Nature_of_Light.pdf - Spectra Student Guide Introduction:...

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Spectra Student Guide
Introduction:
In this lab, you’ll use a high-quality spectrometer, made by Project STAR, to examine
the spectra of a variety of light sources. The main goals are to practice accurately
observing and recording the appearance of a spectrum and to think about the different
types of spectra and the characteristics of the objects that create them.
Background material:
The spectrum of a light source shows how the light intensity varies with the
wavelength of light. Basically, a spectrum records how much light is produced at each
color. There are three main categories of the spectrum, which are produced in the
following three situations:
1. A hot opaque body, such as a dense gas or solid, produces a continuous spectrum
– a complete rainbow of colors. The
intensity varies smoothly with wavelength.
2. A hot, low-density gas produces an
emission line spectrum – a series of bright
spectral lines against a dark background.
Light is only emitted at specific
wavelengths.
3. A cool, low-density gas in front of a hot
opaque body produces an absorption line
spectrum - similar to a continuous
spectrum, except with dark lines (dips in
intensity) at specific wavelengths.
Emission and absorption lines have a characteristic pattern that is determined by the
composition of the gas involved. For a given type of gas, the bright lines in an emission
spectrum, where the hot gas emits light, occur at exactly the same wavelengths as the
dark lines in the absorption spectrum, where the cool gas absorbs some of the
continuous spectrum’s light.
By measuring the spectrum of stars and nebula, and comparing them to spectra
observed in labs on Earth, astronomers are able to learn about the temperature and
composition of distant objects.
Review the background material on light and spectra:

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Using the Spectrometer:
Figure 1: The STAR Spectrometer. Note the locations of the eye hole, the calibrated
scale that you look at through the eye hole and the position of the source with respect
to the spectrometer. This is explained in detail below.
Hold the spectrometer so that you can look through the grating in the narrow end. You
should be able to see two rows of calibration marks and numbers. Pay attention to the
lower row, which gives the wavelength (in nanometers, or nm) of the light in the
spectra above it.