Chapter 4 Spectroscopy

Kirchhoff’s First Law Hot, dense gases or solids produce a continuous spectrum. Example: Light bulb filament Continuous Spectrum

Kirchhoff’s Second Law Hot, rarefied gas produces an emission line spectrum. Example: Neon sign Emission Line Spectrum

Kirchhoff’s Third Law Cool gas in front of a continuous source of light produces an absorption line spectrum. Example: The Sun Absorption Spectrum

The Kirchhoff-Bunsen Experiment These two scientists found that burning chemicals over an open flame resulted in a spectrum with bright lines. They found that each chemical element produced its own characteristic pattern of bright spectral lines.

Structure of the Atom Proton Neutron Electron Quantized Energy Levels

Excitation There are two ways to get electrons excited. Add Heat to the Atoms This causes collisions. Shine Light on the Atoms Atoms can absorb light

De-excitation When an electron makes a transition to a lower energy level a photon is released.

Emission Spectra Spectral lines occur when an electron jumps from one energy level to another. Each chemical element produces its own unique pattern of spectral lines.

Example Question What two things can you do to atoms to cause electrons in the ground state to jump to the first excited state? Add Heat Add Light

Matching Questions Type of Spectrum Appearance 1. Emission Spectra a. All Colors 2. Continuous Spectra b. Dark Lines 3. Absorption Spectra c. Bright Lines

Matching Questions 1. Emission Spectra a. Hot Solids 2. Continuous Spectra b. Hot Stars 3. Absorption Spectra c. Hot Thin Gases

Spectra Absorption occurs when a photon causes an electron to jump from a low energy level to a high energy level. Emission occurs when a photon is emitted after an electron jumps from a high energy level to a low energy level.

Energy Carried by Light “High frequency radiation carries proportionally more energy than low frequency radiation”. - Page 92

Compare these spectra. What do these spectra tell us about the star? Spectrum of Hydrogen in Lab Spectrum a Star What do these spectra tell us about the star?

Measuring a Star’s Composition Each atom absorbs a unique combination of wavelengths of light. From this we can determine the composition of a star. Star’s are composed of mostly hydrogen.

Compare these spectra. What do these spectra tell us about the star? Spectrum of Hydrogen in Lab Spectrum of a Star What do these spectra tell us about the star?

Measuring a Star’s Motion The spectral lines of a star moving away the Earth exhibit a redshift. The spectral lines of a star moving toward the Earth exhibit a blueshift. These shifts are caused by the Doppler effect.

Compare these spectra. What do these spectra tell us about the star? Spectrum of Hydrogen in Lab Spectrum a Star…..Day 1 Spectrum a Star…..Day 2 Spectrum a Star…..Day 3 Spectrum a Star…..Day 4 What do these spectra tell us about the star?

Mystery Star Properties 1. The star appears as a point of light through a telescope. 2. The absorption lines appear split and move over a 4 day period. 3. The brightness of the star also varies over 4 days. Question: Why do you think the brightness of the star is varying? Answer: This could be an eclipsing binary star system that cannot be resolved by a telescope!

Matching Questions 1. The temperature of a star can be determined from its_____________. 2. The pattern of the absorption spectral lines for a star contains information about a star’s________________. 3. The Doppler shift of a star's spectral lines tells us something about the star’s_______________. (a) composition (b) motion (c) color

End of Chapter 4