Homework portion (question #8) due 9/25

 Spectroscopes:

 To break a beam of light into a spectrum, one separates the wavelengths of light with either a grating or a prism.  But, before the light passes through the grating or prism, it should first pass through a narrow slit. Then the light that passes through the grating or prism is an image of the slit and so an image of the slit will be seen at each wavelength (the lines you may see in the spectrum). Since the slit is narrow, there will be minimal overlap of the images at neighboring wavelengths. The separate wavelengths can then be distinguished from each other. Imagine, for example, if the light source was as large as this building and a slit was not used. The red image of the building would overlap the blue image and all the colors in between would be smeared together. You would not be able to see the individual colors and so you would not have succeeded in breaking up light into a spectrum.

To make you spectroscope, you will need a holographic grating, an eyepiece, a slit, and a tube.  Construct the spectroscope as follows:

1. align the piece of grating to the hole in the eyepiece and tape it securely

2. tape the eyepiece to one end of the tube, preferably with the grating to the inside of the tube

3. tape the slit to the other end of the tube with the slit running in the same direction of the long side of the hole in the eyepiece (the slit should run perpendicular to the direction that the grating spreads out light)

 Your spectroscope is ready to be used. Look through the eyepiece at a bright light source like the overhead lights, and you will see a spectrum on either side of the slit.

Identifying the types of spectra.

 With your spectroscope, look at each of the hot gasses in the Balmer light tubes. Note that each gas emits a series of emission lines and that each has a completely different set of emission lines.

1. Why does the light from the gasses appear as ‘lines’?

2. Draw a simple diagram of each of the spectra.  Take care that you draw the correct pattern in the relative separations of the lines. Also indicate the color of each line. These diagrams represent the spectral fingerprints of these gasses.

3. Try looking at an ordinary fluorescent light in the class or hall and see if you can identify what gas is responsible for the light.

4. Point your spectroscope at an ordinary incandescent lamp. What does its spectrum look like?

5. How does the spectrum differ, in general character, from the spectra of the light sources in the gas tube?

6. Sometime during the day (later in the week) point your spectroscope at the sky, near (but not at!!) the Sun. What type of spectrum do you see (emission line, continuum, continuum with emission lines, or continuum with absorptions lines)?

7. Briefly explain why the Sun has the kind of spectrum that it does (remember Kirchhoff’s laws).

8. Sometime later in the week point your spectroscope at some lights around your home or place of work and determine which ones are incandescent lights and which are not. In your report list at least one incandescent lamp and one non-incandescent lamp.

You can gain extra credit by attending any of the events listed below and then writing a short essay ( 300 - 500 words) about the event. If you attend a lecture you must write a brief discussion of the topic of the lecture. If you attend a telescope observing session, you must describe the telescope(s) you looked through and discuss what kind of objects you viewed. Each essay will be worth up to five points (5% of your total grade), and I will accept one essay.

If you discover some other event that is related to astronomy that you would like to write about, please contact me with the details so I can determine whether it is acceptable.

Page created by C. Gino on 09/11/2003

Page last updated 10/20/2003