Light is such a complicated phenomena that no one model can be devised to explain its nature. Although light is generally thought of as acting like an electric wave oscillating in space accompanied by an oscillating magnetic wave, it can also act like a particle. A “particle” of light is called a photon, or a discrete packet of electromagnetic energy.
Most visible objects are seen by reflected light. There are a few natural sources of light, such as the Sun, stars, and a flame; other sources are man‐made, such as electrical lights. For an otherwise non‐luminous object to be visible, light from a source is reflected off the object into our eye. The property of reflection, that light can be reflected from appropriate surfaces, can most easily be understood in terms of a particle property, in the same sense that a ball bounces off a surface. A common example of reflection is mirrors, and in particular, telescope mirrors that use curved surfaces to redirect light received over a large area into a smaller area for detection and recording.
When reflection occurs in particle‐particle interactions (for example, colliding billiard balls), it's called scattering — light is scattered (reflected) off molecules and dust particles that have sizes comparable to the wavelengths of the radiation. As a consequence, light coming from an object seen behind dust is dimmer than it would be without the dust. This phenomena is termed extinction. Extinction can be seen in our own Sun when it becomes dimmer as its light passes through more of the dusty atmosphere as it sets. Similarly, stars seen from Earth seem fainter to the viewer than they would if there were no atmosphere. In addition, short wavelength blue light is preferentially scattered; thus objects look redder (astronomers refer to this as reddening); this occurs because the wavelength of blue light is very close to the size of the particles that cause the scattering. By analogy, consider ocean waves — a row boat whose length is close to the wavelength of the waves will bob up and down, whereas a long ocean liner will scarcely notice the waves. The Sun appears much redder at sunset. The light of stars also redden in passing through the atmosphere. You can see the scattered light by looking in directions away from the source of the light; hence the sky appears blue during the day.
Extinction and reddening of starlight are not caused by just the atmosphere. An exceedingly thin distribution of dust floats between the stars and affects the light that we receive as well. Astronomers must take into account the effect of dust on their observations to correctly describe the conditions of the objects that emit the light. Where interstellar dust is especially thick, no light passes through. Where dust clouds reflect starlight back in our direction, the observer may see blue interstellar wispiness like thin clouds surrounding some stars, or a nebula (to use the Latin word for cloud). A nebula formed by scattering of blue light is called a reflection nebulae.
Wave properties of light
Most properties of light related to astronomical use and effects have the same properties as waves. Using an analogy to water waves, any wave can be characterized by two related factors. The first is a wavelength (λ) the distance (in meters) between similar positions on successive cycles of the wave, for example the crest‐to‐crest distance. The second is a frequency (f) representing the number of cycles that move by a fixed point each second. The fundamental characteristic of a wave is that multiplication of its wavelength by its frequency results in the speed with which the wave moves forward. For electromagnetic radiation this is the speed of light, c = 3 × 10 8 m/sec = 300,000 km/sec. The mid‐range of visible light has a wavelength of λ = 5500 Å = 5.5 × 10 −7 m, corresponding to a frequency f of 5.5 × 10 14 cycles/sec.
When light passes from one medium to another (for example, from water to air; from air to glass to air; from warmer, less dense regions of air to cooler, denser regions and vice‐versa) its direction of travel changes, a property termed refraction. The result is a visual distortion, as when a stick or an arm appears to “bend” when put into water. Refraction allowed nature to produce the lens of the eye to concentrate light passing through all parts of the pupil to be projected upon the retina. Refraction allows people to construct lenses to change the path of light in a desired fashion, for instance, to produce glasses to correct deficiencies in eyesight. And astronomers can build refracting telescopes to collect light over large surface areas, bringing it to a common focus. Refraction in the non‐uniform atmosphere is responsible for mirages, atmospheric shimmering, and the twinkling of stars. Images of objects seen through the atmosphere are blurred, with the atmospheric blurring or astronomical “seeing” generally about one second of arc at good observatory sites. Refraction also means that positions of stars in the sky may change if the stars are observed close to the horizon.
Related to refraction is dispersion, the effect of producing colors when white light is refracted. Because the amount of refraction is wavelength dependent, the amount of bending of red light is different than the amount of bending of blue light; refracted white light is thus dispersed into its component colors, such as by the prisms used in the first spectrographs (instruments specifically designed to disperse light into its component colors). Dispersion of the light forms a spectrum, the pattern of intensity of light as a function of its wave length, from which one can gain information about the physical nature of the source of light. On the other hand, dispersion of light in the atmosphere makes stars undesirably appear as little spectra near the horizon. Dispersion is also responsible for chromatic aberration in telescopes — light of different colors is not brought to the same focal point. If red light is properly focused, the blue will not be focused but will form a blue halo around a red image. To minimize chromatic aberration it is necessary to construct more costly multiple‐element telescope lenses.
When two waves intersect and thus interact with each other, interference occurs. Using water waves as an analogy, two crests (high points on the waves) or two troughs (low points) at the same place constructively interfere, adding together to produce a higher crest and a lower trough. Where a crest of one wave, however, meets a trough of another wave, there is a mutual cancellation or destructive interference. Natural interference occurs in oil slicks, producing colored patterns as the constructive interference of one wavelength occurs where other wavelengths destructively interfere. Astronomers make use of interference as another means of dispersing white light into its component colors. A transmission grating consisting of many slits (like a picket fence, but numbering in the thousands per centimeter of distance across the grating) produces constructive interference of the various colors as a function of angle. A reflection grating using multiple reflecting surfaces can do the same thing with the advantage that all light can be used and most of light energy can be thrown into a specific constructive interference region. Because of this higher efficiency, all modern astronomical spectrographs use reflection gratings.
A number of specialized observing techniques result from application of these phenomena, of which the most important is radio interferometry. The digital radio signals from arrays of telescopes can be combined (using a computer) to produce high‐resolution (down to 10 −3 second of arc resolution) “pictures” of astronomical objects. This resolution is far better than that achievable by any optical telescope, and thus, radio astronomy has become a major component in modern astronomical observation.
Diffraction is the property of waves that makes them seem to bend around corners, which is most apparent with water waves. Light waves are also affected by diffraction, which causes shadow edges to not be perfectly sharp, but fuzzy. The edges of all objects viewed with waves (light or otherwise) are blurred by diffraction. For a point source of light, a telescope behaves as a circular opening through which light passes and therefore produces an intrinsic diffraction pattern that consists of a central disk and a series of fainter diffraction rings. The amount of blurring as measured by the width of this central diffraction disk depends inversely on the size of the instrument viewing the source of light. The pupil of the human eye, about an eighth of an inch in diameter, produces a blurring greater than one arc minute in angular size; in other words, the human eye cannot resolve features smaller than this. The Hubble Space Telescope, a 90‐inch diameter instrument orbiting Earth above the atmosphere, has a diffraction disk of only 0.1 second of arc in diameter, allowing the achievement of well‐resolved detail in distant celestial objects.
The physical cause of diffraction is the fact that light passing through one part of an opening will interfere with light passing through all other parts of the opening. This self‐interference involves both constructive interference and destructive interference to produce the diffraction pattern.
Kirchoff's three types of spectra
Both dispersive and interference properties of light are used to produce spectra from which information about the nature of the light‐emitting source can be gained. Over a century ago, the physicist Kirchoff recognized that three fundamental types of spectra (see Figure 2) are directly related to the circumstance that produces the light. These Kirchoff spectral types are comparable to Kepler's Laws in the sense that they are only a description of observable phenomena. Like Newton, who later was to mathematically explain the laws of Kepler, other researchers have since provided a sounder basis of theory to explain these readily observable spectral types.