Physics 202: Introduction to Astronomy – Lecture 5

Physics 202: Introduction to Astronomy – Lecture 5

Carsten Denker

Physics DepartmentCenter for Solar–Terrestrial

Research

February 1, 2006 Center for Solar-Terrestrial Research

Chapter 2.1 – 2.6 Light and radiation Spectrum: radio,

infrared, visible, ultra-violet, X-rays, gamma-rays

Waves: wavelength, frequency, amplitude, and period

Electromagnetic waves

Electromagnetism

Blackbody radiation Radiation laws Spectroscopy Emission and

absorption lines Spectral line

formation Bohr model of

atoms Hydrogen spectrum Kirchoff’s laws


Coulomb’s Law

Superposition principle

Coulomb’s law

Gravitational force

Coulomb’s law

1 22

q qF k

r

1 22

mmF G

r

1 22

0

1

4

q qF

r

9 2 2

0

18.99 10 N m C

4k

12 2 1 2

0 8.85 10 C N m

1,net 12 13 14 1nF F F F F


Electromagnetic Waves


The Interaction of Light and The Interaction of Light and MatterMatter


Infrared Radiation

In 1800, William Herschel (1738 –1822) extended Newton's experiment of separating chromatic light components via refraction through a glass prism by demonstrating that invisible "rays" existed beyond the red end of the solar spectrum.


Electromagnetic Spectrum


Kirchhoff’s LawsA hot (< 0 K), dense gas or solid object

produces produces a continuous spectrum with no dark spectral lines.

A hot, diffuse gas produces bright spectral lines (emission lines).

A cool, diffuse gas in front of a source of a continuous spectrum produces dark spectral lines (absorption lines) in the continuous spectrum.


Spectroscopy The English chemist and

physicist William Hyde Wollaston (1766 – 1828) noticed dark lines in the spectrum of the Sun while investigating the refractive properties of various transparent substances

Joseph von Fraunhofer (1787-1826) independently rediscovered the “dark lines” in the solar spectrum


Spectroscopy Prisms Diffraction gratings

Transmission grating Reflection grating

sin and 1, 2, 3, ...d n n

nN

nN

Resolving power


The Bohr Model of the Atom Wave–particle duality of light Rutherford 1911 Au: It was quite the most

incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15–inch shell at a piece of tissue paper and it came back an hit you. discovery of a minute, massive, positively charged atomic nucleus

Proton: mp = 1836 me


Hydrogen Atom1

2

1 1 1 and 109677.585 0.008 cm

4H HR Rn

2 2

1 1 1 and HR m n

m n

m = 1 UV [122, 103, 97, …] nm Lyman

m = 2 Visible [656, 486, 434, …] nm Balmer

m = 3 IR [1875, 1282, 1094, …] nm Paschen

m = 4 IR [4051, 2625, 2165, …] nm Brackett

m = 5 IR [7458, 4652, …] nm Pfundt

Planetary model of the hydrogen atom?


Kirchhoff’s Laws Revisited A hot, dense gas or hot solid object produces a continuous

spectrum with no dark spectral lines. This is the continuous spectrum of black body radiation, described by the Planck functions B(T) and B(T), emitted at any temperature above absolute zero. The wavelength max at which the Planck function B(T) obtains its maximum is given by Wien’s displacement law.


Kirchhoff’s Laws Revisited (cont.)

A hot, diffuse gas produces bright emission lines. Emission lines are produced when an electron makes a downward transition from a higher to a lower orbit. The energy lost by the electron is carried away by the photon.

A cool, diffuse gas in front of a source of continuous spectrum produces dark absorption lines in the continuous spectrum. Absorption lines are produced when an electron makes a transition from a lower to a higher orbit. If the incident photon in the continuous spectrum has exactly the right amount of energy, equal to the difference in energy between a higher orbit and the electron’s initial orbit, the photon is absorbed by the atom and the electron makes an upward transition to the higher orbit.

Physics 202: Introduction to Astronomy – Lecture 5

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