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?  Each person in the group should do this for his/herself.

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.  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.  Record the order and color for these lines. For each line you measured calculate the angular distance from the central maximum. Average the right and left angular distance for the corresponding lines. (If you only saw a particular line on one side, then you don't have any to average.)  This averages out any misalignment in the grating.

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. If you have data for a second or third order line, does its wavelength agree with the corresponding line in the first order?