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Unformatted text preview: Review Ch 4: On Orbits
• Orbital energy is conserved.
• Orbits cannot change spontaneously.
• An object’s orbit can only change if it
gains or loses energy:
– Gravitational encounter.
– Atmospheric drag. } Light: The Cosmic Messenger
How to use light to learn what a star is
made out of and how hot it is Examples What is light? Solar Spectrum A Spectrum Spectra Intro to spectroscopy 1 A Spectrum Light is a
peaks Plot of light intensity vs. wavelength Light is a
Wave Frequency: the
number of times
per second the
wave bobs up &
down (Hertz) Light is a
wave: The speed of light
in a vacuum is a
constant: Light interacts with
and magnets c=300,000 km/sec Wavelength x
Frequency = c The Electromagnetic Spectrum Light is a Particle
Photons: particles of light.
wavelength, frequency, and energy.
Bluer light, shorter wavelength, higher frequency, &
higher energy. Visible Gamma
Rays UV IR Radio Redder light, longer wavelength, lower frequency, &
lower energy. Electromagnetic_spectrum 2 What is matter? Elements and Atomic Structure
• Elements: examples H, He, C, N, O.
• Elements are made of atoms.
– Protons (+) & Neutrons (0) = Nucleus
– Electrons (-) Atomic Terms
• Atomic Number: # of protons in nucleus
• Atomic Mass Number: # of protons and
neutrons in nucleus (4 He)
• Isotope: Same atomic #, different mass #. (4He
and 3 He) Molecules: two or more bound atoms (H2 O, CO2 ) Ion: an charged atom (# of protons not = # of
electrons) How do light and matter
interact? Matter How do light & matter
• Light Emission
The Sun, electric lights, you
Anything opaque, you
Windows, glass ﬁlters
Reflection or Scattering Transparent vs. Opaque
• Transparent: transmits light
• Opaque: blocks (absorbs or reflects) light You, movie screen, walls,
mirror 3 What is color?
An object reflects
light and absorbs,
What we see
depends on our
eyes & brain. Why is the rose red?
d) The rose absorbs most light.
The rose transmits some light.
The rose emits infrared light.
The rose reflects red light. HINT: all of these statements
are true. What types of light can we
observe? Types of Light Spectra
4. Thermal radiation (continuous rainbow)
Absorption lines (dark lines on rainbow)
Emission lines (bright lines)
Combinations of the above How does light tell us what
things are made of? (Production of absorption lines anim) 4 Electrons and Light Interaction of Light with Matter
absorbed. Hydrogen Chemical Fingerprints Hydrogen
Levels • Only specific
• Absorption or
Emission Solar Spectrum • Every atom, ion, and molecule has a
unique spectral “fingerprint” Composition mystery gas A Spectrum
How does light tell us the
temperatures of planets and
stars? 5 Thermal Radiation Thermal Radiation
• All everyday objects emit it (including you)
• Only diffuse, almost transparent objects
don’t emit it.
• The spectrum depends on only one
property: temperature Temperature (repeat)
Kelvins (K): temperature unit
T (in K) is directly proportional to the
random Kinetic Energy of the gas particles
“Absolute zero” means zero motion Thermal Radiation Light
area Wienʼs law
animation Two Properties of Thermal
1. Hotter objects emit more light at all
frequencies per unit area.
2. Hotter objects emit photons with a
higher average energy. Summary (Ch 5, part 1)
• Properties of light: order of the EM spectrum,
relationships between wavelength, frequency,
• Properties of matter and elements and atoms
• How light and matter interact.
• How to identify elements with emission or
• What thermal radiation is and how to estimate
the temperature from the thermal radiation. 6 ...
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- Fall '08