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Light. Solar 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.
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Light Solar 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, & 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
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 – 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 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 … • Moving toward us? • Moving away from us?
Light & Temperature Solar 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 • 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
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
