Chapter 5 Notes: Jessica Horn

Chapter 5 Light: The Cosmic Messenger

• What is light? Light is an electromagnetic wave.
• Wavelength and Frequency: wavelength x frequency = speed of light = constant
• Particles of Light: Particles of light are called photons. Each photon has a wavelength and a frequency. The energy of a photon depends on its frequency. 
• The higher the photon energy, the shorter its wavelength.
• Atomic terminology: atomic number = # of protons in nucleus. Atomic mass number = # of protons + neutrons. Isotope: same number of protons but different number of neutrons. Molecules consist of two or more atoms.
• How do light and matter interact? Emission, absorption, transmission-transparent objects transmit light, opaque objects block (absorb) light. Reflection or scattering.
• What are the three basic types of spectra? Emission Line Spectrum, Continuous Spectrum, and Absorption Line Spectrum.

1. Continuous Spectrum-The spectrum of a common light bulb spans all visible wavelengths without interruption.
2. Emission Line Spectrum-A thin or low density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.
3. Absorption Line Spectrum-A cloud of gas between us and a light bulb can absorb light a specific wavelengths  leaving dark absorption line in the spectrum.

• How does light tell us what things are made of? Chemical fingerprints  Each type of atom has a unique set of energy levels. Each transition corresponds to a unique photon energy, frequency, and wavelength. Downward transitions produce a unique pattern of emission lines. Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths. Each type of atom has a unique spectral fingerprint. Observing the fingerprints in a spectrum tells us which kinds of atoms are present. Example: Solar Spectrum.
• How does light tell us the temperatures of planets and stars? Thermal Radiation: Nearly all large or dense objects emit thermal radiation, including stars, planets, and you. An object's thermal radiation spectrum depends on only one property: its temperature.
• Properties of Thermal Radiation. 1. Hotter objects emit more light at all frequencies per unit area. 2. Hotter objects emit photons with a higher average energy.
• Interpreting an Actual Spectrum: By carefully studying the features i a spectrum, we can learn a great deal about the object that created it. 
• Measuring the Shift: We generally measure the Doppler effect from shifts in the wavelengths of spectral lines. Doppler shift tells us ONLY about the part of an object's motion toward or away from us. 
• How do telescopes help us learn about the universe? Telescopes collect more light than our eyes = light-collecting area. Telescopes can see more detail than our eyes = angular resolution. Telescopes/instruments can detect light that is invisible to our eyes (ex infrared, ultraviolet).
• Why do we put telescopes into space? It is NOT because they are closer to the stars! 1. Light Pollution 2.Turbulence causes twinkling = blurs images. 3. Atmosphere absorbs most of EM spectrum, including all UV and X ray and most infrared. 
• How is technology revolutionizing astronomy? Adaptive optics: rapid changes in mirror shape compensate for atmospheric turbulence. Interferometry: This technique allows two or more small telescopes to work together to obtain the angular resolution of a larger telescope.