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

Wavelength, Frequency, & Speed Speed of light in a vacuum 3×10 8 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 – 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 & 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 (m 2 ) Hotter objects emit MUCH MORE ENERGY

Wien’s Law Temperature relates to λ peak 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