Experiment 7

SPECTROSCOPY

In most branches of science, scientists can take a sample of whatever they're studying for closer examination in the laboratory. Astronomers, however, work with the handicap of being separated from the objects we study by enormous distances.

Given that we are mostly restricted to observing the light that arrives here from distant objects, astronomers make as much use as possible from this light, using every method available.

One such method is spectroscopy. Spectroscopy is a technique that breaks down light into its constituent parts. The "parts" are the light at different wavelengths (different colours).

A spectrometer is a device that breaks down incoming light into these separate colours. (In this sense, there are "spectrometers" in nature, such as the rain droplets that break down sunlight into a rainbow.) At an observatory, spectrometers record and measure the spectrum of the light that is fed into them from the telescopes. By studying the spectrum of a star, a galaxy, or any celestial object, we can gain much valuable information about the object.

For one, the spectrum of a moving object can be used to measure its speed using the Doppler Effect (redshift). This uses the same principle that allows the police to measure the speed of a moving car.

Another application of spectroscopy is to determine the chemical composition of an object. This is possible because every element and every molecule emits light at specific wavelengths in a way that is unique to that substance. For example, if you see light from a star appearing at the wavelengths for Oxygen, then you can conclude that this star contains Oxygen.

There are many types of spectrometers, but the two main types used for visible wavelengths are the grating spectrometer and the prism spectrometer.

A prism uses the fact that light of different wavelengths will refract ("bend") by slightly different angles when they pass through a glass wedge (the prism).

A diffraction grating is a transparent material that has very fine lines etched into it at regular spacings. Some of the light that passes through a grating also changes direction in a way that depends on wavelength, allowing the diffraction grating to also spread out light into its different colours.

Part A: Understanding Emission and Absorption Spectra

Objective

To learn what a line spectrum is, and to make qualitative observations about emission and absorption spectra.

Theory

A "white" light source contains every colour of light (although not usually of equal intensities).

This means when you view the spectrum of a white light source, you see a complete "rainbow" of all the colours. Furthermore, because every colour is present, you see the colours blurred together into what is called a continuous spectrum, with no dark areas between them, like this:

Move the uponan Facts About: KPU Light Spectrometer Telescope View through source the telescope Diffraction Mini-Encyclopedia gratingWhite Light Emission spectrum White Light Mystory Gas #1 Mystory Gas #2 Mystery Gas #3 Mystory Gas #4 Mystery Gas #5 Mystery Gas to Mystery Gas #7 1200 lingWhite Light Emission spectrum Emission spectrum Absorption spectrumMystory Gas #1 - Emission apoctrum Move the spectrometer telescope to see the light source's spectrum. Mydory Can 1 1200 Ines/mm what the eye sees: Angle: 4 4.7* KWANTLEN y POLYTECHNIC UNIVERSITY KPU