase” combination
(“crest meets crest”)
constructive interference
amplitude ↑
• “out-of-phase” combination
(“crest meets trough”)
destructive interference
cancellation of wave (“darkness”)
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HT Fig. 3.4
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HT Fig.3.5
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Pet. Fig. 8.5
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2) Refraction/Dispersion
2-1 Refraction
light rays are bent at boundary of media
(e.g., at air-to-glass boundary,
rays are bent towards normal)
2-2 Dispersion
different λ’s are bent to diff. extent
allows separation ( & isolation) of
diff. λ’s ( e.g., red’s, green’s, ...)
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HT Fig 3.6
3-12
EMR as Wave Phenomenon
EMR consists
oscillating of electrical & magnetic fields
which travel in wave fashion
see Pet Fig. 8.2
Note:
travel speed is same for all λ’s of EMR
c = “speed of light” in vacuum
m
= 2.998 × 108 s
km
≈ 300 000 s
value very close for travel in air
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Pet. Fig. 8.2
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Classification of EMR
• wavelengths (λ’s) range from
10–16 m ..... 10+8 m
• most relevant for CHEM101/3 is “light”
ultraviolet
UV
visible
VIS
infrared
IR
approximately
< 400 nm
400 - 800 nm
> 800 nm
VIBGYOR
(reverse of Roy G. Biv)
for details see Pet. Fig.8.3
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Pet. Fig. 8.3
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Particulate Nature of EMR
(Evidence of Quantization of EMR Energy)
(1. Blackbody Radn: skip)
2. Photoelectric Effect
Basic Experimental Set-up (SB Fig. 7.7)
vary ν, observe current I
can determine when electrons are released
also KE of released electrons
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3-18
photoelectric effect .....
Pet. Fig. 8.12
Pet. Fig. 8.12 shows more sophisticated set-up
includes grid for “counter voltage”, Vs ,
which can be adjusted to prevent current I
in turn, Vs , is used to determine KE of e–’s
(physics)
important: KE of e–’s is prop. to Vs
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photoelectric ......
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photoelectric ......
Interpretation
Observations
•
νo = min. frequency requ to release e
•
at higher ν, ΚΕ of e– ↑
•
at lower ν, no e ’s are released,
d
•
at metal surface (at C),
e– ’s are held by “binding energy”,
“work function, Φ ”
•
light ( or EMR in general)
–
–
does not come as a diffuse package of
energy,
no matter how intense the radiation
but rather as individual energy particles
= “photons”
•
each individual photon
must have sufficient energy to liberate an e–
•
energy of photon:
E = hν
Planck equn
where h = 6.63 × 10–34 Js
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interpretation .....
•
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Main Message
photons with νo ( → Eo)
have the min. energy to release an e–
•
EMR (“light”) energy
does not come as a “diffuse wave”,
but rather as an assembly of discrete, small
energy packets = photons
•
thus EMR has “particulate character”
Φ = Eo = hνo
•
if ν > νo
then xs energy is converted to KE of e–
Overall, EMR has dual nature:
KE = E – Eo = h (ν – νo);
i.e.,
this relationship can be used to predict KE of e–
it has wave characteristics
( e.g., interference phenomenon)
&
particulate characteristics
( e.g., photoelectric effect)
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Summary of Lesson AS III
• terms
& symbols: wavelength, frequency,
period, velocity
• v = λν
• definition/description of diffraction/interference
• definition/description of refraction/dispersion
• electromagnetic spectrum: γ rays – radio waves
• sequence of colors in visible light
• photoelectric effect: experimental setup,
results and interpretation
• Planck equn : E = hν
• EMR has wave & particle nature
• HMWK 2.1 – 2.4