1	Light Properties of light are fundamental Almost everything we know about the universe outside our solar system comes from interpreting the light from distant objects. Light is weirder than you think Here are some slides I’ve used for introductory astronomy as a guide to some properties of light. The math here might be useful in figuring out your own stories involving light.
2	Radiation: Two different kinds Something that “radiates”, or spreads out in “rays” High speed particles (eg. high speed neutrons ejected from a disintegrating atomic nucleus) Electromagnetic radiation: Towards shorter “wavelength” and higher energy: Visible light, Ultraviolet light, X-Rays, Gamma-Rays Towards longer “wavelength” and lower energy: Visible light, Infrared radiation, microwaves, radio waves
3	Properties of Light Light has both wave and particle properties Travels like a wave Interacts with matter like a particle: “photon” Full explanation involves quantum mechanics For most cases we can just choose the right “model” from the above two choices Photons, unlike particles in other kinds of “radiation,” are particles of “pure energy”
4	Light is an electromagnetic wave Changing electric fields generates magnetic fields Changing magnetic fields generates electric fields Can set up a cycle where one field causes the other: The E and B fields oscillate in strength, and the disturbance moves forward. To describe the wave you need to specify Direction it is moving Strength of the fields (its intensity) Frequency or Wavelength of the oscillation (u and l are inversely related) Orientation of the electric E field: up or sideways (polarization) You do not need to specify its speed In a vacuum all lightwaves move at the same speed c = 3´10⁸ m/s
5	The Electromagnetic Spectrum Radio waves Microwaves Infrared Visible Ultra-violet X-Rays Gamma rays
6	Relationship between Energy and Wavelength of Light Short wavelength Þ High energy photons Long wavelength Þ Low energy photons Intensity µ total energy (per area per second) µ (# of photons per area per second) ´ (energy per photon) Example with falling rain: Amount of rain µ (# of raindrops) ´ (volume per drop)
7	Why is energy per photon so important? Real life example: Ultra-Violet light hitting your skin Threshold for chemical damage set by energy (wavelength) of photons Below threshold (long wavelengths) energy too weak to cause chemical changes Above threshold (short wavelength) energy photons can break apart DNA molecules Number of molecules damaged = number of photons above threshold Very unlikely two photons can hit exactly together to cause damage
8	Numerical Relationship between wavelength and photon energy Inverse relationship: Smaller l means more energetic c = speed of light = 3.00 ´ 10⁸ m/s h = Planck’s constant = 6.63 ´ 10^-34 joule/s Note: Joule is a unit of energy 1 Joule/second = 1 Watt Energy of a single photon of 0.5 mm visible light? Seems very small, but this is roughly the energy it takes to chemically modify a single molecule. Photons from a 100 W lightbulb (assuming all 100W goes into light?)
9	Temperature and Heat Thermal energy is “kinetic energy” of moving atoms and molecules Hot material energy has more energy available which can be used for Chemical reactions Nuclear reactions (at very high temperature) Escape of gasses from planetary atmospheres Creation of light Collision bumps electron up to higher energy orbit It emits extra energy as light when it drops back down to lower energy orbit (Reverse can happen in absorption of light)
10	Temperature Scales Want temperature scale where energy is proportional to T Celsius scale is “arbitrary” (Fahrenheit even more so) 0^o C = freezing point of water 100^o C = boiling point of water By experiment, available energy = 0 at “Absolute Zero” = –273^oC (-459.7^oF) Define “Kelvin” scale with same step size as Celsius, but 0K = -273^oC = Absolute Zero Use Kelvin Scale for most of work in this course Available energy is proportional to T, making equations simple (really! OK, simpler) 273K = freezing point of water 373K = boiling point of water 300K approximately room temperature
11	Planck “Black Body Radiation” Hot objects glow (emit light) Heat (and collisions) in material causes electrons to jump to high energy orbits As electrons drop back down, some of energy is emitted as light. Reason for name “Black Body Radiation” In a “solid” body the close packing of the atoms means than the electron orbits are complicated, and virtually all energy orbits are allowed. So all wavelengths of light can be emitted or absorbed. (In a gas with isolated atoms, only certain orbits are permitted so only certain wavelengths can be absorbed or emitted.) A black material is one which readily absorbs all wavelengths of light. These turn out to be the same materials which also readily emit all wavelengths when hot. The hotter the material the more energy it emits as light As you heat up a filament or branding iron, it glows brighter and brighter The hotter the material the more readily it emits high energy (blue) photons As you heat up a filament or branding iron, it first glows dull red, then bright red, then orange, then if you continue, yellow, and eventually blue
12	Planck and other Formulae Planck formula gives intensity of light at each wavelength It is complicated. We’ll use two simpler formulae which can be derived from it. Wien’s law tells us what wavelength has maximum intensity Stefan-Boltzmann law tells us total radiated energy per unit area
13	Example of Wien’s law What is wavelength at which you glow? Room T = 300 K so This wavelength is about 20 times longer than what your eye can see. Camera in class operated at 7-14 μm. What is temperature of the sun – which has maximum intensity at roughly 0.5 mm?
14	Kirchoff’s laws Hot solids emit continuous spectra Hot gasses try to do this, but can only emit discrete wavelengths Cold gasses try to absorb these same discrete wavelengths
15	Atoms – Electron Configuration Molecules: Multiple atoms sharing/exchanging electrons (H₂O, CH₄) Ions: Single atoms where one or more electrons have escaped (H⁺⁾ Binding energy: Energy needed to let electron escape Permitted “orbits” or energy levels By rules of quantum mechanics, only certain “orbits” are allowed Ground State: Atom with electron in lowest energy orbit Excited State: Atom with at least one atom in a higher energy orbit Transition: As electron jumps from one energy level orbit to another, atom must release/absorb energy different, usually in form of light. Because only certain orbits are allowed, only certain energy jumps are allowed, and atoms can absorb or emit only certain energies (wavelengths) of light. In complicated molecules or “solids” many orbits and transitions are allowed Can use energy levels to “fingerprint” elements and estimate temperatures.
16	Hydrogen Lines Energy absorbed/emitted depends on upper and lower levels Higher energy levels are close together Above a certain energy, electron can escape (ionization) Series of lines named for bottom level To get absorption, lower level must be occupied Depends upon temperature of atoms To get emission, upper level must be occupied Can get down-ward cascade through many levels