The Sun crossed the celestial equator heading south at 11:09 EDT last night. Known as an Autumnal equinox, this astronomical event marks the first day of autumn in the northern hemisphere and spring in the south.

This image of the Sun in extreme ultraviolet light was recorded yesterday with the Solar Dynamics Observatory. The false-color image shows emission from highly ionized iron atoms. Loops and arcs trace the glowing plasma suspended in magnetic fields above solar active regions.

Homework #3 is due Tuesday, Sept. 28, 5:00 pm – extra credit question due at class time that day

Exam #1, Thursday, Sept. 30

Review session: Tuesday (9/28), 7:00 pm.

Light as a Particle Light can also be treated as photons – packets of energy.

The energy carried by each photon depends on its frequency

(color)

Energy: E = hf = hc/ [“h” is called Planck’s Constant]

Shorter wavelength light carries more energy per photon.

higher energy

lower energy

The Electromagnetic Spectrum

Light as Information Bearer

The spectrum of an object can reveal the object’s:

Composition

Temperature

Velocity

Spectrum: light separated into its different wavelengths.

Spectroscopy: The quantitative analysis of spectra

Four Ways in Which Light can Interact with Matter

1. emission – matter releases energy as light

2. absorption – matter takes energy from light

3. transmission – matter allows light to pass through it

4. reflection – matter reflects light

The type of interaction is determined by characteristics of

the “matter” and the wavelength of light.

Different wavelengths

of light interact

differently with the

atmosphere

Three ways in which spectra manifest themselves:

Continuous spectra

Absorption spectra

Emission line spectra

Continuous spectra are usually related to the temperature of an object that is emitting radiation.

Absorption & emission line spectra are related to the composition of the material absorbing or emitting radiation.

Thermal Emission

A hot, dense glowing object (solid or gas) emits a continuous spectrum.

1. Hotter objects emit more total radiation per unit surface area.

2. Hotter objects have peak emissions at shorter wavelengths (they will appear “bluer”)

Rules for Thermal Emission by Opaque Objects

5000 K

3000 K

4000 K

Wavelength

Ene

rgy

emitt

ed p

er s

quar

e m

eter

The sun emits its peak radiation in the yellow portion

of the visible spectrum.

The human eye has its peak sensitivity at the same wavelength.

Coincidence?

At “room temperature”, or

“body-temperature”, objects emit their

peak radiation in the infrared.

The surface of the Earth emits

radiation in the infrared.

infrared

visible

Extremely hot objects will emit most of their radiation in the ultraviolet, x-ray or even the

gamma ray portion of the spectrum

“Matter” and Light

Atomnucleuselectron

e-

(proton,neutrons)

p+n

● 10,000,000 atoms can fit across a period in your textbook.● The nucleus is nearly 100,000 times smaller than the entire atom (if

atom filled the classroom auditorium, the nucleus would be barely visible at its center).

● Although it is the smallest part of the atom, most of the atom’s mass is contained in the nucleus.

Incorrect view

better view

Electrons do not “orbit” the nucleus; they are “smeared out” in a cloud which give the atom its size.

Atomic Number Element1 Hydrogen (H)2 Helium (He)3 Lithium (Li)4 Beryllium (Be)5 Boron (B)6 Carbon (C)7 Nitrogen (N)8 Oxygen (O)

The number of protons in the nucleus, i.e., the “atomic number”, determines the element

Hydrogene-

p+

atomic number = 1atomic mass number = number protons + neutrons = 1

Helium

e-

p+

n

e-

np+

atomic number = 2atomic mass number = number protons + neutrons = 4

Relative abundances of elements in the universe

Every element has multiple isotopes (same number of protons, different numbers of neutrons) some of which may not be stable (“radioactive”)

Carbon-14 half-life = 5,730 yrs

Hydrogen

e-

p+

n

Deuterium

isotopeof hydrogen

atomic number = 1atomic mass number = number protons + neutrons = 2

Unstable (“radioactive”) isotopes “decay”, producing a new type of atom, i.e., an atom of a different element OR a different isotope of the original element.

One half of the atoms of an unstable isotope decay in one “half-life” of that isotope.

Unstable isotopes and radioactivity

Three isotopes of Carbon, two stable, one unstable.

5730 yrs

14C 14N + electron + antineutrino + energy

Mass (14C) > Mass (14N + electron + antineutrino)

difference in mass is converted into energy: E = mc2

p+

n

e-

np+

atomic number = 2

What if an electron is missing?

ion

He+1

atomic mass number = number protons + neutrons = 4

What if two or more atoms combine to form a particle?

p+ p+

8p+

8n

molecule H2O (water)

Sharing of electrons (chemistry) is involved in the construction of

molecules

Absorption & Emission Line spectra

Electron Energy Levels

● Electrons cannot have just any energy while orbiting the nucleus.

● Only certain energy values are allowed (like the floors of an aprtment building).

● Electrons may only gain or lose certain specific amounts of energy (equal to differences in energy levels).

Electron Orbits / Absorption & Emission● Electrons can gain or lose energy while they orbit the nucleus.

● When electrons have the lowest energy possible, we say the

atom is in the ground state.

● When electrons have more energy than this, we say the atom

is in an excited state.

● When electrons gain enough energy to escape the nucleus,

we say the atom is ionized.

• Each element has its own distinctive set of energy levels for its electrons.

• This diagram depicts the energy levels of Hydrogen.

1 eV = 1.60 x 10-19 joules

Emission/Absorption Spectra

• Each electron is only allowed to have certain energies in an atom.

• Electrons can absorb light and gain energy or emit light when they lose energy.

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

Hydrogen

Kirchhoff’s Laws #2

2. A hot, low density gas emits light of only certain wavelengths – an emission line spectrum.

Kirchhoff’s Law #3

3. When light having a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum – an absorption line spectrum.

Absorption Spectra

• If light shines through a gas, each element will absorb those photons whose energy match their electron energy levels.

● The resulting absorption line spectrum has all colors minus those that were absorbed.

• We can determine which elements are present in an object by identifying emission & absorption lines.

Molecules have rotational & vibrational energy levels

(less energetic than electron energy levels, energies correspond with infrared, microwave, and radio radiations)

The Doppler Shift:

A shift in wavelength due to a wave emitter moving towards (shorter wavelength) or away (longer wavelength) from an observer.

v c

=

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.

BLUESHIFTBLUESHIFT

REDSHIFTREDSHIFT

Measuring Radial Velocity● We can measure the Doppler

shift of emission or absorption lines in the spectrum of an astronomical object.

● We can then calculate the velocity of the object in the direction either towards or away from Earth. (radial velocity)

v c

=

Measuring Rotational Velocity