Optical Sources and Atomic Structure, Lecture notes of Optics

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w.wang

Optical Sources

Wei-Chih Wang

Department of Mechanical Engineering

University of Washington

ME

When sub-atomic particles were first discovered in 1897, theywere thought to orbit the nucleus like planets orbit the sun.Unfortunately, this is the atomic model still taught in manysecondary schools. In the mid-20th. century, it was discoveredthat the structure of the electronic shells was somewhat morecomplex consisting of:- up to at least seven capital-lettered shells (1=K, 2=L, 3=M,4=N, 5=O, 6=P, 7=Q)- up to at least 4 small-lettered orbitals (s, p, d, f)- each orbital is further divided into sub-orbitals: s has 1, p has 3,d has 5, f has 7- each of these sub-orbitals can accommodate two electrons.

,Helium (He) withits two electrons,has filled its s-

The Hydrogen Emission Spectrum

The hydrogen emission spectrum is a

line spectrum

, i.e. only

particular frequencies (wavelengths, energies) are observed.

WHAT IS GOING ON HERE?

Atom in 'excited state', i.e. with increasedEnergy

H

2



2 H (energy)

Molecule

Atoms

H

H*

H*



H + h



The excited atom returns to the 'normal' state (ground state) byreleasing the excess energy as electromagnetic radiation of energy(E= h



) corresponding to the energy difference between the

ground state and the excited state.The fact that the hydrogen emission spectrum is a line spectrum –only radiation of very specific frequencies being emitted – meansthat only excited states of very specific energies are being formed.

A theory was proposed to account for

these observations.

The Bohr Theory of Atomic Structure

- Quantum Mechanics

The

energy

of

electrons

in

atoms

is

Broadband light source

Incandescent Filament Lamps

In conventionalfilament lamps orlight bulbs, electriccurrent ispassed through acoiled tungstenfilament, containedin a glass envelopethat is filled with an

Halogen

Halogen lamps arealso filament lamps.However, halogen isaddedto the fill gas toprevent evaporatedtungsten from

Gas discharged lamp

In a gas dischargelamp, once asufficient voltage isapplied, electrons areemitted from aheated electrode,creating a plasma ora gas capable of

high intensity discharge lamp (e.g.

metal halide lamp)

High pressure gasdischarge lamps emitradiation directly asvisible light. In thistype of lamp thecombination ofdifferent elementatoms in the hot gas

LED lecture

Wei-Chih Wang

University of Washington

Linear and Nonlinear electronics

Vaccum tube (i.e. type 2A3)

Thermistor

voltage

voltage

voltage

Diode

(i.e. PN diode, LED,laser diode, phtodiode)

(large negativetemperaturecoefficient ofresistivity)

Normalresistor

volt

current

curren

Ohm’s Law:

i=

 







1

T

D

nV v

S

D

e

I

i