# Astronomy Picture of the Day

## Presentation on theme: "Astronomy Picture of the Day"— Presentation transcript:

Astronomy Picture of the Day

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

CPS Question The color of visible light is determined by its ____.
A) brightness B) amplitude C) speed D) wavelength

CPS Question If the wavelength of light increases, the frequency must ____. A) increase also B) decrease C) remain unchanged

CPS Question The bending of light that occurs when moving between media of different densities is called ___. A) reflection B) refraction C) diffraction D) distortion

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

The Electromagnetic Spectrum
1 nm = m , 1 Angstrom = m c = f

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

What does the spectrum of an astronomical object's radiation look like?
Many objects (e.g. stars) have roughly a "Black-body" spectrum: Brightness Frequency also known as the Planck spectrum or Planck curve.

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

Spectroscopy and Atoms
How do you make a spectrum?

When you bend light, bending angle depends on wavelength, or color.
Refraction of light When you bend light, bending angle depends on wavelength, or color.

Questions How is temperature related to the amount of energy radiated?
How is temperature related to the color of the object? (Blackbody Demo)

The wavelength of peak emission tells us the temperature of the object!
"cold" dust "cool" star Sun "hot" stars frequency increases, wavelength decreases

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

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) video Video

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)

Pattern of lines is a fingerprint of the element

Sodium emission and absorption spectra
For a given element, emission and absorption lines occur at the same wavelengths. Helium discovered in Sun’s spectrum before being found on Earth! Sodium emission and absorption spectra

Spectrum of the Sun Absorption spectrum
What causes emission/absorption of light at specific wavelengths? Interactive Video 1, 2, 3

Types of Spectra 1. "Continuous" spectrum 2. "Emission" spectrum 3. "Absorption” Spectrum video

The Particle Nature of Light
Light interacts with matter as individual packets of energy, called photons. c photon energy is proportional to frequency: E f (or E  example: ultraviolet photons are more harmful than visible photons. 1 

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

The Nature of Atoms (Fair Analogy)
The Bohr model of the Hydrogen atom: electron _ _ + + proton "ground state" an "excited state" (Fair Analogy)

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)

When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system!

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

Other elements Helium Carbon
neutron proton Each element has its own allowed energy levels yielding a unique spectral fingerprint.

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 video

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

CPS Question Which ONE of these is constant for all forms of EM radiation in a vacuum? A) amplitude B) wavelength C) frequency D) speed E) energy

CPS Question Which ONE is NOT a property of a blackbody?
A) It appears black, regardless of its temperature. B) It emits radiation in a continuum of wavelengths. C) Its spectrum peaks at a wavelength determined by its temperature. D) The total energy that it radiates increases rapidly with temperature.

CPS Question The Sun's observed spectrum is _____.
A) A continuum with no lines, like the rainbow. B) A continuum with bright emission lines. C) Only absorption lines on a black background. D) Nearly a continuum with some absorption lines.

Ionization Hydrogen _ _ + + _ _ Helium + + + + _ _ "Ion"
Absorbing a high energy photon and atomic collisions can both lead to ionization.

Spectrum of the Sun Complicated objects => many different elements
Nearly continuous absorption spectrum What causes emission/absorption of light at certain wavelengths?

. . . . . . Why emission lines? hot cloud of gas
- Photon absorption/atomic collisions excite atoms - Electron drops back to lower level - Photons at specific frequencies emitted

Why absorption lines? . . . . . cloud of gas . . . . . . (Shockwave Demo) (Web Link)

Fusion generates continuous spectrum
Stellar Spectra Sun's 'atmosphere' absorbs specific wavelengths Fusion generates continuous spectrum Star

Kirchhoff's Laws 1. Continuous spectrum 2. Emission spectrum 3. Absorption spectrum

Question How does the pitch or tone of a sound wave change when the source of the sound is moving towards or away from you? What about when you are moving towards or away from the source? Does this effect occur for all types of waves or just for sound waves?

Doppler Shifted Atomic Spectra
Why don’t we see the color of everyday objects change as they move?

We've used spectra to find planets around other stars! (Ch. 4)

Star wobbling causes Doppler shift of its absorption lines.
Only gives information about velocity along line of sight!