Chapter 5: Light and Matter: Reading Messages from the Cosmos

5.1How Do We Experience Light?

•The warmth of sunlight tells us that light is a form of energy.

•We can measure the flow of energy in light in units of watts: 1 watt = 1 joule/second.

Colors of Light

•White light is made up of all the colors of the rainbow.

(LECTURE)- So it's not that the light is made up of colors, but the different wavelengths. The way our eyes and brains perceive the different wavelengths is in color

.red is a longer wavelength and then blue and violet are smaller wavelengths. And wavelengths longer than red or smaller than violet

our eyes are unable to perceive, they don't interact in our eyes in a way that gives us any signal. So we just can't see them. Those are the only range of wavelengths, to which are sensitive.

How Do We Experience Light? (2 of 2)

•Emission
•Absorption
•Transmission
-Transparent objects transmit light.
-Opaque objects block (absorb) light.
•Reflection/scattering

Reflection and Scattering

•A mirror reflects light in a particular direction.
•A movie screen scatters light in all directions.

Interactions of Light with Matter

•Interactions between light and matter determine the appearance of everything around us.

Thought Question 1 Why is a rose red?

D.The rose reflects red light.

5.2 Properties of Light(what is Light)

•Light can act either like a wave or like a particle.
•Particles of light are called photons.

Waves

•A wave is a pattern of motion that can carry energy without carrying matter along with it.

Properties of Waves

•Wavelength is the distance between two wave peaks.
•Frequency is the number of times per second that a wave vibrates up and down.
Wave speed = wavelength × frequency

Light: Electromagnetic Waves

•A light wave is a vibration of electric and magnetic fields.
•Light interacts with charged particles through these electric and magnetic fields.

Wavelength and Frequency

Longer the wavelength, lower the frequency
Shorter the wavelength, higher the frequency
wavelength × frequency = speed of light = constant

Particles of Light

•Particles of light are called photons.
•Each photon has a wavelength and a frequency.
•The energy of a photon depends on its frequency.

Wavelength, Frequency, and Energy

λ × f = c
λ = wavelength, f = frequency
E = h × f = photon energy!
joule × s = Planck's constant
c=3.00 x 10 to the 8th m/s= speed of light
h=6.626 x 10 to the -34th

Thought Question 2- Which of the following light waves has the most energy

C- bottom wave

Special Topic: Polarized Sunglasses

•Polarization -describes the direction in which a light wave is vibrating.
•Reflection can change the polarization of light.
•Polarized sunglasses block light that reflects off of horizontal surfaces.

Thought question 3- The higher the photon energy

B-the shorter its wavelength.

What is the structure of matter

Ten million atoms could fit end to end across a dot

The nucleus is nearly 100,000 times smaller than the atom but contains nearly all of its mass

Atom: Electrons are "smeared out" in a cloud around the nucleus

Nucleus: Contains positively charged protons (red) and neutral neutrons (gray)

Atomic Terminology

•Atomic number = # of protons in nucleus
•Atomic mass number = # of protons + neutrons

•Molecules: consist of two or more atoms (H2O, CO2)

Hydrogen (1H)

Atomic number= 1
Atomic mass number= 1
(1 electron)

Helium (4HE)

Atomic Number= 2
Atomic Mass Number=4
(2 electrons)

Carbon(12C)

Atomic Number=6
Atomic Mass Number=12
(6 electrons)

Atomic Terminology (2 of 2)

•Isotope: same # of protons but different # of neutrons (4 H e, 3 H e)

Different Isotopes of a given element contain the same number of protons, but different number of neutrons

Isotopes of Carbon

Carbon-12- (6 protons + 6 neutrons)
Carbon-13-(6 protons + 7 neutrons)
Carbon-14-(6 protons+ 8 neutrons)

Thought Question 4-The symbol 4 H e represents helium with an atomic mass number of 4, and is the most common form of helium, containing two protons and two neutrons. What does the symbol 3 H e represent?

B. Helium with two protons and one neutron

What are the Phases of Matter?

•Familiar phases:
-Solid (ice)
-Liquid (water)
-Gas (water vapor)

•Phases of same material behave differently because of differences in chemical bonds.

Phase Changes

•Ionization: stripping of electrons, changing atoms into plasma
•Dissociation: breaking of molecules into atoms
•Evaporation: breaking of flexible chemical bonds, changing liquid into solid
•Melting: breaking of rigid chemical bonds, changing solid into liquid

Fully ionized plasma

atoms in plasma become increasingly ionized

Plasma phase

free electrons move among positively charged ions

molecular dissociation

molecules break apart into component atoms

Gas Phase

atoms or molecules move essentially unconstrained

Liquid Phase

atoms or molecules remain together but move relatively freely

Solid Phase

atoms or molecules are held tightly in place

Phases and Pressure

Atmosphere- The pressure of water vapor determines how frequently water molecules return to the ocean by colliding with its surface

Ocean- At the same time, some water molecules are always evaporating from the ocean.

•Phase of a substance depends on both temperature and pressure.
•Often more than one phase is present.

How is Energy Stored in Atoms?

•Electrons in atoms are restricted to particular energy levels.

Energy Level Transitions

•The only allowed changes in energy are those corresponding to a transition between energy levels.

Thought Question 6-According to the following diagram for the electron energy levels of a hydrogen atom, which of the following is an energy that a hydrogen atom can lose through an electron transition?

B. 1.9 e V

5.4 Learning from Light(What Are the Three Basic Types of Spectra?)

•Spectra of astrophysical objects are usually combinations of three basic types
-Continuous
-Emission line
-Absorption line

Continuous Spectrum

•The spectrum of a common (incandescent) lightbulb spans all visible wavelengths, without interruption.

Emission Line Spectrum

•A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.

Absorption Line Spectrum

•A cloud of gas between us and a lightbulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

How Does Light Tell Us what Things are Made of?
Chemical Fingerprints (1 of 6)

•Each type of atom has a unique set of energy levels.
•Each transition corresponds to a unique photon energy, frequency, and wavelength.

Chemical Fingerprints (2 of 6)

•Downward transitions produce a unique pattern of emission lines

Chemical Fingerprints (3 of 6)

•Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.

Chemical Fingerprints (4 of 6)

•Each type of atom has a unique spectral fingerprint.

Chemical Fingerprints (5 of 6)

•Observing the fingerprints in a spectrum tells us which kinds of atoms are present.

Energy Levels of Molecules (1 of 2)

•Molecules have additional energy levels because they can vibrate and rotate.
•The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated.
•Many of these molecular transitions are in the infrared part of the spectrum.

Thought question 7-•Which letter(s) label(s) absorption lines?

Thought Question 8-Which letter(s) label(s) the peak (greatest intensity) of infrared light?

Thought Question 9 -Which letter(s) label(s) emission lines?

Thought Question 10-Which is hottest?

Thought Question 11-Why don't we glow in the dark?

Thought Question 12-I measure a line in the lab at 500.7 nanometers. The same line in a star has a wavelength of 502.8 nanometer. What can I say about this star?

D- Infrared
E- further infrared
A-ultraviolet
A. a blue star
B. People only emit light that is invisible to our eyes.
A. It is moving away from me.

Thermal Radiation

•Nearly all large or dense objects emit thermal radiation, including stars, planets, you.
•An object's thermal radiation spectrum depends on only one property: its temperature.

Properties of Thermal Radiation

1.Hotter objects emit more light at all frequencies per unit area.
2.Hotter objects emit photons with a higher average energy.

How does Light tell us the Speed of a Distant Object?

•The Doppler shift can tell you about the relative motion of distant objects because photons behave as waves
•If an object is moving toward an observer, the observed wavelength of the light decreases
-Light appears blueshifted
•If an object is moving away from an observer, the observed wavelength of the light increases
-Light appears redshifted

Measuring the shift

Laboratory Spectrum- Lines at rest Wavelengths

Object 1- Lines redshifted- object moving away from us

Object 2- Greater redshift object moving away faster than object 1.

Object 3- Lines Blueshifted: Object moving toward us.

Object 4 Greater blueshift Object moving toward us faster than object 3
•We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.
•The amount of blueshift or redshift tells us an object's speed toward or away from us.
•Doppler shift tells us only about the part of an object's motion toward or away from us: