Radiation - School of Physics - The University of Sydney · Web view- diffraction & interference of light Emission and Absorption of electromagnetic radiation by atoms and molecules

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ELECTROMAGENTIC RADIATION

ELECTROMAGNETIC WAVES

Electromagnetic spectrum

diameter of an atom ~ 10-10 m diameter of nucleus ~ 10-15 m

Radiation

f (Hz)

(m)

Photon

Energy (eV)

Comments

power lines

50

6106

210-13

Currents. Radiation emitted from 50 Hz power lines are they dangerous?

radio

AM FM TV

< 109

> 0.3

< 410-6

Currents, electronic circuits. AM FM radio, TV.

microwaves

109

31011

0.3

110-3

410-6

110-3

Magnetron. ~ 10 mm to 30 m can penetrate atmosphere satellite communications, mobile phones, police radar, microwave cooking (polar water molecule), medical diathermy.

infrared

31011

41014

110-3

810-7

110-3

2

Molecular vibrations & rotations, all objects. Why do you get warm standing in the sun? Thermography, IR satellite images, detecting tumors & cancers.

light

3.91014

7.71014

7.810-7

3.910-7

2

3

Outer electrons, incandescent lamps, lasers, arcs. Eye sensitive to this narrow band. White light mixture of all colours of visible spectrum. TV (red blue green). If a an uranium atom enlarged to a size of a pea , red ~ 20 m long.

Sunlight ~ 1017 photons.s-1.m-2. Blue light photons sufficient energy to disrupt chemical bonds.

ultraviolet

810-14

31016

410-7

110-8

3

1102

Inner & outer electrons. Ionising (4 eV photons break C-C bonds). Skin cancers, tans the skin, vitamin D synthesis, damage to eyes. O3 layer absorbs UV protection layer. Can kill micro-organisms.

x-rays

31016

1020

110-8

310-12

1102

4105

Inner electrons, x-ray tubes. Diagnostic medical-rays. Computed tomography CT scan. Crystal & molecular structures.

-rays

> 1018

310-10

> 4103

Nuclei, accelerators. Sterlisation, food preservation, cancer treatment, medical diagnosis, flaw detection.

Very penetrating can pass through large thickness of concrete.

MODELS FOR LIGHT

Wave model

c = f speed of light c = 3.0108 m.s-1

Thomas Young & Augustin Fresnel emr waves

- diffraction & interference of light

Emission and Absorption of electromagnetic radiation by atoms and molecules

Most of the chemical and optical properties of a substance are dependent upon the outer most bound electrons in atoms. Each electron is usually in its ground state the lowest energy state. Only when an atom absorbs specific and sufficient energy can the atom move to a well-defined higher energy state (excited state).

Resonant Absorption

Spontaneous emission

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Microwaves

ground state

excited states

energy

E

1

E

2

h f

=

E

2

-

E

1

E

1

E

2

h f

E

2

-

E

1

E

1

E

2

h f

=

E

2

-

E

1

Excited state

Oscillating

mixed state

Ground state

~ 10

-

9

s later