Chapter 5Basic Properties of Light The chapter on Light is important for all studies in astronomy. Thus, it is important to review those properties in ASTR 1100. 1. Examine the nature of light as a wave and particle phenomenon in terms of its ability to carry packets of energy. 2. Note the various types of information ― line of sight speed, radiation temperature, chemical composition, etc. ― that can be established from the study of light from planets, stars, galaxies, gas clouds, etc.
Light is a phenomenon that is characterized by photons, which have both wave characteristics and particle characteristics. Because light has the properties of both a particle (the photoelectric effect) and a wave (narrow slit interference), it is usually pictured as a sine wave with an arrow attached, e.g. :
Light is a form of electromagnetic radiation since it carries both a varying electric field and a varying magnetic field at right angles to each other. It is characterized by its wavelength, λ.
Light’s Particle-Like Nature: … is demonstrated by: The Photoelectric Effect. Light incident on alkali metals and other solids (potassium, bismuth, calcium, antimony, etc.) is able to release electrons from the surface, if they are energetic enough.
Wave Nature of Light: The wave nature of light is demonstrated by the interference effects in straight edge diffraction, single slit diffraction, and double slit diffraction experiments using a point source with either monochromatic or white light. Straight edge diffraction Single slit diffraction (top: monochromatic, bottom white light) Double slit diffraction (top: monochromatic, bottom white light)
Low energy radiation (longest λ, shortest ν) is dispersed the least, high energy radiation (shortest λ, longest ν) the most.
The spectrum of the Sun is an absorption spectrum, i.e. a hot gas viewed against a brighter continuum source.
A high resolution view. Most of the dark spectral lines are caused by atoms of gaseous iron in the Sun’s atmosphere.
How the colour of an object corresponds to its temperature. Initially hot objects glow red, then yellow-orange, and finally white, i.e. “white hot,” as the temperature increases. The resulting radiation is referred to as black body radiation.
Properties of black body radiation 1. Energy (light) is emitted at all wavelengths, except for = 0 and = . 2. The form of the continuous energy distribution is given by the Planck function. 3. As temperature T increases, the energy output increases at all wavelengths . 4. As T increases, the energy output increases most rapidly for small . Wien’s law:
The brightness of a distant object decreases in proportion to the inverse square of its distance.
According to Einstein’s famous relationship: For light: Thus, since light carries energy, photons can be considered to have “mass” as they travel at the speed of light. At rest they are “massless.” And, since mass is affected by gravity, light rays can be deflected when passing near massive objects: stars, massive galaxies, clusters of galaxies, etc.
A plexiglass simulation of a gravitational lens created by Charles Dyer and Robert Roeder (1981).
Double quasar, QSO 0957+561. The image of a single background quasar split into two halves by an intervening galaxy.
Another property of light arising from its wave properties is that it is subject to the Doppler Effect. The wavelengths of photons from distant objects moving relative to the observer appear to be either “stretched” to longer wavelengths or “compressed” to shorter wavelengths when viewed by us, if they are moving either away from us (“red shift”) or towards us (“blue shift” ), respectively. The Doppler Effect was first noted for sound waves, but applies to light waves as well by extension.
The atomic and ionic lines seen in stellar spectra provide a measure of a star’s temperature. Helium lines denote hot stars (20,000K), molecular bands cool stars (3000K)
The spectral lines in the spectra of stars are from atomic species in the gas of stellar atmospheres that produce electronic transitions in the specific temperature ranges applicable to stars.