PHYSICS 183 for Fall 2009

Atomic Spectra

Caution:  Do not touch the metal ends of the discharge tubes while they are turned on.  The tubes get quite hot, so turn them off when not in use.

In this experiment you will use an instrument called a spectrometer to measure the wavelengths of the light emitted by excited hydrogen atoms.  The spectrometer allows light to pass through a small slit and then through a diffraction grating.  The different wavelengths will form maximums at different angles.  You can keep track of the different wavelengths by their color.  The spectrometer allows us to very accurately measure the angular position of each maximum.  You will use a vernier scale to measure all angles to the nearest tenth of a degree.  You read the scale just like a vernier caliper.

Set the telescope so that the central maximum is in the cross-hair.  You may need to focus the telescope to see it clearly.  You do this by moving the eyepiece in or out.  Record this angle.  Move the telescope to the left.  How many lines and of what color do you see before the color sequence begins to repeat?  This is what is called the first order (m=1).  Keep going left.  Can you see the complete second order?  If so, do you see any third order lines?  Do you see all the same lines on the right side?

Return to the central maximum.  Move the telescope to the left until you see the first spectral line. Carefully adjust the position of the telescope so that the cross-hair is centered on the line. Record the angle.  Then calculate its angular distance from the central maximum.  Continuing to move to the left, repeat the measurement for each first order line you can see.  Then repeat the process on the right side.     Measure the angle for the last line you can see on each side.  Average the right and left angular distance for the corresponding lines.

Now use your data to calculate the wavelength of each line.  Compare your calculated wavelengths to the Balmer lines of hydrogen given in your book. Does the wavelength of the last line you saw agree with the corresponding line in the first order?