Properties of light: Light can be reflected from a surface, if the surface is smooth and shiny, as with a mirror, and can be accurately calculated from knowledge of the shape of the reflecting surface. knowledge of the shape of the reflecting surface. Light also bends or refracts when it passes from one type of transparent material to another, e.g. from air to a glass lens. air to a glass lens. Reflection and refraction of light are the basic properties that make all optical instruments possible. all optical instruments possible. In 1672, Isaac Newton discovered an experiment in which he allowed light from the sun to pass through a glass lens. from the sun to pass through a small hole and then through a prism, and thus discovered that sunlight thus discovered that sunlight is actually made up of a mixture of all the colours of the rainbow. of all the colours of the rainbow, the light is separated into different colours with a prism. prism. On leaving the opposite side of the prism, the light is bent again and scattered further. more. Because this matrix of colours is a spectrum of light. The value of stellar spectra Newton looked at the solar spectrum, all he could see was a continuous band of colours. of colours. In 1802, however, William Wollaston built an improved spectrometer that included an spectrometer that included a lens to focus the spectrum of the sun on a screen. on a screen. Wollaston saw that the colours were not evenly distributed, but that some colour ranges were missing, appearing as dark bands in the solar spectrum. the solar spectrum. In 1815, Fraunhofer, after a more careful examination of the solar spectrum, found around 600 spectrum, about 600 such dark lines (missing colours) were found. (missing colours). Types of spectra -A continuum spectrum (formed when a solid or very dense gas emits radiation) is a radiation) is a matrix of all the wavelengths or colours of the rainbow. - Absorption spectrum, consisting of a series or pattern of dark lines (missing colours) superimposed on each other. (missing colours) superimposed on the continuous spectrum of a source. Occurs when white light passes through a thin, cold gas. - Emission spectrum, appears as a pattern or series of bright lines; consists of light in which only certain discrete wavelengths are present. wavelengths are present. - In 1860, the German physicist Gustav Kirchoff became the first person to use spectroscopy to to use spectroscopy to identify an element in the Sun when he found the spectral signature of sodium gas. Found the spectral signature of sodium gas.
- Perhaps the most important characteristic of a star is its luminosity, the total amount of energy in all wavelengths that is total amount of energy in all wavelengths that it emits per second. second. - Of the energy emitted by a star actually reaches an observer on Earth. Earth. We call the amount of a star's energy that reaches a given area (say, a star's apparent brightness). reaches a given area (say, one square metre) every second here on Earth. on Earth. - He classified the stars into six categories of brightness, each of which he called a magnitude. - The blue colours dominate the visible light output of very hot stars (with a lot of additional radiation in the ultraviolet). On the other hand, cool stars emit most of their visible light energy in the red wavelengths (with more radiation coming from the wavelengths (with more radiation coming from the infrared). - The difference between two of these magnitudes, say, between the blue and the visual magnitudes (B - V). visual magnitudes (B - V), is called the colour index. - The main reason why stellar spectra look different is because stars have different temperatures. because stars have different temperatures. Most stars have almost the same composition as the Sun. - At some periods in their lives, stars can expand to enormous dimensions. enormous dimensions. Stars of such exaggerated size are called giants. - The absorption lines of most of the known chemical elements have now been identified in the spectra of the Sun and stars. have now been identified in the spectra of the Sun and stars. The temperature and pressure in a star's atmosphere will determine which types of atoms can produce absorption lines. can produce absorption lines.