Spectroscopy spectroscopy: breaking up light into its component colors to study how atoms and light interact dispersion: spreading out of white lightdispersion.

Spectroscopy

• spectroscopy: breaking up light into its component colors to study how atoms and light interact

• dispersion: spreading out of white light

• spectrum: the spectra of colors produced by sending white light through a prism

• spectroscope: instrument used to see spectra (ex. in class)

Types of Spectra

• continuous spectrum: a spectrum with no breaks. A continuum of unmixed shades of color.

• spectral (emission) line: a well defined, thin line of one specific color

• emission-line spectrum: thin lines of specific colors against a dark background

• absorption line: a dark line on a continuous spectrum marking a lack of intensity of a specific color (not necessarily a complete absence of that color)

• absorption-line (dark-line) spectrum: dark lines of missing color against a continuous background (ex: solar spectrum)

More on spectra

• polarized light: all light propagating (waving) in the same direction. A spectroscope polarizes light prior to its dispersion. This ensures that all spectral lines are vertical and parallel.

• uniform speed of light: light travels at same speed through the same medium. Light travels fastest through a vacuum.

Kirchoff’s Rules• 1. A hot, opaque solid liquid, or highly compressed gas

emits a continuous spectrum.Example: filament of a incandescent light bulb

• 2. emission-line spectrum => A hot, transparent gas produces a spectrum of bright lines (emission lines). The number and colors of these lines depend on which elements are present in the gas.Example: a neon sign

• 3. absorption-line spectrum => If a continuous spectrum (from a hot opaque solid, liquid, or gas) passes through a gas at a lower temperature, the cooler gas causes the appearance of dark lines (absorption lines). Their colors and numbers depend on the elements in the cool gas. (99% of stars have this spectrum)Example: sunlight

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Kirchoff’s Rules

Spectral Summary

• every element /isotope displays a unique arrangement of lines in its spectra

• emission and absorption lines for the same element are in identical positions

Balmer Series• The set of hydrogen absorption or emission lines that lie

in the visible part of the spectrum, the first of which is the H-α line (in the red visible region)

• This very orderly pattern of spectral lines led scientists to look for a cause in the internal structure of the atom

Particle nature of light

• quanta: “chunks” of light energy (photons) emitted by radiating matter

• photon: a quanta (piece) of light• photoelectric effect:

Ephoton = hf = hc / λ (since f = c / λ)

• E = energy of the photon• f = frequency of the photon• h= Planck's constant (6.63 x 10-34 J·s) • λ: wavelength of photon• c : speed of light (3 x 108 m / s in a vacuum)

Bohr’s atomic model• explains and predicts why absorption and

emission of photons take place. • Emission and absorption of light is due to

transitions between electron energy levels• energy levels: electrons have a large number of

levels with specific energies (stair step example)• ground state: the 1st (lowest) energy level.

When the electron occupies this level, the atom is at its minimum energy.

• the higher the total energy of the atom, the closer the orbits (or electron energy levels) are to each other.

Excitation & De-excitation

Model of atomic energy levels

• excitation: moving an electron to a higher level (yields an absorption line)– 2 methods of excitation: collision with another atom or

absorption of a specific photon with sufficient energy.• de-excitation: the electron descends to a lower level

(yields an emission line)– The electron loses energy and this exact energy is given off by

the emission of a photon.• electrons only move to exact energy levels (don't take

half-steps)• Ionization: if an atom gains enough energy, the electron

flies away from (escapes) the nucleus. It is no longer bound to the atom.

• Ionization energy: the amount of energy needed to ionize an electron.

Balmer series explained

• Emission or absorption lines arising from transitions between the 2nd level and all others (except the 1st)

• Photons at visible wavelengths

Hydrogen-specific series

• Lyman series• Emission or absorption lines

arising from transitions between the 1st level and all others.

• Photons at ultraviolet wavelengths

• Balmer series• Emission or absorption lines

arising from transitions between the 2nd level and all others (except the first).

• Photons at visible wavelengths

• Paschen series• Emission or absorption lines

arising from transitions between the 3rd level and all others (except the first and second).

• Photons at infrared wavelengths

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Balmer Thermometer

• Balmer Lines: spectral lines of H caused by bound-bound transitions to and from the 2nd energy level– from 3->2: red line – from 4->2: blue line – from 5->2: violet line – from 6->2: violet line

• If a star is too cool; few electrons are excited to or above the 2nd energy level (3000K; 6000° F)

• If a star is too hot; most electrons are excited above the 2nd level

• "In between" stars have the strongest Balmer Lines (Spectral Class: A)

Spectral Classification• -Original Method:

– depended on Balmer line strength only; – no understanding of relationship to temperature; – classified from strongest (A) to weakest (Z) Balmer lines

• But, weak Balmer lines can be from hot and cool stars...• Modern Stellar Spectra Sequence: reorganized classes by temperature:

(hot) O B A F G K M (cool) b b w w y o r l l h h e r e u u i i l a d e e t t l n / / e e o g w w / / w e h h b y i i l e t t u l e e e l o w

• Also a color scale!

Spectral sub-classes

• Spectral subclasses. (0-9) differ between intensities of specific absorption lines

• ex: (hotter) G0, G1, G2... K0, K1 (cooler)• Strengths of Balmer lines suggest

differences in stellar spectra & reflect differences in temperatures

• (the hotter the) Temperature --> (the more) Collisions ---> (the more) Ionization