PTYS/ASTR 206Telescopes and Imaging 2/1/07 Plus … Telescopes and Imaging Blackbody Radiation and Spectroscopy

PTYS/ASTR 206 Telescopes and Imaging2/1/07

Plus … Telescopes and Imaging

Blackbody Radiation and Spectroscopy


Announcements

• Second homework is due on Tuesday

• Next-week’s reading assignment– Sections 7-1, 7-2, 7-4, 7-5, and 7-6 (pp. 146-160)


Today’s topics

• Origin of light– Blackbody radiation– Wien’s Law– Stefan-Boltzman Law

• Light as a particle

• Emission and absorption spectra and Kirchoff’s Laws

• Telescopes -- basics


The Intensity of Light decreases with distance from the source and obeys the Inverse Square Law


Origin of Light

• Continuum– Everything that’s heated glows

– Color depends on temperature

• Atomic, molecular emissions– Every atom, molecule

has a characteristic spectrum (like a fingerprint)

– Caused by transitions from one energy level to another


Temperature Scales• Kelvin

– used by most astronomers and planetary scientists

– 0 K is “absolute” zero

• Kelvin Celsius TC = TK - 273

• Celsius Fahrenheit TF = (9/5)TC + 32

• Fahrenheit Celsius TC = (5/9)(TF-32)


Blackbody radiation

• A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths

• The Sun closely approximates the behavior of blackbodies, as do other hot, dense objects

• The intensities of radiation emitted at various wavelengths by a blackbody at a given temperature are shown by a blackbody curve


The Sun is like a Blackbody


Wien’s Law

• The higher the temperature, the smaller the wavelength of maximum emission

Example: A heated metal rod will start to glow red, then get brighter and glow yellow, then get brighter still and turn blue and then white


Wien’s Law

Wien’s law states that the dominant wavelength at which a blackbody emits electromagnetic radiation is inversely proportional to the Kelvin temperature of the object


Stefan-Boltzmann Law

• The Stefan-Boltzmann law states that a blackbody radiates electromagnetic waves with a total energy flux F directly proportional to the fourth power of the Kelvin temperature T of the object:

F = T 4

σ is called the Stefan-Boltzmann constant

This law can be used to determine the temperature of the Sun, starting with a measurement of the amount of light arriving at Earth.(see Box 5-2 of the textbook)


Light also behaves like a particle

• Max Planck was able to derive the blackbody spectrum by assuming that light was made up of tiny, discrete packets of energy – called photons

• Energy of a photon (light) with a wavelength, λ

ħ = Planck’s constant = 6.625 x 10-34 J • s


Photoelectric effect

• When UV light strikes a metal plate, electrons are emitted by the metal and can be detected

• When the plate is illuminated by visible light, no electrons are emitted.

• In the light-as-a-particle picture, this can be understood !– UV light has a shorter

wavelength and a higher energy compared to visible


The Modern View of Atomic Structure

• Protons, Neutrons, Electrons• Size

– About 10-10 m (1 Å – or 1 Angstrom)

– Nucleus is only 10-14 m !!• Mass

– Protons, Neutrons ~10-27 kg– Electrons ~10-31 kg

• The nucleus has most of the mass, but is less than 0.03% by volume of the entire atom!


What do Atoms have to do with Planetary Science?

• Spectroscopy!

• Emission and Absorption Lines– Each element emits/absorbs

at a specific wavelength that is unique to that element

– This fact can be used to infer the composition of a body or its atmosphere


Kirchoff’s Laws• A hot opaque body (blackbody)

produces a smooth continuous spectrum– Example: stars

• A cool transparent gas in front of a source of a continuous spectrum produces an absorption-line spectrum– Example – planetary

atmospheres, solar photosphere and chromosphere

• A hot transparent gas radiates an emission-line spectrum (against a dark background)– Example: the solar corona


Absorption lines in the Solar Spectrum

Indicates the presence of Iron in the Sun


Light Scattering:The reason the sky is blue (on Earth!)


Look how dark it is in the shadow of the Apollo 11 lander On Earth (Tucson Barrio), we

can see just fine in the shadows


The “Green Flash”


Telescopes and Astrophotography

• Basic telescope types and how they work

• Magnification and Resolution

• Atmospheric Turbulence– Hubble– Adaptive Optics

• Basics of Astrophotography



Basic Telescope Types

• Refractor

• Reflector– Newtonian – Schmidt-

Cassegrain

(adjacent photo)


Magnification• The amount of magnification depends on the focal

length of the primary and the eyepiece

Documents

PTYS/ASTR 206Telescopes and Imaging 2/1/07 Plus … Telescopes and Imaging Blackbody Radiation and Spectroscopy