Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 23: Light and Matter

Key Ideas:

Temperature (Kelvin Scale): Measures internal energy content
Kirchoff's Laws of Spectroscopy: A hot, dense object produces a continuous spectrum (blackbody spectrum); A hot, low-density gas produces an emission-line spectrum; A cool, dense gas produces an absorption-line spectrum

Interation of Light and Matter

Light and Matter can interact in a number of different ways:: Matter can transmit light (glass, water); Matter can reflect light; Matter gains energy by absorbing light; Matter loses energy by emitting light
The last two (absorption and emission) bear on the internal energy of the matter

Temperature

Temperature is a measurement of the internal energy content of an object.
Solids:: Higher temperature means higher average vibrational energy per atom or molecule
Gases:: Higher temperature means more average kinetic energy (faster speeds) per atom or molecule

Kelvin Temperature Scale

An absolute temperature system: Developed by Lord Kelvin (19th century); Uses the Celsius temperature scale
Absolute Kelvin Scale (K):: 0 K: Absolute Zero (all motion stops); 273 K: pure water freezes (0 Celsius); 310 K: human body (37 Celsius); 373 K: pure water boils (100 Celsius)
Advantage:: The total internal energy is directly proportional to the temperature scale in Kelvin.

What is a Spectrum?

A spectrum is the distribution of photon energies emitted by a light source:: Asks: how many photons of each energy are emitted?
Spectra are observed by passing light through a spectrograph: Breaks light into its component colors; Uses either prisms or diffraction gratings

Kirchoff's Laws of Spectroscopy

1) A hot solid or hot, dense gas produces a continuous spectrum.

2) A hot, low-density gas produces an emission-line spectrum

3) A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line spectrum

Blackbody Radiation

A Blackbody is an object that absorbs all light: Absorbs at all wavelengths; As it absorbs light, it heats up; Characterized by its Temperature
It is also a perfect radiator: Emits at all wavelengths (continuous spectrum); Energy emitted depends on Temperature; Peak wavelength depends on Temperature

Stefan-Boltzmann Law

Energy emitted per second per area by a blackbody with Temperature T:: E = sigma T⁴; sigma is Boltzmann's constant (a number)
In words:: Hotter objects are Brighter at all wavelengths

Wien's Law

Examples:

Heat a piece of iron from 300K to 600K: Temperature increases by 2 times; Brightness increases by 2⁴ = 16 times; Peak wavelength shifts towards the blue by 2 times from about 10 microns to about 5 microns
Hotter objects get brighter at all wavelengths and get bluer in color

Emission-Line Spectra

A hot, low-density gas emits an emission-line spectrum: Emits only at particular wavelengths, giving the appearance of bright, discrete "emission lines"; No light between the emission lines

Absorption-Line Spectrum

Light from a continuous spectrum through a vessel containing a cooler gas shows:: A continuous spectrum from the lamp crossed by dark "absorption lines" at particular wavelengths; The wavelengths of the absorption lines exactly correspond to the wavelengths of emission lines seen when the gas is hot!
Light is being absorbed by atoms in the gas

Why does it work?

Question:: Why does each element have a characteristic line spectrum?
Answer:: It reflects the detailed structure of the atom; Depends on the number and arrangement of electrons in orbit around the nucleus
Discovering why unlocked the secrets of the atom.

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.