NATS From the Cosmos to Earth Light as a Wave For a wave, its speed: s = l x f But the speed of light is a constant, c. For light: l x f = c The higher f is, the smaller l is, and vice versa. Our eyes recognize f (or l) as color! Visible Light Waves Animation

NATS From the Cosmos to Earth Visible light ranges through 7 major colors from long wavelengths (low frequency - red) to short wavelengths (high frequency - violet) - Red, orange, yellow, green, blue, indigo, violet (Roy G Biv)

NATS From the Cosmos to Earth Light as a Particle (Photon) Light propagates as quanta of energy called photons Photons move with speed of light have no mass are electrically neutral Energy of a photon or electromagnetic wave: E = hf = h c/ l where h = Planck’s constant f = frequency of a light wave - number of crests passing a fixed point in 1 second c = velocity of light l = wavelength of a light wave - distance between successive crests

NATS From the Cosmos to Earth The Electromagnetic Spectrum Most wavelengths of light can not be seen by the human eye. The visible part of the electromagnetic spectrum lies between ultraviolet and infrared light (between about 400 and 700 nm). The higher the frequency (shorter the wavelength), the higher the photon energy. Radio waves are at the long wavelength end of the spectrum and gamma rays are at the short wavelength end of the spectrum.

NATS From the Cosmos to Earth Light as Information Bearer Spectrum of a distant object - a spectrum is the amount of energy or intensity at different wavelengths. By studying the spectrum of an object, we can learn its: 1Composition 2Temperature 3Velocity We can separate light into its different wavelengths (spectrum).

NATS From the Cosmos to Earth Electron Energy Levels Electrons can not have just any energy while orbiting the nucleus. Only certain energy values are allowed. Electrons may only gain or lose certain specific amounts of energy. Each element (atom and ion) has its own distinctive set or pattern of energy levels - holds the key to studying of distant objects in the universe. This diagram depicts the energy levels of Hydrogen. 1 eV (electron volt) = 1.6 X J Electron jumps to higher energy levels can only occur with addition of the particular amounts of energy representing differences between possible energy levels. Energy levels are quantized - study of electron energy levels called quantum mechanics. Atom gains this energy either from KE of another atom colliding with it or from absorption of energy carried by light - falls to lower energy level by emitting light or transfer of energy by collision.

NATS From the Cosmos to Earth Absorption and Emission. When electrons jump from a low energy shell to a high energy shell, they absorb energy. When electrons jump from a high energy shell to a low energy shell, they emit energy. This energy is either absorbed or emitted at very specific wavelengths, which are different for each atom. When the electron is in a high energy shell, the atom is in an excited state. When the electron is in the lowest energy shell, the atom is in the ground state.

NATS From the Cosmos to Earth The Hydrogen Atom. The hydrogen atom is the simplest of atoms. Its nucleus contains only one proton which is orbited by only one electron. In going from one allowed orbit to another, the electron absorbs or emits light (photons) at very specific wavelengths. Note - wavelength is often written as and the unit used is an angstrom (A) = m

NATS From the Cosmos to Earth De-excitation an d Emission Animation

NATS From the Cosmos to Earth Excitation and Absorption Animation

NATS From the Cosmos to Earth Interaction of Light with Matter So each electron is only allowed to have certain energies in an atom. Electrons can absorb light and gain energy or emit light when they lose energy. It is easiest to think of light as a photon when discussing its interaction with matter. Only photons whose energies (colors) match the “jump” in electron energy levels can be emitted or absorbed. Hydrogen So visible emission spectrum is created when a gas is heated and collisions in gas continually bump electrons to higher energy levels - emit photons of specific wavelength as they fall back to lower levels. Absorption spectrum is produced when white light is passed through cloud of cool gas. Photons of specific wavelengths absorbed as electrons jump to higher energy levels. Emission Spectrum Absorption Spectrum

NATS From the Cosmos to Earth Emission Spectra The atoms of each element have their own distinctive set of electron energy levels. Each element emits its own pattern of colors, like fingerprints. If it is a hot gas, we see only these colors, called an emission line spectrum. Orion Nebula in Ultraviolet

NATS From the Cosmos to Earth Absorption Spectra If light shines through a cool gas, each element will absorb those photons whose colors match their electron energy levels. The resulting absorption line spectrum has all colors minus those that were absorbed. We can determine which elements are present in an object by identifying emission and absorption lines. Hydrogen

NATS From the Cosmos to Earth Spectrum of a Gas Cloud Animation

NATS From the Cosmos to Earth Thermal Radiation Animation Thermal/Blackbody Radiation Photons are produced whenever charged particles are accelerated - A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field - electromagnetic radiation - photons In an opaque object or dense gas cloud, photons can’t easily escape - they “bounce around” in the object. This randomizes their radiative energies and resulting photon energies depend only on the body’s temperature - produces a continuous spectrum called a thermal radiation or blackbody spectrum. Blackbody - a hypothetical body that completely absorbs all wavelengths of thermal radiation incident on it - does reflect light - appears black if temperature low enough so as not to be self-luminous. - all blackbodies heated to a given temperature emit thermal radiation with the same spectrum - required by thermal equilibrium - distribution of blackbody radiation as a function of wavelength - the Planck law, cannot be predicted using classical physics. - the first motivating force behind the development of quantum mechanics

NATS From the Cosmos to Earth Key Features of a Blackbody Spectrum - a dense object produces light at all possible wavelengths if the object is above absolute zero. - since everything in the universe is above 0 K, all dense objects (solids, liquids, thick gases) will produce a thermal spectrum. - the shape of a continuous spectrum depends on only the temperature of the object not its chemical composition. - as the temperature of an object increases, more light is produced at all wavelengths - as the temperature of an object increases, the peak of thermal spectrum curve shifts to shorter wavelengths (higher frequencies)---cool things appear red or orange, hotter things appear yellow or white, and very hot things blue or purple.

NATS From the Cosmos to Earth Temperature (K) of Black Body Wavelength ( max) at Which Most Radiation is Emitted Type of Radiation 30.1 cmRadiowaves cm"Far" Infrared 3, nm"Near" Infrared 4, nmRed Light 6, nmYellow Light 8, nmViolet Light 10, nm"Near" Ultraviolet 30, nm"Far" Ultraviolet 300,00010 nm"Soft" X-Rays 1.5 million20 nm"hard" x-rays 3 billion0.001 nmGamma rays

NATS From the Cosmos to Earth Hotter objects emit more total radiation per unit surface area. E =  T 4 (  = 5.7 x watts/m 2 K 4 ) - Stephan-Boltzmann Law Hotter objects emit photons with a lower wavelength (higher average energy.) max = 2.9 x 10 6 / T(K) [nm] - Wien’s Law The Stefan-Boltzmann Law, Wein’s Law, and Newton’s Universal Law of Gravitation together allow us to determine a star's type, its mass, its temperature, its rate of energy production, its diameter, its life expectancy and its future fate.

NATS From the Cosmos to Earth

Kirchhoff’s Laws of Radiation First Law. A luminous solid, liquid or gas, such as a light bulb filament, emits light of all wavelengths thus producing a continuous spectrum of thermal radiation. Second Law. If thermal radiation passes through a thin gas that is cooler than the thermal emitter, dark absorption lines are superimposed on the continuous spectrum. The gas absorbs certain wavelengths. This is called an absorption spectrum or dark line spectrum. Third Law. Viewed against a cold, dark background, the same gas produces an emission line spectrum. It emits light of discrete wavelengths. This is called an emission spectrum or bright line spectrum..

NATS From the Cosmos to Earth So what astronomical body has this spectrum?