# Radiation and Spectra Chapter 5

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Light Radiation and Spectra Chapter 5

What is Light? Newton Maxwell Quantum Mechanics
Prism shows white light contains all colors Light made of particles (photons) Maxwell Theory of electricity and magnetism Light is electromagnetic waves Produced by wiggling electrons Radiation = production of light Quantum Mechanics Light is both: particle and wave

Waves Wavelength ( l ) Frequency ( f )
Distance between crests (or troughs) Frequency ( f ) How often it repeats (wiggles up and down) Measured in Hertz (Hz) number of times per sec

Waves Speed c = 3 x 108 m/s Wavelength inversely related to frequency
c = lf Wavelength inversely related to frequency l = c / f high frequency = short wavelength low frequency = long wavelength

Particles as Waves “Wave Packet” particle/photon = localized wave

Properties of Light Color Carries energy (heat) Depends on frequency
blue = high frequency = short wavelength red = low frequency = long wavelength Carries energy (heat) Photon energy E = h f high frequency = high energy = blue low frequency = low energy = red h = Planck’s constant

Red light has ____ than blue light.
A. larger frequency, energy, and wavelength B. smaller frequency, energy, and wavelength C. larger frequency and energy, but smaller wavelength D. smaller frequency and energy, but larger wavelength

All types of light travel at the same speed -
Which of the following travels fastest? } radio waves infrared (heat) waves microwaves blue light waves none of the above All are types of light! All types of light travel at the same speed - the “speed of light”, c

Propagation of Light Photons travel in straight lines
energy spread over larger area at larger distances produces 1/r2 decrease in brightness Double distance - brightness decreases by 4

If a 100-watt light bulb is placed 10 feet away from
you, and an identical 100-watt light bulb is placed 100 feet away from you, which will appear brighter? The closer one The farther one They will appear the same brightness How much fainter will the far one appear compared to the close one? Twice as faint 10 times fainter 100 times fainter 1000 times fainter ~ 1/r2

Electromagnetic Spectrum
Visible light: red, orange, yellow, green, blue, indigo, violet (ROYGBIV) Invisible Light: Ultraviolet = bluer than blue Infrared = redder than red Other wavelengths: Short: X-rays, gamma-rays Long: microwave, radio

Which kind of electromagnetic radiation has a
wavelength longer than that of visible light? A. infrared B. ultraviolet C. x-rays D. gamma rays E. none of the above

What’s the wavelength of my favorite radio station?

Objects made of atoms Atoms (and their electrons) vibrate Wiggling electrons radiate, producing light Bigger objects produce more light Higher temperature = stronger vibration Hotter objects emit more light Perfect absorber is black Absorbed light (energy) heats object Temperature increases until emitted energy = absorbed energy Emitted radiation called Blackbody Radiation Thermal radiation emitted by most objects similar to blackbody

Luminosity, L L = energy emitted per second Luminosity for a spherical object (a star) L = 4p R2 s T4 R = radius (size) of star; T = temperature double size, luminosity increases by 2x2 = 4 double temperature; luminosity increases by 2x2x2x2 = 16 Stefan-Boltzmann Law

Luminosity Temperature and Size Part I
WORKBOOK EXERCISE: Luminosity Temperature and Size Part I (pages in workbook)

Color Wavelength where most light emitted lmax = 3 x 106 / T T in Kelvin; lmax in nanometers (1 nm=10-9m) Cool stars are red Hot stars are blue Color indicates temperature! Wien’s Law As T , Wavelength , Color = redder As T , Wavelength , Color = bluer

Homework: WORKBOOK EXERCISE

The graph above shows blackbody spectra for three different stars
The graph above shows blackbody spectra for three different stars. Which of the stars is at the highest temperature? Star A Star B Star C Because peak energy emission occurs at shortest wavelength

Doppler Shift Originally discovered using sound waves Moving object
emits light with slightly different color Frequency (pitch) of approaching object is higher Blueshift Wavelength shorter (shifted blueward) Frequency (pitch) of receeding object is lower Redshift Wavelength longer (shifted redward)

Doppler Shift Redshift Blueshift

Spectroscopy Prism separates light into different colors
Continuous spectrum contains all colors Example: blackbody spectrum

Spectroscopy Absorption Line Spectrum
Some colors are missing (discrete lines) Solar Spectrum N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

Spectroscopy Emission Line spectrum
Only certain colors are present (discrete lines) Spectrum for each element unique (like fingerprints)

Model Atom Nucleus contains protons and neutrons
number of protons = element (1 proton = hydrogen, 2 protons = helium, etc.) number of neutrons about same as protons Isotope = different number of neutrons hydrogen helium Isotopes of hydrogen

Model Atom Electrons orbit nucleus
Number of electrons = number of protons Ionization = removing electrons Only certain orbits are allowed hydrogen helium

Atomic Absorption Atom absorbs photon energy
electron “jumps” to higher energy orbit only certain discrete orbits are allowed Atom can absorb only discrete colors (energies)

Atomic Emission Electron “jumps” to a lower energy orbit
Atom emits photon can emit only discrete colors same colors (wavelengths/energies) as absorption

Atomic Energy Levels Energy Levels Different for each element
each element has unique set of absorption/emission lines

Kirchoff’s Laws Continuous spectrum Emission line spectrum
Produced by hot solid (or dense gas) Emission line spectrum Produced by hot, low density gas Absorption line spectrum Produced when continuous source is viewed through cooler low density gas

Kirchoff’s Laws Absorption lines same wavelengths as emission lines
Gas can only absorb and emit at certain discrete frequencies/wavelengths/energies

WORKBOOK EXERCISE: “Types of Spectra” (pages in workbook)

If you analyze the light from a low density object (such as a cloud of interstellar gas), which type of spectrum do you see? dark line absorption spectrum bright line emission spectrum continuous spectrum

Imagine that you observe the Sun while in your space ship far above Earth’s atmosphere. Which of the following spectra would you observe by analyzing the sunlight? dark line absorption spectrum bright line emission spectrum continuous spectrum