Assignments •For Mon.

– Do Practice online quiz 03

– Next week – Lab is: Lab04-Mars-Season.pdf

– Also complete Moon-Observing.pdf

•1st Midterm is Friday, Oct. 16

Moon-Observing.pdf

Chapter 5 Light: The Cosmic Messenger

Wavelength, Frequency, and Energy

λ × f = c

λ = wavelength, f = frequency c = speed of light = 300,000 km/s

E = h × f = photon energy h = 6.626 × 10-34 joule × s = photon energy

Interaction of Light with Matter

• Electron can only have certain energies in an atom.

• Electrons can – absorb light & gain energy – emit light & lose energy.

• Only photons with energies (colors) matching the “jump” in electron energy levels can be emitted or absorbed.

Hydrogen

Kirchoff’s laws

1. hot solid/dense-gas: continuous spectrum

2. hot gas: emission line spectrum

3. cold gas + continuous source: absorption line spectrum

Spectral lines & Doppler shift

• Motion of atoms shifts frequency by Doppler effect

• Atoms of a gas absorb & emit light at discrete frequencies

The Doppler Effect

1. Light emitted from an object moving towards you will have its wavelength shortened.

2. Light emitted from an object moving away from you will have its wavelength lengthened.

3. Light emitted from an object moving perpendicular to your line-of-sight will not change its wavelength.

BLUESHIFT

REDSHIFT

The Doppler Effect

observe infer

A star’s spectral lines are shifted to longer wavelengths than measured in the lab. This implies that the star must be

A. Hotter than the sun B. Colder than the sun C. Moving away from us D. Moving toward us

A star’s spectral lines are shifted to longer wavelengths than measured in the lab. This implies that the star must be

A. Hotter than the sun B. Colder than the sun C. Moving away from us D. Moving toward us

A spectral line with rest wavelength 600.00 nm is observed in a star to be at a wavelength 600.06 nm. What fraction of the speed of light is the star moving relative to us?

A. 10-1

B. 10-2

C. 10-3

D. 10-4

A spectral line with rest wavelength 600.00 nm is observed in a star to be at a wavelength 600.06 nm. What fraction of the speed of light is the star moving relative to us?

A. 10-1

B. 10-2

C. 10-3

D. 10-4

A spectral line with rest wavelength 600.00 nm is observed in a star to be at a wavelength 600.06 nm. Given that the speed of light is c=3x105 km/s, what is the star’s speed toward or away from us?

A. 30 km/s toward us

B. 30 km/s away from us

C. 60 km/s toward us

D. 60 km/s away from us

A spectral line with rest wavelength 600.00 nm is observed in a star to be at a wavelength 600.06 nm. Given that the speed of light is c=3x105 km/s, what is the star’s speed toward or away from us?

A. 30 km/s toward us

B. 30 km/s away from us

C. 60 km/s toward us

D. 60 km/s away from us

How does light tell us the temperatures of planets and stars?

Thermal Radiation

• Nearly all large or dense objects emit thermal radiation, including stars, planets, and you.

• An object’s thermal radiation spectrum depends on only one property: its temperature.

Thermal Radiation1. Hotter objects emit photons with a higher average

energy. 2. Hotter objects emit more light at all frequencies per unit area.

Thought Question

Which is hottest? A. A blue star B. A red star C. A blue planet like the Earth D. A red planet like Mars

Thought Question

Which is hottest? A. A blue star B. A red star C. A blue planet like the Earth D. A red planet like Mars

Temperature dependence of wavelength at peak intensity

λmax = λmax,sunTsunT

= 500nm 6000KT

observed colordepends on temperature

Wien’s law

A star’s spectrum has it peak at a wavelength of 250 nm. About what is its surface temperature?

A. 1500 K B. 3000 K C. 6000 K D. 12000 K

A star’s spectrum has it peak at a wavelength of 250 nm. About what is its surface temperature?

A. 1500 K B. 3000 K C. 6000 K D. 12000 K

T = Tsunλpeak ,sun

λpeak

= 6000K 500nm250nm

= 12000K

Temperature dependence of total energy flux

F =σ T 4

flux=power/areadepends on temperature

Stefan-Boltzman law

σ = Stefan-Boltzman constant

Luminosity

L = F A=σT 4 4πR2luminosity=total power=flux*area depends on temperature and radius

LLsun

=TTsun⎛

⎝⎜⎞

⎠⎟

4RRsun

⎛

⎝⎜⎞

⎠⎟

2

Using the sun as reference gives: