Light, Spectra, and Matter. Why will we spend so much time discussing the electromagnetic spectrum? We rely on remote sensing of EM radiation. Tells us.

Light, Spectra, and Matter

Why will we spend so much time discussing the electromagnetic spectrum?

We rely on remote sensing of EM radiation.

Tells us the temperature and composition

This gives us important clues to the origins of these objects.

Not easy to visit astrophysical objects (the Sun, planets, other stars) and make direct in situ measurements

Light properties

• Energy• Power• Intensity• Wavelength• Frequency• Speed

There are many properties of light that can be measured and quantified. Each of these properties will have its own units.

Which of the following is NOT a unit of energy?

1. Joule2. Kilowatt3. Kilowatt-hour4. Electron volt

Which of the following is NOT a unit of energy?

1. Joule2. Kilowatt3. Kilowatt-hour4. Electron volt (eV)

BTU = British thermal unit

1 eV = 1.6 × 10–19 J

Intensity

Intensity is a measure of how much power from a light source is distributed over an area.

Its units are Watts per square meter (W/m2), which is also written W m-2

Solar IntensityThe Solar Energy Output is 4 x 1026 W.

How much of that hits us?

When the Sun is directly over head, it delivers the equivalent of 22 × 60 watt light bulbs over each square meter (m2) of ground!!!

This amount, 1368 W m-2, is known as the solar constant

How is solar energy delivered from the Sun to the Earth?

As light!!!!

Electromagnetic Wave: propagating wave of electric and magnetic fields that oscillate perpendicular to each other and the direction of propagation

Light travels as an electromagnetic wave

Electric field

Magnetic field

In a vacuum, wave propagates with speed = 3.00 x 108 m/s(cosmic speed limit)

Wave Properties

frequency (f or ν (nu)): number of peaks that pass a location in a given time (units: Hertz (Hz) = 1/s = s-1)

speed (v): how much distance the wave moves per unit time for an EM wave the speed is always the speed of light. In other words, v = c = 3 x 108 m/s.

Wave Propertieswavelength (): distance between two consecutive peaks

(units: km, m, cm, mm, m, nm…)

amplitude: height of the wave (or depth of the trough); related to intensity but we won’t use it

Wave Propertiesspeed (v): how much distance the wave moves per unit time

(for an EM wave v = c = 3 x 108 m/s)

frequency (f): number of peaks that pass a location is a given time (units: Hertz (Hz) = 1/s = s-1)

wavelength (): distance between two consecutive peaks (units: km, m, cm, mm, m, nm…)

These three properties are related:c

f

If wavelength is 10 m and frequency is 100 Hz (oscillations / seconds), what would be the speed of the wave?

1. 10 m/s 2. 1 m/s 3. 1000 m/s 4. 100 m/s

If wavelength is 10 m and frequency is 100 Hz (oscillations / seconds), what would be the speed of the wave?

1. 10 m/s 2. 1 m/s 3. 1000 m/s 4. 100 m/s

If wavelength is 10 m and frequency is 100 Hz (oscillations / second), what would be the speed of

the wave?

The PhotonLight behaves like both a particle and a wave!

Photon: smallest bundle of light energy (i.e., a particle of light)

Photons carry light energy:1. A photon’s energy is proportional to frequency

(Eph f).• A photon’s energy is inversely proportional to

wavelength (Eph -1).

Plank’s constant (h) = 6.602 x 10-34 Js

Matter actually a wave too!

• All matter exhibits particle and wave properties (DeBroglie, 1921)

• For ordinary objects, the wave nature of matter is much too small to measure– The wavelength of a baseball

moving at 80 mph would be about 10-34 meters

• But for small particles, this is wave nature of matter is measurable– The wavelength of an electron is

about 10-10 meters

Electron diffraction pattern showing its wave nature

How is a difference in the frequency or wavelength of light observed?

The Visible Spectrum:

For visible wavelengths COLOR

How does a prism work?• Dispersion: Speed of light in

the prism (glass or plastic) depends on the frequency (color)

• Refraction: Change in speed of light causes a change in its direction

• Result: Blue changes direction most since its speed is the lowest inside the prism. And red changes direction least since its speed is highest inside the prism.

Red R Orange O Yellow Y Green G Blue B Indigo IViolet V

Herschel Thinks Outside the Box:In 1800 William Herschel made a discovery when he tried to

determine the temperature of light.

• He noticed that a thermometer recorded energy from the Sun`s spectrum even when placed beyond the red end of the visible rainbow. •He called this emission Calorific Rays and it was the first discovery that light had colors invisible to the human eye. •These rays are known today as Infrared light.

Herschel’s work color is associated with a temperature

Visible light is just a small part of the electromagnetic (EM) spectrum

Fraunhofer’s Surprise

In 1813, Joseph von Fraunhofer, the owner of a glass manufacturing firm in Munich, made an even more interesting discovery.

Using a precision dispersing prism, he discovered that the `solar blackbody` was cut by thousands of dark bands.

Fraunhofer’s Surprise

Fraunhofer tried to test whether this effect was real.

1) He tested with different optics.

2) He tested by looking at different objects (moon and planets).

Bunsen and Kirchhoff`s solution:Robert Bunsen (Univ. of Heidelberg)

turned pyromania into one of the great discoveries of modern physics. Bunsen set fire to things in order to figure out their elemental composition

Iron

A colleague there, Gustav Kirchhoff, suggested using a prism to break the light apart. They quickly discovered (1860) that burning substances produced light in narrow bands with unique patterns.

Blueprint to Composition:

Bunsen and Kirchhoff`s trick was the key to finding out the composition of anything from the light it produced.

Many of the lines they found had the same wavelength as those of Fraunhofer`s dark bands. They were seeing the composition of the Sun!

Absorption by (and re-emission from) a cooler gas!

Kinds of Spectra:

Why were Bunsen`s heated gas spectra composed of bright lines while Fraunhofer`s exhibited a continuous spectrum with dark bands?

Bunsen`s fires were stimulating light emissions in the hot gas. So what are Fraunhofer`s bands?

Bunsen found that he could identify the signature of different elements in the Fraunhofer spectrum of the Sun.

Types of SpectraContinuous: black body radiation

continuous

Absorption: requires a cool object in front of a hot background(ex: Fraunhofer) discrete

Emission: requires a hot object with a cool background (ex: Bunsen) discrete

Spectroscopy is the use of light’s interaction with matter to identify or characterize properties of matter.

Basic definitions:• Element: a substance that cannot be broken

down by chemical means (defined by number of protons)

• Atom: the smallest piece of matter that is still an element

• Molecule: two or more atoms that are bound together by chemical bonds

• Nucleus: the protons and neutrons bound together at the center of an atom

Why do elements have the `discrete` interactions that Bunsen saw?

This has to do with the nature of atoms and how they are put together.

Atoms and light

Why do different elements (and molecules) have different interactions?

A brief history of the atomFirst discussion of the nature of matter (~400 BCE):

• Leucippus, a Greek philosopher, all matter consisted “tiny and indivisible bodies called atoms”.

• The word atom comes from the Greek word `atomos` (not divisible).

• Democritus, another Greek philosopher, these atoms were not all alike, but had different shapes and sizes to make different matter.

• Opposed (Aristotle): The prevailing view that everything was made up of four basic elements: earth, fire, air, and water, not atoms.

• Views such as Aristotle`s dominated science for many centuries, until the Renaissance.

Dividing the ‘indivisible’:The plum pudding model

• J. J. Thomson discovers cathode rays are made of electrons (he called them ‘corpuscles’ – 1897). Electrons are shown to have a negative charge

• Thomson proposes model of the atom (1904):– Atom has smaller components– Negatively charged corpuscles/electrons

(plums)– Positive ‘soup’ to balance negative charge

(pudding)

Discovering “nothing”Meanwhile, Ernest Rutherford (Cambridge) discovers two new types of radiation emitted by uranium (1899):

1. Alpha particles (): later found to be the helium nucleus2. Beta particles (): later called the “electron” by Thomson

Positive Nucleus

Negative electron

In 1909, Rutherford fires alpha particles at gold foil.

Expected only small angle scattering due to gold atoms’ “plum pudding”.

Saw mostly no scattering with occasional back scattering

Matter is mostly empty space!!!!

Rutherford’s atom1. Mass is highly concentrated in the positively-

charged nucleus at the center of the atom.

2. Electrons (negatively charged) “orbit” the nucleus.

3. Lots of empty space in-between

4. Similar to today’s atom– Number of protons determine

the element identity

– Number of electrons determine

the chemical properties of the atom

Positive Nucleus

Negative electron

The nucleus is not uniform

James Chadwick (1932): Discovers the neutron. Neither positive nor negative, it has about the same mass as a proton.

Nuclei are made up of protons and neutrons.

Rutherford (1918): Discovers the proton. The proton is about 2000 times as massive as the electron and has a positive charge, exactly the same magnitude as the electronic charge.

Atoms, elements, and isotopesAtoms (below - periodic table (Mendeleev, Meyer, 1867))

– Nucleus

• Protons – number determines the element (atomic #)

• Neutrons – number determines the isotope (mass #)

– Electrons – number determines the chemical properties

Atoms, elements, and isotopesIsotopes:Atoms with the same number of protons but different numbers of neutrons are called isotopes.

Isotopes have the same chemical properties, but different masses, different emission spectra, and participate in different nuclear reactions.

e-

p+

hydrogen (1H)

p+

e-

deuterium (2H)

n

A stable isotope of hydrogen – 0.02% natural abundance

p+

e-

nn

Radioactive isotope of hydrogen

tritium (3H)

p+

e-

nn

p+

e-

New element; not an isotope of H

helium (4He)

A stable isotope of hydrogen – 99.98% natural abundance

Bohr atomic model Niels Bohr (Copenhagen Univ.), based

on Rutherford’s work, suggested a quantized structure of electronic orbits in an atom (1913)

Bohr and Werner Heisenberg later (1926) modify structure to account for the wave properties of electrons.

Electron distances and energies are discrete valuesp+

e-

Energy States:

E1

E2

E3

p+

e-

Electrons exist in `orbits` (much like planets in the solar system) that are stable at specific separations from the nucleus.

The spacing of these energy levels is not even.

Atoms and light

The distance from the nucleus determines the energy of the electron (lower E is closer).

Energy States:

E1

E2

E3 E1E2 > E2E3 > E3E4 etc…

So what does all of this have to do with Bunsen and Franhofer lines?

Atoms and light

If you heat the atom up to high enough temperatures, the electron will jump to higher orbits (higher energy state).

How does `heating` do this? Collisions

Atoms and light

p+

e-

Energy States:

E1

E2

E3

If you heat the atom up to high enough temperatures, the electron will jump to higher orbits (higher energy state).

How does `heating` do this? Collisions

Atoms and light

p+

e-

Energy States:

E1

E2

E3

Atoms and light

p+

e-

Energy States:

E1

E2

E3

After a time, the electron falls back to the lowest energy state.

A photon is given off.

The energy of the photon is exactly equal to the energy difference between the two energy states.

Atoms and light: absorption

p+

e-

Energy States:

E1

E2

E3

Process of emission is fully reversible.

The energy of the photon must be exactly equal to the energy difference between the two energy states.

Electron can absorb a photon and jump to a higher energy level.

Atoms and light: absorption

p+

Energy States:

E1

E2

E3

Process of emission is fully reversible.

The energy of the photon must be exactly equal to the energy difference between the two energy states.

Electron can absorb a photon and jump to a higher energy level.

e-

Conservation of energyThe energy difference between electron orbital states is

exactly equal to the energy of the photon emitted or absorbed.

E2 – E1 = h f

Where E1 and E2 are the energies associated with the electronic orbital states, f is the frequency of light, and h

is Planck’s constant = 6.62 × 10–34 J•s

hydrogen energy level diagram

Quantized energy• Different frequencies are perceived as different

colors• Atoms of different elements have different

allowable energy level transitions and thus emit and absorb different discrete colors.

• Example: Each line in the spectrum of iron is different energy level transition

Iron

Types of spectra

Continuous: black body radiation continuous

Absorption: requires a cool object in front of a hot background(ex: Fraunhofer) discrete

Emission: requires a hot object with a cool background (ex: Bunsen) discrete

e-

What happens if a very energetic photon interacts with an atom?

Ionization:

Such a photon can give enough energy to the electron that it can escape the atom.

The amount of energy necessary to do this is called the binding energy of the atom.

e-

p+

Energy States:

When an atom absorbs light (or thermal) energy greater than the binding energy, the electron escapes.

Ionization:

The atom is left with a positive charge and is called an ion.

Together, ions and free electrons are called plasma.

e-

e-

p+

Energy States:

Plasma is found in stars, space, and parts of our atmosphere.

e-

Molecules are atoms that are connected by bonds (electrons). At a basic level a molecule will behave similarly to an atom.

What about molecules?

e-

p+

-

e-

p+

Molecules also have discrete electron energy levels.

p+ p+

Like atoms, electrons in molecules can absorb a photon and move to a higher energy level

Electronic Energy States:

A photon with enough energy can free an electron by overcoming the binding energy.

What about molecules?e-

p+

e-

p+

p+ p+

This produces a molecular ion.

Plasmas can also contain molecular ions.

Electronic Energy States:

Photo-dissociation

e-

p+

e-

p+

Or overcome the molecular binding energy and break the molecule up (photo-dissociation).

Electronic Energy States:

e-

e-

p+ p+

Spectrum of HCl, a diatomic molecule

Frequency increases right along the x-axis; intensity is the y-axis

We can use light spectra to identify molecules!