Chapter 2 (My Take)

It’s what we see

(or don’t see for that matter)

that counts

Astro 101(including the scientific method)

in a nutshell….

Chapter 2Light and Matter

Units of Chapter 2

Information from the Skies

Waves in What?

The Electromagnetic Spectrum

Thermal Radiation

Spectroscopy

The Formation of Spectral Lines

The Doppler Effect

Summary of Chapter 2

2.1 Information from the Skies

Electromagnetic radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields

Example: Light

Wave motion: Transmission of energy without the physical transport of material

Example: Water wave

Water just moves up and down.

Wave travels and can transmit energy.

Frequency: Number of wave crests that pass a given point per second

Period: Time between passage of successive crests

Relationship:

Period = 1 / Frequency

Wavelength: Distance between successive crests

Velocity: Speed at which crests move

Relationship:

Velocity = Wavelength / Period

2.2 Waves in What?

Diffraction: The bending of a wave around an obstacle

Interference: The sum of two waves; may be larger or smaller than the original waves

Water waves, sound waves, and so on, travel in a medium (water, air, …).

Electromagnetic waves need no medium.

Created by accelerating charged particles

Magnetic and electric fields are inextricably intertwined.

A magnetic field, such as the Earth’s shown here, exerts a force on a moving charged particle.

Electromagnetic waves: Oscillating electric and magnetic fields; changing electric field creates magnetic field, and vice versa

2.3 The Electromagnetic Spectrum

The visible spectrum is only a small part of the total electromagnetic spectrum.

Different colors of light are distinguished by their frequency and wavelength.

Different parts of the full electromagnetic spectrum have different names, but there is no limit on possible wavelengths.

Note that the atmosphere is only transparent at a few wavelengths – the visible, the near infrared, and the part of the radio spectrum with frequencies higher than the AM band. This means that our atmosphere is absorbing a lot of the electromagnetic radiation impinging on it, and also that astronomy at other wavelengths must be done above the atmosphere.

Also note that the horizontal scale is logarithmic – each tick is a factor of 10 smaller or larger than the next one. This allows the display of the longest and shortest wavelengths on the same plot.

2.4 Thermal Radiation

Blackbody spectrum: Radiation emitted by an object depending only on its temperature

More Precisely 2-1: The Kelvin Temperature Scale

Kelvin temperature scale:

• All thermal motion ceases at 0 K.

• Water freezes at 273 K and boils at 373 K.

Radiation laws:

1. Peak wavelength is inversely proportional to temperature.

Radiation laws:

2. Total energy emitted is proportional to fourth power of temperature.

Compared to optical photons

A. Radio photons have longer wavelength.B. X-ray photons have a larger frequency.C. Infrared photons have a smaller energy.D. All of the above.E. None of the above.

Which of the following has the LONGEST wavelength?

A. Red light.B. Blue light.C. Green light.D. Infrared light.

If Star A is hotter than Star B, and Star A is emitting most of its light at a wavelength corresponding to yellow light, which of the following statements is true?

A. Star B will emit most of its light at a wavelength longer than yellow.

B. Star B will emit most of its light at a wavelength shorter than yellow.

C. Star B will emit most of its light at the same wavelength as Star A.

D. More information is required to answer this question

The energy of a photon is

A. proportional to the wavelength and inversely proportional to the frequency.

B. proportional to the wavelength and proportional to the frequency.

C. inversely proportional to the wavelength and inversely proportional to the frequency.

D. inversely proportional to the wavelength and proportional to the frequency.

Electromagnetic radiation will be created by any charged particle that

A. accelerates (changes speed or direction).

B. moves in a straight line at constant speed.

C. remains at rest.D. is subjected to a gravitational field.

2.5 Spectroscopy

Spectroscope: Splits light into component colors

Emission lines: Single frequencies emitted by particular atoms

Emission spectrum can be used to identify elements.

Absorption spectrum: If a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emit.

Absorption spectrum of the Sun

Kirchhoff’s laws:

• Luminous solid, liquid, or dense gas produces continuous spectrum.

• Low-density hot gas produces emission spectrum.

• Continuous spectrum incident on cool, thin gas produces absorption spectrum.

Kirchhoff’s laws illustrated

Existence of spectral lines required new model of atom, so that only certain amounts of energy could be emitted or absorbed.

Bohr model had certain allowed orbits for electron.

2.6 The Formation of Spectral Lines

Emission energies correspond to energy differences between allowed levels.

Modern model has electron “cloud” rather than orbit.

Atomic excitation leads to emission.

(a) Direct decay

(b) Cascade

Absorption spectrum: Created when atoms absorb photons of right energy for excitation

Multielectron atoms: Much more complicated spectra, many more possible states

Ionization changes energy levels.

Molecular spectra are much more complex than atomic spectra, even for hydrogen.

(a) Molecular hydrogen (b) Atomic hydrogen

Spectral lines unique to each type of atom are caused by

A. each atom having a unique set of protons.

B. the unique sets of electron orbits.C. the neutron-electron interaction being

unique for each atom.D. each type of photon emitted by the

atom being unique.E. none of the above; spectral lines are

not unique to each type of atom.

If an electron moves from a lower energy level to the next higher energy level, then

A. the atom has become excited.B. the atom has become ionized.C. the atom's light will be blue shifted.D. the atom's light will be red shifted.

In general, the observed spectra of stars appear as what kind of spectrum?

A. Absorption.B. Continuous.C. Emission.D. Nonthermal.

If you view the light from an opaque, hot gas through a spectroscope, you would expect to see what kind of spectrum? A. Continuous.B. Emission line.C. Absorption line.D. Combination of emission line and

continuous.

When an atom is excited, then it has: A. more electrons than protons.B. the same number of electrons as

protons.C. one or more electrons stripped off.D. one or more electrons move to higher

energy levels.

When you see a spectrum with absorption lines in it, you can infer that:

A. the light passed through ionized atoms.B. electrons moved up in energy levels to

absorb the light.C. electrons moved down in energy levels

to absorb the light.D. all the atoms were in excited states.

2.7 The Doppler Effect

If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer.

Relationship between frequency and speed:

Depends only on the relative motion of source and observer

The Doppler effect shifts an object’s entire spectrum either toward the red or toward the blue.

If a star is moving towards us, its speed can be determined by measuring it’s

A. red shift.B. Doppler shift.C. parallax shift.D. average photon speed.

The Doppler effect

A. is a measure of a star's velocity in space.

B. is a shift of the star's spectrum which depends on its velocity in the line of sight.

C. is a shift in the stars apparent position with respect to background stars.

D. is a shift in the star's spectrum which depends on its temperature.

If a yellow spectral line is Doppler shifted toward the blue end of the spectrum:

A. the source is moving away from you.

B. the source is moving toward you.

C. the distance between you and the source is increasing.

D. the distance between you and the source is decreasing. 

Summary of Chapter 2

• Wave: period, wavelength, amplitude

• Electromagnetic waves created by accelerating charges

• Visible spectrum is different wavelengths of light.

• Entire electromagnetic spectrum:

• includes radio waves, infrared, visible light, ultraviolet, X-rays, gamma rays

• can tell the temperature of an object by measuring its blackbody radiation

Summary of Chapter 2, cont.

• Spectroscope splits light beam into component frequencies.

• Continuous spectrum is emitted by solid, liquid, and dense gas.

• Hot gas has characteristic emission spectrum.

• Continuous spectrum incident on cool, thin gas gives characteristic absorption spectrum.

Summary of Chapter 2, cont.

• Spectra can be explained using atomic models, with electrons occupying specific orbitals.

• Emission and absorption lines result from transitions between orbitals.

• Doppler effect can change perceived frequency of radiation.

• Doppler effect depends on relative speed of source and observer.