Lecture
The Origin and Nature of Light; Reflectance Spectroscopy

Learning Objectives

Upon completion of this lesson, you should be able to

==State what electromagnetic radiation is, how it generated and propagates, how we gather and record it
==Describe in easily understoon language what is physically happening when some material reflects light; state why some material might have a "preference" for reflecting one color or another.
==Explain what is meant by reflectance spectroscopy.
==Define wavelength, frequency, reflectance, absorption, scattering, imaging, color
==Using an image showing color-coded reflectance, interpret the kinds and approximate quantities of surface materials.
==Identify the asteroids certain meteorites came from on the basis of the reflectance spectra.

Concepts Covered

Atoms
Electromagnetic Radiation
Electromagnetic Spectrum
Light as a wave and a particle
Continuous Spectra
Where does light come from?
Reflectance Spectroscopy
Take the Quiz == page down to the bottom of these lecture notes

How different the world would look if we could see at infrared wavelengths:

"How different would the world look if we could see infrared, or IR light? Well, for one thing, we would be able to see our hand in front of our face in the dark! Our bodies actually emit IR light, which we experience as heat. If we could see in the IR, everything that gives off heat would suddenly be apparent to us, even if there were no visible light! Since our eyes cannot see in the IR, something like night vision (or infrared) goggles can be used to see differences in temperature and to assign different brightnesses or false colors to the different temperatures or energies of IR light. This provides a picture that our eyes can interpret.
If we looked at a person with infrared goggles, we would see something similar to the image on the left showing a Jet Propulsion Lab engineer holding a lighted match. The image is color-coded to show differences in temperature. The flame and the engineer's palm (a place where warm blood vessels are close to the surface of the skin) are warmer than his glasses. This shows how infrared images show heat energy and its distribution." from NASA/GSFC

Atoms

We start with a brief summary of what, exactly, atoms are:

Atoms, in the classical sense, are made up of a nucleus containing neutrons and protons. The nucleus is held together by the strong force.
Electrons form a "smeared out" charged cloud around the nucleus -- due to something called the Heisenberg Uncertainty Principle.

The electrons are held to the nucleus by the electromagnetic force.
Electrons can "jump" between energy states.
Only a certain number of electrons can occupy each energy state.

For a neutral atom, there are equal numbers of protons and electrons.
For positively charged ions (the usual case), one or more electrons are missing and the atom is positively charged.
Isotopes of atoms carry different numbers of neutrons.
Everything in the universe is made up of atoms. Our understanding of the Periodic Chart leads us to believe that the universe is made up of exactly the same atoms found here on Earth.

Electromagnetic Radiation

We can describe the properties of light:

light consists of the oscillation of electric and magnetic fields
light can be measured in wavelengths (meters)
light can be measured according to its frequency (cycles per second)
light is characterized by its velocity, c, in a vacuum that's the fastest speed there is. The speed of light is a FUNDAMENTAL CONSTANT OF THE UNIVERSE.
light can be measured according to its frequency (cycles per second)

Wavelength, frequency, and the speed of light are related by:

c = wavelength*frequency

The whole electromagnetic spectrum consists of (from highest energy, highest frequency, shortest wavelength -- to -- lowest energy, lowest frequency, longest wavelength): gamma rays, x-rays, ultraviolet, visible, infrared, microwave, radio.

The wavelengths range from a picometer (0.000000000001 m) for gamma rays, to 10,000 m for radio. (Note, however, that both ends of the scale are unbounded.)

The Electromagnetic Spectrum (Light)

Frequency (Hz)	Wavelength (meters)	Typical Astronomical Sources
Lowest Frequency Lowest Energy	Longest Wavelengths Greater than 1 cm (0.01 meter)	Active galaxies Quasars Alien civilizations
	1 cm -- 1 mm (approximately)	Cosmic Microwave Background Interstellar molecules
	1 mm -- 700 nm	Interstellar dust
	700 nm -- 400 nm	Stars Interstellar gas
	400 nm -- 10 nm	Hot stars
	10 nm -- 0.1 nm	Black hole accretion disks Pulsars
Highest frequency Highest energy	Shorter than 0.1 nm (0.0000000001 m) Shortest wavelength	Supernova explosion Mergers of neutron stars or black holes?

For a wonderful and informative presentation of light, check out Imagine the Universe: Electromagnetic Radiation. All of the above images were obtained from that site.

Light as a wave and a particle

Almost everything we know about our cosmos comes from light: either given off by the object or reflected by the object. We talk about light coming to us from across the Universe. Light is the propagation of electric and magnetic fields. In a vacuum, it travels at an astounding 300,000 kilometers per second (or, more accurately, exactly 299,792.458 km/sec). The changing electric field generates a magnetic field, and the changing magnetic field generates an electrical field -- nature's most perfect dynamo-electromagnet.

Light can behave either as a wave or as a particle. As a wave, it's characteristics are described by its wavelength, velocity (speed), and frequency. These three properties are related by (from Imagine the Universe):

As a particle, light behaves as tiny packets of energy. Photons at radio frequencies have the lowest energy -- that's why we can live with radio waves passing by us continuously. Photons at gamma ray frequencies have the highest energy -- that's why you do not want to meet up with an atomic bomb or a nuclear meltdown. Gamma ray photons destroy life. The energy of a photon is related to its frequency by:

is the energy of the photon; is a very small constant called Planck's constant (of order 7 x 10^-34 in units of energy * time!); and is the Greek letter nu representing frequency.

Light can behave as a particle, a small energy packet called a photon or a wave. It just depends on how it is measured -- what the interaction is. Light enters the lenses of our eyes as waves, and is refracted and focused on the retinas. There, the light acts as particles, transmitting their energy to the rods and cones that send the nerve impulses to our brain.

Light behaves as a particle when it runs our light-powered calculators.

(Thought questions: if infrared, microwave, and radio waves are all forms of electromagnetic radiation, why can't we operate our calculators by holding them up to a stove or our cell phones or towards the Queen Anne radio towers? In a pinch during a nuclear war, would gamma rays work?)

Light behaves as a wave when refracted through raindrops forming a rainbow.

Continuous Spectra

A continuous spectrum is what we normally think of as all of the colors or the rainbow. A blackbody radiates a continuous spectrum.

What is the definition of a blackbody? Is there such a thing as a perfect blackbody? A perfect blackbody would emit and reabsorb all light. A blackbody can be viewed as an object giving off only thermal radiation -- a thermal radiator. Hot objects give off higher energy light, cool objects give off lower energy light. It's a concept you are all very familiar with. Let's say you had a choice of touching a red-hot stove or a white-hot stove. Which would you choose? You can look directly at a red-hot burner that is emitting red and infrared light, but a welder must wear dark welder's glass (No. 13 at least). The welders are protecting their eyes not only from sparks but from the ultraviolet light being produced by temperatures of 1000's of degrees.

The Sun can be viewed as a near-perfect thermal radiator (if it were a perfect radiator or a blackbody, we would not get any light as it would all be reabsorbed). It's surface temperature of about 6000 Kelvin means it radiates most of its light in the visible part of the spectrum. Our atmosphere also lets visible light pass through to the surface. Our eyes are nicely tuned to see visible light. These things are not coincidences.

Where does light come from?

Light is emitted or absorbed when electrons of an atom change their energy states -- or in the classical sense, when the electron changes its "orbit."

A photon is emitted when an electron "jumps" from a higher energy state to a lower energy state. The photon must somehow get rid of that energy and it does so by emitting energy in the form of electromagnetic radiation. (Think of potential energy: an object up 8-m high has more potential energy than an object a few cm off the floor.)

Would you expect a big energy jump to give off a blue photon or a red photon? Would you expect a small energy jump to give off a blue or a red photon?

Reflectance Spectroscopy

Spectroscopy is the study of light as a function of wavelength that has been emitted, reflected or scattered from a solid, liquid, or gas.

Absorption and Scattering

As photons enter a mineral, some are reflected from grain surfaces, some pass through the grain, and some are absorbed. Those photons that are reflected from grain surfaces or refracted through a particle are said to be scattered. Scattered photons may encounter another grain or be scattered away from the surface so they may be detected and measured. Photons may also originate from a surface, a process called emission. All natural surfaces emit photons when they are above absolute zero. Emitted photons are subject to the same physical laws of reflection, refraction, and absorption to which incident photons are bound.
Photons are absorbed in minerals by several processes. The variety of absorption processes and their wavelength dependence allows us to derive information about the chemistry of a mineral from its reflected or emitted light. The human eye is a crude reflectance spectrometer: we can look at a surface and see color. Our eyes and brain are processing the wavelength-dependent scattering of visible-light photons to reveal something about what we are observing, like the red color of hematite or the green color of olivine. A modern spectrometer, however, can measure finer details over a broader wavelength range and with greater precision. Thus, a spectrometer can measure absorptions due to more processes than can be seen with the eye.

Imaging Spectroscopy

Today, spectrometers are in use in the laboratory, in the field, in aircraft (looking both down at the Earth, and up into space), and on satellites. Reflectance and emittance spectroscopy of natural surfaces are sensitive to specific chemical bonds in materials, whether solid, liquid or gas. Spectroscopy has the advantage of being sensitive to both crystalline and amorphous materials, unlike some diagnostic methods, like X-ray diffraction. Spectroscopy's other main advantage is that it can be used up close (e.g. in the laboratory) to far away (e.g. to look down on the Earth, or up at other planets). Spectroscopy's historical disadvantage is that it is too sensitive to small changes in the chemistry and/or structure of a material. The variations in material composition often causes shifts in the position and shape of absorption bands in the spectrum. Thus, with the vast variety of chemistry typically encountered in the real world, spectral signatures can be quite complex and sometimes unintelligible. However, that is now changing with increased knowledge of the natural variation in spectral features and the causes of the shifts. As a result, the previous disadvantage is turning into a huge advantage, allowing us to probe ever more detail about the chemistry of our natural environment.

Quoted directly from: US Geological Survey Spectroscopy Lab. This link has lots of images and the scientific process of remote sensing via reflectance spectroscopy.

Color

What does it mean when an object "looks blue" or "looks red"? It means that a blue object absorbs all colors except blue and a red object absorbs all colors except red: blue and red are reflected.

At the atomic or molecular level what is happening is that the electrons of the atom or the energy levels of the molecule (electrons, vibration and rotation energies) get excited. If the frequency of the light is such that the electrons get excited only for an extremely brief period of time, then the light is reflected, it "bounces" right back away from the surface. If the light has just the right frequency or energy (interchangeable terms, right?), the atoms or molecules "hang onto" that energy for just a bit longer than if they are not excited. The energy is then released in a random direction, which may or may not be in the direction from which the light came. This light is not reflected back or only partially reflected from the object.

The energy of the light, if absorbed, serves to increase the energy of the atoms and molecules, leading to an increase in the temperature of the material. We stay relatively cool in white colors because all wavelengths of visible light are reflected; we heat up in black colors because all wavelengths of light are absorbed. This radiative energy goes to thermal energy of the atoms or molecules making up the dye in the fabric. A shirt looks red because all colors except red are absorbed. A banana looks yellow because red and green are reflected.

Application to Earth Studies

The following charts are from the USGS Spectroscopy Lab and the articles given there.

Take a look at these two images and compare the "true" color image to the one taken through various filters ranging from visible to infrared wavelengths. Note that the one identifying various types of hematite and other iron-bearing minerals is in false colors. Since we cannot see at infrared wavelengths, color is not defined in that region of the electromagnetic spectrum.

The reflectance of a material is dependent upon the wavelength of the incident light. Take a look at the following spectra. If you were to measure the reflectance of these substances and minerals at wavelength intervals, would you be able to tell which was which? Would you be able to identify water ice separate from carbon-black? What logic would you use? After viewing the six different spectra, page further down and take a look at the spectra of Callisto (a moon of Jupiter), Mercury, and Jupiter.

The wavelengths of visible light, what we call light we can detect with our eyes, fall between 400 and 700 nanometers (nm), roughly. A nanometer is one-billionth of a meter. Shorter than 400 nm, and the light enters the ultraviolet, x-ray, and gamma ray region. Longer than 700 nm, and the light enters the infrared, microwave, and radio region.

Application to Planetary Studies

Rocks, metals, water, gas, clouds, etc. all have components that will reflect or absorb different frequencies. By knowing the wavelength signatures of the light reflected by rocks, metals, etc., we can unravel the composition of planets, moons, asteroids, and comets.

Compare the following three reflectance spectra from Mercury, Jupiter, and Jupiter's moon Callisto. Note that the wavelength coverage (x axis, in microns or 10^-6 meters) is not the same for all three images, and that scale for the amount of reflectance (percentage of light reflected) is also different (y axis).

Are these three bodies made of the same materials? How do you know?
Which object is the brightest over all wavelengths?
Which object is the darkest? How do you know?
Which object is bright at red wavelengths (at about 0.6 - 0.7 microns), but dark at infrared wavelengths (at about 2 microns). How can you tell?

From The Meteorite Market Adapted from the text of Morrison, Wolff, and Fraknoi (Saunders 1995)

We will soon be studying meteorites. There is strong evidence that most of the meteorites come from the asteroid belot. Take a look at the reflectance spectra shown in the two figures above. Do you find this particular evidence convincing?


From The Meteorite Market	Adapted from the text of Morrison, Wolff, and Fraknoi (Saunders 1995)
We will soon be studying meteorites. There is strong evidence that most of the meteorites come from the asteroid belot. Take a look at the reflectance spectra shown in the two figures above. Do you find this particular evidence convincing?