1. Review Waves

Frequency and wavelength are inversely related

1/

(a) higher frequency shorter wavelengths

In terms of sound: higher pitch

In terms of light: bluer color

(b) lower frequency longer wavelengths

In terms of sound: lower pitch

In terms of light: redder color

Components of a wave:

shape

amplitude

frequency () or wavelength ()

speed of propagation ( c )

2. Doppler Shift

You experience it almost every day with sound

When does it happen?

whenever waves are being emitted from a source,
(light waves or sound waves)

and ...

there is some component of radial motion between the source and observer.
(some portion of the motion is straight towards you or away from you)

Why does it happen?

If the observer moves away from the source, because all waves (light, sounds, or other) have a finite speed, it takes longer for a full wavelength to get to you.

the frequency goes down

the wavelength appears to get longer

`` REDSHIFT ''

If the observer moves towards the source, it takes less time for a full wavelength to get to you.

the frequency goes up

the wavelength appears to get shorter

`` BLUESHIFT ''

3. Continuum and discrete (line) radiation and absorption

How can you tell if something is a blackbody?

Kirchhoff's laws

1. A luminous solid or liquid (or dense gas) emits light at all wavelengths. This produces a continuous spectrum of radiation

Why? Because such material is close to being a blackbody!

However, does all continuum radiation have a blackbody spectrum (Planck spectrum)? Even if we ignore line emission and absorption?

2. A low-density gas emits light at discrete wavelengths (or frequencies) that are called emission lines

3. A low-density gas absorbs light from a continuum source at the same discrete wavelengths. This produces absorption lines

The specific type of radiation you see depends on:

the type of material

the geometry

the environment

In astronomy, frequently one observes both continuum, absorption, and emission line spectra all at once.

4. Why there is discrete (line) radiation

Emission and Absorption

Analogy with gravity and orbits:

atomic nucleus as the sun

electrons as the planets

BUT! quantum mechanics:

energy comes in discrete packets or quanta

the energy levels of atoms and molecules are discrete
(electrons can't be in just any 'orbit' around the nucleus)

Whenever there is an energy transition, light (a photon) is either given off (emitted), or absorbed by the electron.

Emission: electron emits photon, and moves to a lower (less energetic) orbit

Absorption: electron absorbs a photon, and moves to a higher (more energetic) orbit.

If the photon has enough energy, it may even 'ionize' the atom, i.e. give the electron enough energy to 'escape' from the nucleus.

5. Putting it all together

temperature - from the shape of the spectrum, if it's a blackbody

composition - from the types of emission and absorption lines

``characteristic fingerprint''

geometry - emission or absorption lines

temperature and density - relative strengths of various emission or absorption lines

distance - doppler shift of identified emission or absorption lines

Why does this work?

When does this work?

I.e., when and why are distance and velocity related?

(a) a blackbody spectrum

(b) a Planck spectrum with absorption lines superimposed

(c) a blackbody spectrum with emission lines superimposed

(d) a Planck spectrum

(e) absorption lines

(a) Kirchhoff

(b) Wien

(c) Stefan

(d) Doppler

(e) Planck