What is light? Light is electromagnetic radiation. Technically, light is the part of electromagnetic (e.m.) radiation that humans (and other animals) see. Although incorrect, we usually call “light” all types of electromagnetic radiation, like X-ray light or UltraViolet light Light is made of energy. Light always travels at the speed of light: c. Light is a wave. 1

What is Electromagnetic Radiation? Made of propagating waves of electric and magnetic fields. It carries energy with it Sometimes called “radiant energy” Think – solar power, photosynthesis, photo-electric cells, the fireplace … 2

What is an electromagnetic wave? It is electricity and magnetism moving through space. Electric magnetic force So, when we say the speed of light is “c” what we really mean is that the speed of the electromagnetic wave is “c”, regardless of its frequency. 3

Light is an electromagnetic wave Light as a wave Waves you can see: e.g., ocean waves Waves you cannot see: sound waves electromagnetic waves Pitch and intensity Light is an electromagnetic wave 4

Properties of Waves For light in general: speed = c = d/t = λ  Wavelength – the distance between crests (or troughs) of a wave. Frequency – the number of crests (or troughs) that pass by each second. Speed – the rate at which a crest (or trough) moves. For light in general: speed = c = d/t = λ  λ  = c λ = c/ wavelength frequency speed of light = 3x108 m/s in vacuum Boxcars and speed 5

Light as a Wave Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å): 1 nm = 10-9 m 1 Å = 10-10 m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm). 6

Different colors of visible light correspond to different wavelengths. Wavelengths and Colors Different colors of visible light correspond to different wavelengths. 7

Visible Light Shorter Wavelength Longer Wavelength 8

Remember: visible light isn’t the whole story Remember: visible light isn’t the whole story. It’s just a small part of the entire electromagnetic spectrum Short Wavelength Long Wavelength (high frequency) (high energy) (low frequency) (low energy) 9

Wavelengths and size of things Roy G. Biv 10

Reminder #1: E.m. Radiation generally contains bundles of waves of different wavelengths (colors) How much of each color is present in a given bundle of e.m. radiation, i.e. the distribution of intensity of each wavelength, is called the spectrum Here is an example of optical (visible) light:

The Multi-wavelength Sun infrared Radio X-ray optical 12

Optical Sky 13

Near-infrared sky Boldt et al. 14

Radio Sky 15

Soft X-ray Sky 16

Different wavelengths carry different types of information Visible light: the glow of stars (dust blocks the light) Infrared: the glow of dust Visible light (top) and infrared (bottom) image of the Sombrero Galaxy 17

Light as particles Light comes in quanta of energy called photons – little bullets of energy. Photons are massless, but they have momentum and energy. Couldn’t explain things if light is always a wave. Gasoline vs. apple 18

Wave-particle duality All types of electromagnetic radiation act as both waves and particles. The two views are connected by the relation E = h = h c /  h is the Planck's constant, c is the speed of light is the frequency, is the wavelength You can't keep dimming down light of a given color (say, red) indefinitely and see a steady red signal. Double the frequency and E doubles, etc The energy of a photon does not depend on the intensity of the light!!! 19

Intensity A photon's energy depends on the wavelength (or frequency) only, NOT the intensity. But the energy you experience depends also on the intensity (total number of photons). 20

In Summary: properties of Light All light travels through (vacuum) space with a velocity = 3x108 m/s The frequency (or wavelength) of a photon determines how much energy the photon has (E=h). The number of photons (how many) determines the intensity. Light can be described in terms of either energy, frequency, or wavelength. 21

Compared to visible light, radio waves have: higher energy and longer wavelength higher energy and shorter wavelength lower energy and longer wavelength lower energy and shorter wavelength all light has the same energy 22

Origin of light Light (electromagnetic radiation) is just varying electric and magnetic fields that propagate through space. Now, two very important things happen in nature: An electric field that varies in strength (e.g., owing to the acceleration of an electron) generates electromagnetic radiation. Electromagnetic radiation, in turn, accelerates electrons (or any electrically charged particle) We discuss two major mechanisms of light production: Blackbody Radiation, a.k.a. thermal radiation Spectral Line Emission by atoms and molecules

Heat and Temperature Temperature refers to the degree of agitation, or the average speed with which the particles move (T~v2). All atoms and molecules are moving and vibrating unless at absolute zero Temp = -273 C = 0 K = -459.7 F Heat refers to the amount of energy stored in a body as agitation among its particles and depends on density as well as temperature. Things are composites of molecule, which are made of atoms. They are bound together by the e-M force…

Scale of Temperature Fahrenheit Centigrade Kelvin F C K 212 Water Boils 100 Water Boils 373 32 Water Freezes Water Freezes 273 -459.7 Absolute Zero -273 Absolute Zero

Scale of Temperature C = (F-32) / 1.8 F = 1.8*C + 32 C = K - 273.16 K = C + 273.16 Absolute zero of temperature (atoms are still) K = 0 C = -273.16 F = -459.7

Blackbody Radiation Acceleration or deceleration of an electron results in the production of an electromagnetic wave. Heat makes electrons and atoms move: thermal motion. Electrons collide against atoms and each other all the time, i.e. accelerate and decelerate all the time. Makes electromagnetic radiation so also called thermal radiation. Makes a continuous spectrum of radiation so also called continuum radiation.

Blackbody Spectrum It is the spectrum of radiation from thermal motions of matter. Temperature is how fast the electrons and atoms are moving on average. Faster motions means more energetic collisions and higher energy photons.

Hotter objects have electrons moving with higher speed, thus they emit photons with a higher average energy. Wien’s Law: The wavelength at the peak of the blackbody emission spectrum is given by max (nm)= 2,900,000/T (P.S. remember that E = hc/) Thus, the hotter the matter, the higher the energy of the e.m. radiation Introduce temperature units, thermometer

Total emitted power: E = 4  R2 σ T4 Hotter objects emit more total radiation per unit area. However, a big cold object can emit the same or more energy (depending on how big it is) than a small, hotter one Cold Hot Stefan-Boltzmann Law: Emitted power per square meter = σ T4 σ = 5.7 x 10-8 W/(m2K4) Total emitted power: E = 4  R2 σ T4

L=A T4

The graph below shows the blackbody spectra of three different stars The graph below shows the blackbody spectra of three different stars. Which of the stars is at the highest temperature? Which has the highest energy (energy is the total area under the curve)? 1) Star A 2) Star B 3) Star C A B Energy per Second C Wavelength

Reminder: Blackbody Radiation, i. e Reminder: Blackbody Radiation, i.e. a continuum of wavelengths with a characteristic distribution of strengths

is bluer when hotter. is both is neither You are gradually heating up a rock in an oven to an extremely high temperature. As it heats up, the rock emits nearly perfect theoretical blackbody radiation – meaning that it is brightest when hottest. is bluer when hotter. is both is neither

Campfires Campfires are blue on the bottom, orange in the middle, and red on top. Which parts of the fire are the hottest? the coolest? As atoms get hotter, they wiggle faster and collide with each other harder ---> they emit more light and more wiggles per second = frequency goes up = wavelength goes down. Thus bluer. Thus, as temperature goes up light gets stronger and gets blue.

More on Blackbody Radiation (a.k.a. Thermal Radiation) Every object with a temperature greater than absolute zero emits blackbody radiation. Hotter objects emit more total radiation per unit surface area. Hotter objects emit photons with a higher average energy.

Color and Temperature Orion Stars appear in different colors, Orion from blue (like Rigel) Betelgeuse via yellow (like our sun) to red (like Betelgeuse). If the spectra of stars are black bodies, then these colors tell us about the star’s temperature. Rigel

Stars come in different colors

How to Make Light (Part 2): Line Emission/Absorption (light with discrete wavelengths) Structure of atoms Energy levels and transitions Emission and absorption lines Light scattering Please pick up a diffraction grating on your way into class.

The Structure of Matter: Atoms Atoms are made of electrons, neutrons, and protons. The binding force: the attractive Coulomb (electrical) force between the positively charged protons in the nucleus and the negatively charged electrons around the nucleus. A Planetary Model of the Atom A story about atoms To understand the microscopic processes involved and to study another emission mechanism As an introduction to elements

The Structure of Matter: Atoms An atom consists of an atomic nucleus (protons and neutrons) and a cloud of electrons surrounding it. Almost all of the mass is contained in the nucleus, while almost all of the space is occupied by the electron cloud. 43

Electron Orbits Electron orbits in the electron cloud are restricted to very specific radii and energies. r3, E3 r2, E2 r1, E1 These characteristic electron energies are different for each individual atom, i.e. element. 44

Energy Levels In other words, the electron in a given orbit has the energy that corresponds to that orbit. The higher the orbit, the higher the energy. Different atoms have different energy levels, set by quantum physics. Quantum means discrete! Ladder Gasoline vs apple

Spectral Line Emission Collisions (like in a hot gas) can provide electrons with enough energy to change energy levels. A photon of the difference in energy between levels is emitted when the electron quickly falls down to a lower level.

Energy Levels of a Hydrogen Atom Different allowed “orbits” or energy levels in a hydrogen atom. Emission line spectrum Absorption line spectrum

Spectral Lines of Some Elements Argon Helium Mercury Sodium Neon Spectral lines are like a cosmic barcode system for elements.

Atoms of different elements have unique spectral lines because each element has atoms of a unique color has a unique set of neutrons has a unique set of electron orbits has unique photons

Spectral Line Emission again If a photon of exactly the right energy is absorbed by an electron in an atom, the electron will gain the energy of the photon and jump to an outer, higher energy level. A photon of the same energy is emitted when the electron falls back down to its original energy level or of a different energy if it falls down to a different energy level.

Atomic Transitions: excitation of atoms Remember that Energy = Wavelength = Colors Eph = E3 – E1 An electron can be kicked into a higher orbit when it absorbs a photon with exactly the right energy. Eph = E4 – E1 Wrong energy The photon is absorbed, and the electron is in an excited state. (Remember that Eph = h*c/) All other photons pass by the atom unabsorbed. 52

Atomic Transition: Excitation of Atoms To change energy levels, an electron must either absorb or emit a photon that has the same amount of energy as the difference between the energy levels E = h = hc/ --- A larger energy difference means a higher frequency. Different jumps in energy levels means different frequencies of light absorbed, i.e. different colors

Kirchhoff’s Laws of Radiation A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum. 54

Kirchhoff’s Laws of Radiation 2. A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum. Light excites electrons in atoms to higher energy states Transition back to lower states emits light at specific frequencies 55

Light excites electrons in atoms to higher energy states Kirchhoff’s Laws of Radiation 3. If light comprising a continuous spectrum passes through a cool, low- density gas, the result will be an absorption spectrum. Light excites electrons in atoms to higher energy states Frequencies corresponding to the transition energies are absorbed from the continuous spectrum. 56

Sources of spectral lines Emission nebula Reflection nebula

The Spectrum of a star (the Sun) There are similar absorption lines in the other regions of the electromagnetic spectrum. Each line exactly corresponds to chemical elements in the stars.

The spectrum of a star’s light is approximately a black body spectrum. The spectrum of a star: nearly a Black Body The light from a star is usually concentrated in a rather narrow range of wavelengths. The spectrum of a star’s light is approximately a black body spectrum. In fact, the spectrum of a star at the photosphere, before the light passes through the atmosphere of the star, is a nearly PERFECT black body one 60

Again, remember the two Laws of Black Body Radiation. I 1. The hotter an object is, the more energy it emits: L = 4 R2T4 More area, more energy where L = Energy = = Energy given off in the form of radiation, per unit time [J/s];  = Stefan-Boltzmann constant 61

(where TK is the temperature in Kelvin) Again, remember the two Laws of Black Body Radiation. II 2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases.  Wien’s law: max ≈ 2,900,000 nm / TK (where TK is the temperature in Kelvin) 62

The spectra of stars also contain characteristic absorption lines. Stellar Spectra The spectra of stars also contain characteristic absorption lines. With what we have learned about atomic structure, we can now understand how those lines are formed. 63

The Spectra of Stars The inner, dense layers of a star do produce a continuous (blackbody) spectrum. Cooler surface layers absorb light at specific frequencies. The atmosphere also absorbs light at other specific frequencies => Spectra of stars are B.B.absorption spectra. 64

By far the most abundant elements in the Universe Analyzing Absorption Spectra Each element produces a specific set of absorption (and emission) lines. Comparing the relative strengths of these sets of lines, we can study the composition of gases. By far the most abundant elements in the Universe 65

Lines of Hydrogen Most prominent lines in many astronomical objects: Balmer lines of hydrogen 66

The only hydrogen lines in the visible wavelength range The Balmer Absorption Lines n = 1 n = 4 Transitions from 2nd to higher levels of hydrogen n = 5 n = 3 n = 2 H H H The only hydrogen lines in the visible wavelength range 2nd to 3rd level = H (Balmer alpha line) 2nd to 4th level = H (Balmer beta line) 67

Emission nebula, dominated by the red H line Observations of the H-Alpha Line Emission nebula, dominated by the red H line 68

Absorption Spectrum Dominated by Balmer Lines Modern spectra are usually recorded digitally and represented as plots of intensity vs. wavelength 69

The Balmer Thermometer Balmer line strength is sensitive to temperature: Most hydrogen atoms are ionized => weak Balmer lines Almost all hydrogen atoms in the ground state (electrons in the n = 1 orbit) => few transitions from n = 2 => weak Balmer lines 70

Comparing line strengths, we can measure a star’s surface temperature! Measuring the Temperatures of Stars Comparing line strengths, we can measure a star’s surface temperature! 71

Spectral Classification of Stars Different types of stars show different characteristic sets of absorption lines. Temperature 72

Mnemonics to remember the spectral sequence: Spectral Classification of Stars Mnemonics to remember the spectral sequence: Oh Only Be Boy, Bad A An Astronomer s Fine F Forget Girl/Guy Grade Generally Kiss Kills Known Me Mnemonics 73

Stellar Spectra O B A F Surface temperature G K M 74

The Composition of Stars From the relative strength of absorption lines (carefully accounting for their temperature dependence), one can infer the composition of stars. 75