Ch.5 Light and Matter

Photon

Massless packet of electromagnetic energy with wave properties.

Wavelength

Distance between successive wave crests.

Frequency

Number of wave cycles per second.

Wavelength–frequency relation

Shorter wavelength means higher frequency.

Photon energy

Higher-frequency, shorter-wavelength photons carry more energy.

Electromagnetic spectrum order

Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray.

Visible color order (long to short wavelength)

Red, orange, yellow, green, blue, violet.

Radiation in astronomy

Electromagnetic light, including invisible wavelengths.

Kelvin

Absolute temperature scale used in astronomy.

Absolute zero

0 K, the lowest thermodynamic temperature.

Luminosity

Total energy an object emits per second; intrinsic brightness.

Flux / apparent brightness

Light received per area per time; depends on distance.

Inverse-square brightness law

Twice as far means one-quarter the observed flux.

Blackbody

Ideal thermal emitter with a continuous spectrum set by temperature.

Wien's law

Hotter objects peak at shorter wavelengths.

Stefan–Boltzmann law

Emitted power per surface area rises as T to the fourth power.

Double temperature, same area

Luminosity increases 16-fold.

Double radius, same temperature

Luminosity increases fourfold.

Hotter star color

Generally bluer than a cooler star.

Why do humans emit infrared?

Body temperature gives thermal radiation peaking in infrared.

Continuous spectrum

Unbroken range of wavelengths from a hot dense emitter.

Emission-line spectrum

Bright lines produced when atoms emit specific energies.

Absorption-line spectrum

Dark lines where cooler gas absorbs selected wavelengths.

Electron energy levels

Discrete allowed electron energies within atoms.

Emission transition

Electron falls to lower energy and emits photon.

Absorption transition

Electron absorbs photon and moves to higher energy.

What do spectral lines reveal?

Chemical composition and physical conditions of gas.

Ion

Atom that has gained or lost electrons.

Doppler effect

Observed wavelength changes due to line-of-sight motion.

Redshift

Longer observed wavelength; source moving away in simple Doppler case.

Blueshift

Shorter observed wavelength; source moving toward observer.

Motion perpendicular to sightline

No classical line-of-sight Doppler shift.

Rest line 600 nm observed at 594 nm

Blueshift of 1%; object approaching at about 0.01c.

Spectrograph

Instrument that separates light by wavelength.

Why do stars have absorption lines?

Cooler outer gas absorbs specific wavelengths from continuous light.