Class Update• D2L Quiz 5 available; Quiz 6 on Tuesday; Questions on Test 2• Observations

• Moon Phases• Due Tuesday, Mar. 3 (2 weeks)

• Friday, Feb. 20• 8-9:30pm University of Minnesota (Telesc. & Star Gazing [no Moon])

• Sunday, Feb. 22 (Dress Warmly – hat, gloves)• 7:30-10pm Macalester College (Telescopes, Moon Craters, & Star

Gazing with Raquel)• Email to reserve your spot in my car if you want a ride to and from

MCTC. Meet at Café Espresso Royale at 6:45pm.• Macalester Olin Science Building 4th floor, southwest corner• Street Parking off side streets of Snelling (Osceola & Fairmont)

• No class Thu, Feb. 26 for Student Success Day – Please attend• Mars and Venus near Moon Friday night (show in Starry Night)

– Venus brighter than Mars

Blackbody Radiation

Atoms and Light

1

3

2

Spectra

Blackbody Radiation

• Definition:

• Radiation emitted by a heated object• When not heated, the object is black

• When heated, the object varies in color depending on the temperature

• Continuous spectrum

• A black body absorbs and emits radiation

Spectra

Blackbody Radiation

Examples:

• Incandescent light bulb

• Filament changes color as it is heated

• Hot lava

• Sun and other stars

• Heated metal

• Quickly heated ice cube glows

Light Bulb Filament

Solve:Are there vibrations of the atoms in the filament?

1. Are there many atoms or a few atoms in the metal filament?

MANY ATOMS

1. MANY ATOMS in the metal filament.

2. True or False: Each atom vibrates at some frequency. TRUE

Light Bulb Filament

Solve:Are there vibrations of the atoms in the filament?

1. MANY ATOMS in the metal filament.

2. Each atom VIBRATES at some frequency.

Light Bulb Filament

Solve:Are there vibrations of the atoms in the filament?

3. True or False: All atoms vibrate at the same frequency.

FALSE Not all vibrate at the same frequency Many different frequencies

1. MANY ATOMS in the metal filament.

2. Each atom VIBRATES at some frequency.

Light Bulb Filament

Solve:Are there vibrations of the atoms in the filament?

3. Atoms vibrate at the DIFFERENT frequencies.

Different frequencies correspond to different wavelengths and display different colors.Blue light = 400 nmRed light = 700 nm

Spectra

Blackbody Radiation

Examples:http://webphysics.davidson.edu/alumni/milee/java/bb_mjl.htm

Side Note:

Intensity = Energy/surface area

Temperature α Energy

Which star is hottest? How do you know?

Hotter object has: More overall radiation (intensity) Higher peak

Which star is hottest? How do you know?

Spectrum of the Sun

What do you know from this spectrum?

Absorption – hot dense object with a gas cloud

Mix of gases (but which ones?)

SpectraSpectra

Other stars show similar but not identical line patterns.

So why are there lines in the spectra at all?

What do those lines mean?

Spectra of stars

(differences)

Intensity difference

Line differences are element differences

Spectra

Annie Cannon• Harvard, turn of the last century• Sorted thousands of stars by their patterns of

spectra lines.• Discovered patterns among star spectra• Lines are different for different elements

λs?, visible?, Temperatures?

Longer λUV

Hotter

Cooler

SpectraAnnie Cannon

O

B

A

F

G

M

K

{{{{{{

Hotter

Cooler

Lines used to categorize wavelength & temperature

Still used today and extended past M for darker red and brown stars.

Blue

Red

Spectra

SpectraSo why are there lines in the spectra at all?

And What do those lines mean?

Spectra

Definitions

• Atom– Basic unit of a chemical element– Has protons, neutrons, & electrons

• Element– More than 100 substances that cannot

be broken down any simpler• Molecule

– Group of atoms

http://facstaff.gpc.edu/~pgore/PhysicalScience/atom-with-electrons.gif

Spectra

Atoms

• Protons – positive charge ( + ); larger mass• Neutrons – neutral, no charge; larger mass• Electrons – negative charge ( - ); smaller mass • Nucleus

– protons and neutrons• Surrounded by

– “orbiting” electrons

http://facstaff.gpc.edu/~pgore/PhysicalScience/atom-with-electrons.gif

Quantum MechanicsStudy of atoms and subatomic particles

1. Electrons have certain “orbits” and not others

2. Orbits are closer together as you get further from the nucleus

Ground state -Lowest energy levelBohr

Model

Quantum Mechanics

When an electron jumps “down”, it emits all the energy at once in a bundle called a photon (the energy is quantized).

If an electron absorbs a photon of just the right energy, it jumps “up”.

Absorb = take in (ex. Increase energy)Emit = give off (ex. Decrease energy)

Of the electron jumps shown, which number of jump EMITS (gives off) a photon?

2 and 3Decrease in energy

Of the electron jumps shown, which number of jump ABSORBS (takes in) a photon?

1Increase in energy

For the atom shown, a blue and a green photon are emitted and a red photon is absorbed. Which jump corresponds to emitting a blue photon? Which jump corresponds to emitting a green photon? Which jump corresponds to absorbing a red photon?

For the atom shown, a blue and a green photon are emitted and a red photon is absorbed. Which jump corresponds to emitting a blue photon? 2 Which jump corresponds to emitting a green photon? 3 Which jump corresponds to absorbing a red photon? 1

Cecilia Payne-Gaposchkin

• Surveyed star spectra• Discovered stars are about 90% hydrogen

and 10% helium