Upload
jeffery-bailey
View
215
Download
0
Tags:
Embed Size (px)
Citation preview
1
Electromagnetic Radiation
2
3
4
c=
How many wavelengths pass through point P in one second? Frequency!
P
5
Electromagnetic Radiation
6
A radio operator broadcasts at a frequency of 14.2 MHz (megahertz).What is the wavelength of the radio waves put out by the transmitter?
ms
sm
MHz
smc
c
1.21102.14
100.3
2.14
100.316
1818
- Solve example 7.2.
- Calculate the wavelengths of the electromagnetic radiation presented in previous slide.
7
Atomic Spectra
Now, replace white light source with a hydrogen lamp.
8
H2 → 2H
H → H*(excited)
H*(excited) → H (Ground state) + light
ب
أ
عضوالةطاق
cathode
anode
Discharge tube
9
Continuous spectrum
Line spectrum
Problem: No explanation provided by classical physics.
Scientists (such as Lyman, Balmer and Paschen) analyzed the observed lineswith respect to their wavelengths. Rydberg summarized their efforts in the so-called Rydberg’s equation:
22
21
111
nnR
R=1.09678×10-2 nm-1
Rydberg’s constant
n: positive integer.
10
Calculate the wavelength, in nanometers, in the line of spectrum of hydrogenCorresponding to n1=2 and to n2=4!
lightGreen
nmnmcm
nmcm
x
mxm
nmcm
cmcm
cmcm
cmcm
nnR
3.4861010863.410863.4
101
10
10101
?1
10863.46.20564
1
6.2056416
3109678
1
16
1
4
1109678
4
1
2
1109678
1
111
755
7
7
92
51
11
122
1
22
21
11
Energy of Light- Energy of electromagnetic waves
h h h h h h h h h h
-However, Planck (Black body radiation) Einstein (photoelectric effect)
Light composed of tiny particles, called quanta (photons)
Energy of each photon (quantum) = h × Ephoton = h × Number of photons determines light intensity. h=6.6×10-34 J.s
Planck’s constant
12
Bohr’s Theory
Bohr’s Postulates:
•Electron moves in circular orbits around the nucleus.
•Electron can possess only certain energy values corresponding to the orbit.
•Electron can “jump” from one orbit to another, the energy difference will be emitted or absorbed in the form of light quanta.
13
22 1
nZAEn A=2.18×10-18 J=13.6 eV
Z : atomic numbern : positive integer = 1, 2, 3, …
14
The larger n- the larger is the orbit size, the farther is the electron from nucleus- the larger is the electron energy
Comparison to throwing stone upwards.
Negative sign means that the electron is under the influence of the nucleus.
Electron free from nucleus attracting force when n=∞.
15
Explanation of line spectrum
nlow
nhigh
Rch
A
nnch
A
c
c
nnh
A
hnn
AE
n
A
n
AEEE
highlow
highlow
highlow
lowhighlowhigh
22
22
22
22
111
11
11
16
Lyman series Balmer Series Paschen Series ends at n=1 ends at n=2 ends at n=3
UV visible Infra-red
17
Example 7.5
Calculate the energy, frequency and wavelength of the photon emitted when an electron in the hydrogen atom drops from the fifth to the second energy level.
nmms
smc
ssJ
J
h
E
JJE
nnAEE
photon
photon
highlowphoton
5.43210325.41094.6
100.3
1094.6106.6
10578.4
10578.45
1
2
11018.2
11
7114
18
11434
19
1922
18
22
18
De Broglie HypothesisLight behaves: - as waves (electromagnetic waves)
Hertz experiment
- as particles (photons) Photoelectric effect, Compton effect
Why should light be special??!!!!!
Generalization of dual nature (wave nature & particle nature) to all matter.
Any moving object can be considered to be a wave!!!
Energy of that object is E=mc2
If object considered to be a wave, E=h
De Broglie suggested:
19
p
h
vm
h
cm
h
cmh
cmc
hcmh
22
De Broglie relation De Broglie wavelength
Experimental evidence: electron diffraction Diffraction is a phenomenon that only waves can undergo Includes waves interference
20
Example: Calculate the wavelength of a football player weighing 60 kg moving in the yard with 10 km/h velocity.
mmkg
sJs
vm
h
sms
mhkmv
3534
1096.3778.260
106.6
/778.23600
10000/10
too small to be observed experimentally
Example: Calculate the wavelength of an electron moving with a velocity of 1000 km/h.
Angstrommmkg
ssJ
vm
h25.71025.7
1000000)1011.9(
106.6 1031
34
X-ray: diffraction on crystals
21
Wave mechanicsA version of quantum mechanics (modern concepts in physics)
Electron in atoms are considered to be standing waves.
Each electron in atom is described by a set of numbers called Quantum numbers.