Astronomy 103: 6-Astronomical Spectra and the Doppler Effect

Key Concepts

  • Types of Spectra:

    • Thermal Spectrum (Continuous Spectrum):

    • Produced by hot light sources like a light bulb, emitting light across all visible wavelengths (colors).

    • The spectrum appears as a smooth, continuous rainbow without gaps.

    • Graph representation shows smooth intensity variation across different wavelengths.

    • Absorption Spectrum (Discrete Spectrum):

    • Occurs when light from a hot source passes through a cooler gas cloud.

    • Atoms in the gas absorb specific wavelengths of light based on their composition and temperature.

    • Results in dark absorption lines at particular wavelengths, indicated by dips in intensity on the graph.

    • Emission Spectrum (Discrete Spectrum):

    • Produced when atoms in a warm gas cloud emit light at specific wavelengths.

    • Emission lines represent bright spikes at these wavelengths in the spectrum, indicating no other light is present.

Using Spectrographs

  • Spectrograph Components:

    • Eyepiece: To observe spectral lines.

    • Slit: Allows light from the source to enter the device.

    • Light Source: Can be a hot object emitting light.

    • Spectrum Appearance: Observers look at the spectrum through the eyepiece after the light passes through the slit.

Continuous Spectrum Details

  • Produced by:

    • A hot light source (such as a light bulb).

  • Characteristics:

    • Displays a wide range of visible wavelengths smoothly without interruption.

    • The graphical representation shows relatively consistent intensity across various wavelengths with slight variations.

Absorption Spectrum Details

  • Mechanism:

    • Hot light passes through a cooler gas cloud; specific wavelengths are absorbed by atoms in the cloud.

  • Characteristics:

    • Displays dark absorption lines on a continuous spectrum graph, indicating the absence of light at those specific wavelengths.

    • Each absorption line correlates to a dip in intensity at that wavelength.

Emission Spectrum Details

  • Mechanism:

    • Atoms in a warm gas cloud emit light at distinct wavelengths related to their composition and temperature.

  • Characteristics:

    • Forms bright emission lines at particular wavelengths on a spectrum, with no other light present.

    • The graphical representation shows upward spikes at wavelengths where light is emitted.

Energy and Electron Transitions

  • Photon Emission and Absorption:

    • Energy of Light and Atoms:

    • Photons emitted or absorbed correspond to specific energy differences between electron orbitals of an atom.

    • Example: For hydrogen, the energy of red light matches the energy difference between electron orbitals where an electron can transition from level n=2 to level n=3.

    • Light absorbed by an atom can cause an electron to move to a higher energy level, whereas light that does not match any energy level transition does not interact with the atom.

Key Equations

  • Energy of a Photon:

    • Given as: E = \frac{hc}{\lambda}

    • where:

      • $E$ = energy of the photon

      • $h$ = Planck's constant (approximately 6.626 \times 10^{-34} \, J \, s)

      • $c$ = speed of light (approximately 3.00 \times 10^8 \, m/s)

      • $\lambda$ = wavelength of light

Summary of Elemental Spectra

  • Elements observed in spectral analysis include:

    • Hydrogen

    • Helium

    • Oxygen

    • Neon

    • Argon

  • Specific Wavelengths Observed:

    • Example wavelengths include $\lambda = 400 \text{ nm}$ to $\lambda = 700 \text{ nm}$, representing a range of visible light across spectral lines.