light solar system astronomy chapter 4. light & matter light tells us about matter almost all...
TRANSCRIPT
LightSolar System Astronomy
Chapter 4
Light & Matter Light tells us about matter
Almost all the information we receive from space is in the form of light.
The light can tell us the conditions of objects in space – temperature, composition, motions, etc.
Light has many strange properties which stretch our ideas of what is “real.”
Light: Wave or Particle? Light can be both like waves and like
particles (photons) The particle picture is helpful when thinking
about how light is absorbed and emitted The wave picture is best for describing how
light gets from one place to another
Light as a Wave Light is a wave of electric and magnetic
fields – electromagnetic (EM) wave The wavelength (λ) is the length between
crests of the wave The frequency (f) is the number of waves that
pass by each second Different types of light (visible, infrared) have
different wavelengths
Wavelength & Frequency
Wavelength, Frequency, & Speed Speed of light in a vacuum
3×108 m/s (300,000 km/s) Travels more slowly through materials like glass or
water Wavelength and frequency are related:
Visible Light Our eyes see a small range of EM
radiation Red light λ = 700 nm Violet light λ = 400 nm Spectrum:
ROY G BIV
Visible Light Our eyes see a small range of EM
radiation Red light λ = 700 nm Violet light λ = 400 nm Spectrum:
ROY G BIV
Electromagnetic Spectrum Visible light is just a small part…
Electromagnetic Spectrum Visible light is just a small part…
Hydrogen Spectrum A Big Mystery
Kirchoff’s Laws of Radiation
Light as a particle “Photons”
Little packets of energy Atoms can absorb or emit Photons can carry different
amounts of energy High energy
short wavelength high frequency
Low energy long wavelength low frequency
Atoms & Light Neils Bohr
Electrons surround/orbit nucleus can have certain energies;
other energies are not allowed. Each type of atom (carbon, oxygen,
etc.) has a unique set of energies. A good way to represent the energies
is with an energy level diagram.
Atoms & Light
Explained hydrogen spectrum precisely!
Atoms & Light Spectroscopy
Atoms & Light Mystery Gas…
Doppler Effect
Doppler Effect
Doppler Effect – Radial Velocity Radial velocity is part toward or away
from observer Along line of sight Toward gives shorter wavelengths – redshifted Away gives longer wavelengths -- blushifted
Doppler Effect
Doppler Shift – Concept Quiz Hydrogen emits light at λ = 656 nm. You
see a distant galaxy in which the light from hydrogen has λ = 696 nm. Is this galaxy …a) Moving toward us?b) Moving away from us?
Light & TemperatureSolar System Astronomy
… still Chapter 4
Emitted Light All objects emit light (or EM radiation)
What kind depends on temperature and state (solid or gas)
Light carries off energy Rate of loss is called luminosity
Energy Balance Planet’s temperature is a balance
Energy absorbed from sun Energy emitted from planet
From temperature Temperature is a measure of heat radiated
Balance is example of thermal equilibrium Very important concept in many areas of
astrophysics
Temperature All atoms are constantly in thermal motion
Temperature is a measure of average speed (kinetic energy) of atomic motion
Measure in Kelvin Water freezes/boils 273 K/373 K Minimum possible 0 K Sun is 5800 K (10,000˚ F)
Temperature Measure of energy/motion
More temperature is more motion More pressure increases motion and
temperature
Temperature & pressure are CLOSELY linked
Blackbody Radiation Dense objects emit radiation
Blackbody radiation Thermal radiation Continuous radiation/spectrum
For two objects of same size Hotter emits more light at all wavelengths Emit more total energy per second (higher
luminosity) More of the radiation is at shorter wavelengths
Blackbody Radiation
Blackbody Radiation
Blackbody Radiation Of note:
Some light is emitted at all wavelengths Often a negligible amount
Little very short or very long wavelengths There is a peak wavelength
Stefan’s Law Flux is the total energy emitter per area
(m2) Hotter objects emit MUCH MORE ENERGY
Wien’s Law Temperature relates to λpeak
Hotter means bluer Simple measurement to calculate temperature
Brightness Amount of light that arrives at a particular
place Inverse-square law
Equilibrium Temperature Equilibrium reached when
Energy absorbed equals energy emitted Distant planets are cold mainly because of
inverse-square law Actual temperature depends on how well
planet absorbs incoming light albedo
Equilibrium Temperature
Concept Test If the Sun got hotter, which of the
following would be true? The flux from the Sun would increase The peak of its spectrum would shift to redder
colors The brightness at the Earth would decrease
Concept Test Compared to the brightness of the Sun at
the Earth, the brightness at ½ AU would be ¼ as much ½ as much The same as now Twice as much Four times as much
Concept Test In the distant future the Sun will be cooler
but will emit far more energy every second than it does now. What will happen to the Earth’s temperature? It will be hotter It will be the same It will be cooler