Light and Telescopes

LIGHT: THE BASICS

Longer Wavelengths vs. Shorter Wavelengths

Wavelength: the distance from the crest of one wave to the crest of the successive wave

Frequency: longer wavelengths corresponds to lower frequency and lower energy, shorter wavelengths correspond to high frequency and higher energy

Radiation: transmission of energy through space

Transmission: light rays or electromagnetic waves bending through a different medium

  • All waves have a source (e.g. electromagnetic waves originate from vibrating charged particles)
  • All waves, except electromagnetic waves, transmit through a medium

Wavelengths in Visible Light

Electromagnetic Spectrum: electromagnetic waves ranging from low frequency,  low energy, and long wavelength to high frequency, high energy, and short wavelength that originate from vibrating charges from the Sun; all electromagnetic waves travel at the same speed, or the speed of light (c = 300,000 km/sec or 186,000 miles/sec)

PROPERTIES OF LIGHT

Reflection: light rays or electromagnetic waves bouncing off reflective surfaces (e.g. mirror)

Refraction: light rays or electromagnetic waves bending through a different medium (e.g. air to water)

PROBLEMS WITH LIGHT AND MIRRORS

Spherical Aberration

Spherical Aberration: when light rays incident on the edges of the spherical mirror are focused at a different point from light rays incident closer to the center of the mirror –> blurry images; corrected by using parabolic mirrors

Chromatic Aberration

Chromatic Aberration: as light rays travel through a lens, different wavelength rays are bent by different amounts, resulting in different focal points

OPTICAL TELESCOPES

Three Types: 1. Reflective (mirrors), 2. Refractive (lens); 3. Combined or Catadioptic (both mirrors and lens): combines advantages of refractive and reflective telescopes, while avoiding disadvantages

  • Objective: main lens or mirror
  • Eyepiece: lens that magnifies images
  • Focal Length: distance between the center of the lens and the its focus
  • Aperture: diameter of objective

Functions of Telescopes: to collect light, to resolve details, to magnify, to measure, to record

Problems of Optical Telescopes: “seeing” (Earth’s atmosphere refracts light), air transparency, light pollution

Hubble Space Telescope

Unusual Telescopes

  • Radio: Arecibo, VLA, COBE
  • Microwave, or RadarPIONEER, COBE
  • InfraredSIRTF, IRAS, SPITZER
  • UltravioletCOPERNICUS, IUE
  • X-rayHEAO, EXOSAT, CHANDRA
  • Gamma RayGRO, EINSTEIN, COMPTON
  • OrbitalHUBBLE
  • Multiple MirrorsKECK
  • Interferometry: VLA, VLT
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Light and Black Body Radiation

Light is composed of mass-less infinitesimal particles called photons that travel at the speed of light (300,000,000 m/s).

Electromagnetic Spectrum

THE ELECTROMAGNETIC SPECTRUM: depicts the different wavelengths and energies of light

Radio Waves –> Microwaves –> Infrared Light –> Visible Light (ROYGBIV) –> Ultraviolet Radiation –> X-Rays –> Gamma Rays (longest –> shortest wavelengths, lowest –> highest energies)

  • The Electromagnetic Spectrum and Stellar Spectra = continuous spectrum (energy emission over a  broad range of wavelengths – curve)
  • Laser = line spectrum (energy emission at a narrow range of wavelength – peak)

Black-bodies at Different Temperatures

A “black-body” is an object which absorbs all light incident on it and doesn’t reflect or transmit any light. Black bodies are perfect emitters of light. Their classification depends only on temperature, and not other properties such as chemical composition; hence, black-body radiation is also “thermal” radiation. In 1900, Max Planck discovered that a black body emits an energy spectrum of light. Black body radiation includes lava flow (800 K), incandescent light bulbs – tungsten wire heated (2,800 K). Comparing two black bodies of different temperatures, the hotter black-body will: 1) emit more radiation (more luminous); 2) emit more photons; 3) peaks at shorter wavelengths; 4) have a bluer color. Measuring the shape of a star’s spectrum can reveal the star’s temperature.

Wien’s Lawγ peak = 2,900 μm K/ T; using the wavelength of the black-body’s spectrum’s peak to determining the star’s surface temperature

Luminosity: amount of energy radiated by an object per second, in Watts

Brightness: how bright an object appears as seen by an observer; also known as flux received from the star

Stefan- Boltzmann LawL = σT4 x surface area, where L = luminosity, T = temperature, and σ = 5.67 x 10-8 W/ (m²•K4), Stefan-Boltzmann constant; to determine a star’s luminosity

 Apparent Brightness: how bright stars appear to the observer; depends on luminosity and distance

  • considering a set of photons that emerge at the same moment from the star’s surface, the spherical shell of photons is 4∏r², where r = distance from the star
  • L/4∏r² (L = luminosity) = energy per second per surface area of photons
  • apparent brightness or flux: b = L/4∏r²

Absolute Brightness: considering temperature and mass and disregarding distance, how bright the stars actually are

PHOTONS AND THE ATOM

The Atom

The Atom and Its Subatomic Particles

  • Subatomic particles: Electrons (-), Protons (+), and Neutrons (neutral)
  • The mass of a proton is 1830 times the mass of an electron; the mass of a proton is approximately equal to the mass of a neutron
  • While protons and neutrons form the atom’s nucleus, electrons have discrete energy levels in atom
  • The electron can only be on energy levels, not in between
  • Outer orbits have higher energy than inner orbits
  • Most of the space within an atom is empty!

Absorption/ Emission: Photons

Photons: Emission and Absorption

  • Photons are emitted in random fashion (cascade from level to level or all at once – from current level to the ground state, or the lowest energy level, the closest to the nucleus)
  • Absorption of a photon causes the electron to a higher energy level
  • A photon can only be absorbed if its energy is equal to the difference in energy between two energy levels
  • An electron can only stay in a higher energy level for a very short time
  • Ionization: If a photon is large enough, it can kick the electron out of the atom
  • Recombination: When a free electron becomes bound to an atom
  • Electrons give up energy by emitting a photon

Emission Lines from Gas Clouds

Emission Line Spectrum

  • A dilute (non-opaque) gas cloud is not a back-body emitter
  • Atoms in a hot, dilute cloud of ionized gas will emit a characteristic pattern of spectra lines (Emission Line Spectrum)

Absorption Line Spectra

Absorption Spectrum

  • Normal stars have absorption lines
  • Black-body radiation originates from the star’s interior