Science 9: Unit E: Space Exploration Topic 3: Spectroscopy and the Doppler Shift

White Light is Made of a Spectrum of Colours.  White light is made up of a spectrum of colors. The spectrum of colors starts with red, orange, yellow, green, blue, indigo, and violet/purple. A spectroscope is a device used to show the spectrum of colors given off by a light source. It works by having the light pass through a series of prisms which break up the light into colors.

Dark Line Spectrum  When a spectroscope was place in the path of sunlight it was discovered that sunlight did not give off a continuous spectrum. There were dark gaps in the sun’s light spectrum. These lines or were called spectral lines.

Element Spectra  Two German scientists Kirchoff and Bunsen discovered that by heating elements until they glowed (incandescence) they gave off a spectrum of their own. A particular element gives its own signature spectrum with its own spectra lines.

Spectral Analysis  Every star gives off its own signature spectrum. What makes the spectrum unique is the position of the spectral lines in the spectrum.  Just like stars, elements, when heated, give off their own signature spectrums, with their own particular pattern of spectral-lines.  The location of the spectral lines tells us what elements are in the star. By matching the pattern of spectral lines coming from a star to the known spectral patterns of elements you can determine the composition of a star.

Doppler Effect, Big Bang, and Red/Blue Shift  Astronomers noticed that distant stars and galaxies gave off spectral patterns of their own which matched known elements. However, the patterns were all shifted to the red end of the spectrum. At first no one knew what this meant.

Sound and the Doppler Effect  When a source of sound, like a police siren, moves towards an observer, the waves bunch up and the sound goes higher in pitch.  When the police car moves past and away from you, the sound waves spread out and the sound goes lower in pitch.

Light and the Doppler Effect  The same thing happens with light. When a light source moves away from you, the light waves shift to the red end of the spectrum (lower frequency), when the light source moves towards the observer the light waves shift to the blue end of the spectrum (higher frequency).  So What does this mean?  The fact that the spectral lines are all shifted to the red end of the spectrum meant that the stars/galaxies and us are moving away from each other.

The Doppler Effect and the Big Bang Theory  Scientists then theorized that if we’re all moving away from each other that we must have started out together which gave us the Big Bang Theory. Everything in the universe started in a tiny point of space and then exploded apart, the Big Bang. We’re still exploding now, eventually the universe like an elastic- band will stop going out and eventually start pulling back in to that same point in a Big Crunch.

Types of Spectra  I. Emission/Bright Line Spectra – A series of bright colored lines produced against a black background. These spectra are produced when a gas is heated at low/normal pressure. The easiest to produce and see.

Types of Spectra Continued.  II. Continuous Spectrum – The full spectrum that you see in the rainbow. These are produced when a gas is heated at high pressure or a solid or liquid is heated.

Types of Spectra Continued  III. Absorption/Dark Line Spectra – The kind of spectra we’ve been studying. The continuous spectrum with a series of dark gaps (spectral lines). These are produced when a gas is heated at high pressure and then has to pass through a much cooler environment.

Diffraction Gratings  A piece of transparent materials (such as plastic or glass) with thousands of closely spaced slits in it. This allows light waves to bend around the edges and interfere with each other producing areas of constructive interference (bright lines) and destructive interference (black gaps).  Modern spectroscopes use diffraction gratings instead of prisms because they produce much better detail in showing spectral lines from star spectra.