LECTURE 11: Atoms and Light

Key Questions:






ATOMS

All material objects are made of atoms, which are themselves made up of protons, neutrons, and electrons.








CONSTITUENTS OF ATOMS

Atoms are made of protons (p), neutrons (n), and electrons (e).








THE FOUR FORCES

There are only four basic forces in nature: gravity, electromagnetic, and the strong and weak nuclear forces.








CHEMICAL ELEMENTS

The number of protons in an atom determines its chemical behavior.








THE SPEED OF LIGHT

In empty space, light travels at a constant, finite speed,

c = 300,000 km/s = 3 x 108 m/s.







ENERGY

Examples:






WAVES

Characteristics of waves:






ELECTROMAGNETIC WAVES








VISIBLE LIGHT

Visible light is a form of electromagnetic wave. The wavelength of light determines its color.








THE ELECTROMAGNETIC SPECTRUM

There are other forms of electromagnetic radiation. All of them travel (in empty space) at the speed of light c.

From long wavelength (low frequency) to short wavelength (high frequency):

Earth's atmosphere is opaque to many forms of electromagnetic radiation.
Windows of transparency: visible and part of radio.






PHOTONS

EM radiation travels in discrete packets of energy, called photons, which can be thought of as ``particles of light.''

The energy of a photon depends on its wavelength; shorter wavelength = higher energy.

The energy of a photon is
E = h c / \lambda
E = energy of photon
\lambda = wavelength of photon
c = speed of light
h = Planck's constant (a universal number).

For many purposes in astronomy, more useful to think of light (and EM radiation) in terms of photons rather than waves.






TEMPERATURE AND DEGREES KELVIN








BLACKBODY RADIATION

Opaque objects emit EM radiation with a characteristic, continuous distribution of wavelengths called a blackbody spectrum.

The wavelength of peak emission depends on the object's temperature: hotter objects emit more energy at shorter wavelengths.

Consider a body that could absorb all light that falls on it.

Examples:






ATOMIC FINGERPRINTS IN SPECTRA

The spectrum of an object is the distribution of its emitted electromagnetic radiation.
We often show the spectrum with a plot of emitted energy vs. wavelength.

Atoms like to emit and absorb light at particular wavelengths, producing spectra with discrete lines.
Each type of atom has a characteristic spectral ``fingerprint.''

Absorption and emission lines:








WHAT CAN WE LEARN FROM AN OBJECT'S LIGHT?

Spectra of astronomical objects (e.g., the Sun) may have a mix of continuous spectrum, absorption lines, and emission lines.

A complication for objects in solar system: much of light is reflected from Sun, not emitted by body.






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Updated: 2005 April 24 [dhw]