Chapter 4Chapter 4

SpectroscopySpectroscopy

Kirchhoff’s First LawKirchhoff’s First Law

Hot, dense gases or solids produce a continuous spectrum.

• Example: Light bulb filament

Continuous Spectrum

Kirchhoff’s Second LawKirchhoff’s Second Law

Hot, rarefied gas produces an emission line spectrum.

• Example: Neon sign

Emission Line Spectrum

Kirchhoff’s Third LawKirchhoff’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 ExperimentThe 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 QuestionsMatching Questions

Type of Spectrum Appearance

1. Emission Spectra a. All Colors

2. Continuous Spectra b. Dark Lines

3. Absorption Spectra c. Bright Lines

Matching QuestionsMatching Questions

1. Emission Spectra a. Hot Solids

2. Continuous Spectra b. Hot Stars

3. Absorption Spectra c. Hot Thin Gases

SpectraSpectra 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.

hfE

Energy Carried by LightEnergy Carried by Light

“High frequency radiation carries proportionally more energy than low frequency radiation”. - Page 92

Compare these spectra.

Spectrum of Hydrogen in Lab

Spectrum a Star

What do these spectra tell us about the star?

Measuring a Star’s CompositionMeasuring 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.

Spectrum of Hydrogen in Lab

Spectrum of a Star

What do these spectra tell us about the star?

Measuring a Star’s MotionMeasuring 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.

c

v

Compare these spectra.

Spectrum of Hydrogen in Lab

Spectrum a Star…..Day 1

What do these spectra tell us about the star?

Spectrum a Star…..Day 2

Spectrum a Star…..Day 3

Spectrum a Star…..Day 4

Mystery Star PropertiesMystery Star Properties1. 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 QuestionsMatching 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