Light Solar System Astronomy Chapter 4. Light & Matter Light tells us about matter Almost all...

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