[PPT] - Light and Atoms Our goals for learning: Light interacts with atoms PowerPoint Presentation

SLIDE 1

ASTR 1120 General Astronomy: Stars & Galaxies

ATH REVIEW:

Tonight, 5-6pm, in RAMY N1B23

Due THU, Sept. 10, by 5pm, on

Mastering Astronomy

OMEWORK #1

CLASS RECORDED STARTED - INFO WILL BE POSTED on CULEARN

ASTR 1120 General Astronomy: Stars & Galaxies

AST CLASS:

Light: general properties
Matter: general properties
Matter: general properties (cont.)
Interaction between light and matter

ODAY:

Light and Atoms

Light interacts with atoms in specific

ways

Allows us to measure properties of the

gas such as composition & temperature

The key: the spectrum of an object

(intensity as a function of wavelength)

How can we use spectral lines to determine

the composition of a distant object?

How can we determine the temperature of

distant objects?

Can we use spectra to tell us how fast

something is moving?

Our goals for learning:

SLIDE 2

Energy Levels in Atoms

Electrons in atoms do NOT “orbit” around the nucleus like little planets - their position better described by probability waves However, they do move in different “energy states” – some electrons in a given atom have more energy than others These energy states are “quantized”– there are only certain energies that the electrons are allowed to

have. This is quantum physics.

Example of electron energy states in a hydrogen atom

Lower level is lower

energy.

Units: 1 electron-

volt (eV) =

1.6 x 10-19 Joules =

TINY Each electron in each element has its

wn particular

pattern of energy levels: elemental fingerprints!

Electrons can move between levels if they are given or give out the exact amount of energy corresponding to the difference in the energy levels. For hydrogen, if an electron at level 1 (Ground state) is given more than 13.6 eV of energy, the electron will fly free (ionize)

Example: Energy jumps A, B and C allowed; D is not possible for this

atom. E ionizes the atom with an

energy gain of >3.4 eV

How do electrons move between levels?

Where does that energy come from (energy increase) or go to (energy decrease)??

The energy change

between levels is equal to the energy

f the photon.
Larger energy jumps

will be SHORTER wavelength photons!

PHOTONS!

SLIDE 3

Emission Spectra

Emission for thin, hot gas where electrons are “excited” (in high

energy states). Gas glows in specific colors.

– This is our FINGERPRINT of the elements in the gas!

Will eventually lose thermal energy through emitting photons,

and cool!

Each atom has a different set of energy levels

different spectrum

spectra using a diffraction grating

Spectrum shows bright emission lines from various elements The Crab nebula: remains of an exploded star (supernova)

Most common visible light emission line:

Hydrogen Alpha (H) ^
N=3 to n=2 energy jump

at 656.3 nm

The universe is mostly red!!

SLIDE 4

Continuous Spectrum

Hot solids/liquids/dense gases emit a

continuous rainbow of light

– Blackbody Radiation

Absorption Spectrum

If light with a continuous spectrum shines through a cloud of

COOL gas with electrons in low-energy states, the gas can absorb photons OF THE RIGHT ENERGIES to move electrons to excited states

Resulting spectrum shows DARK LINES of

absorption.

– Corresponds to wavelengths where the atom has absorbed a photon and excited an electron to a higher energy state

Why don’t we see those atoms re-emit the same

photon when they de-excite?

– Atoms WILL emit these photons again and electrons fall back to ground state, BUT photons will be scattered in all directions and so most will be lost from

ur sight

What causes spectral lines?

A. Black body radiation.
B. Electron energy transitions in the atom.
C. The Doppler shift of moving objects.
D. High frequency electromagnetic waves.
E. Protons and neutrons spinning in an

atom. Clicker Question

SLIDE 5

What causes spectral lines?

A. Black body radiation.
B. Electron energy transitions in the atom.
C. The Doppler shift of moving objects.
D. High frequency electromagnetic waves.
E. Protons and neutrons spinning in an

atom. Clicker Question

What do we see at position 1?

Clicker Question

A. Absorption

Line Spectrum

B. Continuous

Spectrum C.Emission Line Spectrum

1 2 3

What do we see at position 2?

Clicker Question

A. Absorption

Line Spectrum

B. Continuous

Spectrum C.Emission Line Spectrum

1 2 3

What do we see at position 3?

Clicker Question

A. Absorption

Line Spectrum

B. Continuous

Spectrum C.Emission Line Spectrum

1 2 3

SLIDE 6

Kirchoff’s Laws

1) Hot solid, liquid,

r dense gas

(continuum spectrum) 2) Continuous spectrum viewed through a cooler gas (absorption line spectrum) 3) Thin, hot gas (emission line spectrum)

1 2 3

Solar Spectrum (as seen from Earth) Where could the dark lines in the Solar spectrum be coming from?

A. Absorption in the Sun’s atmosphere
B. Emission from the Sun’s atmosphere
C. Absorption in the interior of the Sun
D. Emission from the interior of the Sun
E. Absorption by the glass mirrors in the

telescope used to collect the light Clicker Question

Where could the dark lines in the Solar spectrum be coming from?

A. Absorption in the Sun’s atmosphere
B. Emission from the Sun’s atmosphere
C. Absorption in the interior of the Sun
D. Emission from the interior of the Sun
E. Absorption by the glass mirrors in the

telescope used to collect the light Clicker Question

SLIDE 7

16 4 million Year-old Cluster + Ionized Nebula + Surviving cloud

1. Hotter objects emit more total radiation per

unit surface area.

Stephan-Boltzmann Law

E is proportional to T4

2. Hotter objects emit bluer photons (with a

higher average energy.)

Wien Law max = 2.9 x 106 / T(Kelvin) [nm]

Rules for Emission by Blackbody Objects uman seen wi an ina-red camer

SLIDE 8

Which is the hottest star? One that appears:

A. Red
B. Yellow
C. Blue
D. White
E. They are all the same temperature.

They just look different colors Clicker Question

Which is the hottest star? One that appears:

A. Red
B. Yellow
C. Blue
D. White
E. They are all the same temperature.

They just look different colors Clicker Question

Quick guide to thermal spectra (be familiar with these)

3 K (coldest natural things): max = 1mm

= Microwaves

300 K (people, planets, warm dust): max = 10-5 m

= 10,000 nm, IR

3000-30,000K (stars): max = 10-6 m to 10-7 m

= 1000 to 100 nm, IR – visible – UV

300,000- 30,000,000K: weird and intense places

(UV through X-rays/gamma rays)

What is this object?

Let’s use its spectral information to determine

what it is.

SLIDE 9

What is this object?

Reflected Sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed - object must look red

What is this object?

Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K

What is this object?

Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere

What is this object?

Ultraviolet Emission Lines: Indicate a hot upper atmosphere

SLIDE 10

What is this object? Mars!

Measuring velocities without a stopwatch: the Doppler Shift

Familiar shift in pitch
f SOUND: higher

when approaching, lower when receding

Similar shift in

frequency of light: higher frequency (blueshift) when approaching, lower frequency (redshift) when receding

Most easily used

with absorption

r emission lines

where you know the zero-velocity (rest) wavelengths. Then, measure redshift or blueshift to get the velocity away or towards you.

Reading/Assignment

Ch. 5 sec. 5.3, 5.4, 5.5
Homework #1 on Mastering Astronomy