4 Light Matter and Telescopes

Astronomy 103: Light and Tools of Astronomy

Introduction to Light

• Understanding Electric Forces:

  • Charges can be positive or negative.

  • Like charges repel; opposite charges attract.

Electric Fields and Light

• Electric Force:

  • The electric field is visualized as outward lines from a positive charge (inward for negative).

  • Fields extend to infinity but weaken with distance.

• Relation to Light:

  • Movement of a charge alters its electric field.

  • The propagation of this change is not instantaneous.

• Speed of Electric Field Changes:

  • Information travels at 300,000 km/s (speed of light).

  • After 1 second, the field's change is observed 1 light-second away (approx. from Earth to Moon).

Wave Formation

• Light as a Wave:

  • Movement of charge creates waves in the electric field traveling at the speed of light.

  • This wave is known as light, also called electromagnetic radiation.

Wave Characteristics

• Defining Light:

  • Lambda (λ) symbolizes light as a wave.

  • Key characteristics:

    • Frequency (f): Number of waves per second (measured in Hertz, Hz).

    • Wavelength (λ): Distance between wave crests.

• Visible Light Spectrum:

  • Wavelengths range from red (longest) to violet (shortest).

  • Visible light spectrum spans from 400 nm (violet) to 700 nm (red).

Frequency-Wavelength Relation

• Relationship:

  • Frequency (f) and wavelength (λ) are inverse; as one increases, the other decreases.

  • The equation: λ = speed x time.

Temperature and Radiation

• Understanding Temperature:

  • Measures of atomic and molecular movement:

    • Hot: fast atoms; Cold: slow atoms.

  • Absolute zero: -273°C (-459°F).

• Continuous Spectrum:

  • Emission occurs across all wavelengths; blackbody spectrum illustrates thermal radiation.

  • Hotter objects emit more energy and shorter wavelengths.

Star Temperature Effects

• Emission Variability:

  • Star temperature determines peak intensity and wavelength:

    • Hotter stars: Brightness and shorter peak wavelengths.

Spectra and Atoms

• Discrete Spectra:

  • Atoms absorb and emit light at specific wavelengths, producing a discrete spectrum.

  • Each element has a unique set of spectral lines, enabling identification.

• Electron Behavior:

  • Electrons can be excited to higher energy orbits when absorbing light.

  • Emission occurs when electrons fall back, releasing light of specific wavelengths.

Electron and Atoms

• When an atom is ionized:
  an electron leaves its atom

• When an electron absorbs light:
  - an electron jumps into a higher energy level

• When an electron emits light:

  • an electron jumps into a lower energy level

Practical Observations

• Spectral Identification:

  • Identification of elements in stars relies on spectral line patterns.

Doppler Effect

• Shift in Wavelength:

  • Moving light sources alter perceived wavelengths:

    • Redshift: object moving away.

    • Blueshift: object moving toward.

Telescopes as Tools

• Types of Telescopes:

  • Optical Telescopes: Use mirrors (reflecting) or lenses (refracting) to gather light.

  • Light travels fastest in space; slower in matter.

• Modern Telescopes:

  • Reflecting telescopes preferred due to issues with chromatic aberration in lenses.

  • Largest telescopes today utilize segmented mirrors to mitigate weight and improve performance.

Observational Techniques

• Image Acquisition:

  • Use of charge-coupled devices (CCDs) for capturing light information, preferable to photographic plates.

• Resolving Power:

  • Defined by the smallest angle distinguishable between two objects; larger telescopes demonstrate better resolution.

Atmospheric Considerations

• Limitations from Earth:

  • Atmospheric turbulence impacts image quality; solutions include mounting on high altitudes or using space-based telescopes.

Radio Astronomy

• Radio Telescopes:

  • Focus on longer wavelengths, less sensitive to imperfections.

  • Arrays provide superior angular resolution compared to single units.

Advanced Techniques

• Interferometry:

  • Combines data from multiple radio telescopes, achieving higher resolution similar to optical telescopes.

• Multispectral Observations:

  • Observing celestial objects across various wavelengths can yield comprehensive data (e.g., the Milky Way).

Observations of High-Energy Events

• X-ray and Gamma-ray Astronomy:

  • Requires specialized techniques; X-rays reflect at shallow angles.