Understanding Spectroscopy by Looking

The goal of this lab is to understand how a spectrograph works, you will discover how the spectrum changes when you adjust basic properties of the spectrograph.

The basic components of a spectrograph are, in the order the light reaches them are:

Each of you will have provided your own eyeball spectrograph (EBS) which map to the elements above as follows:

Your arms and legs provide the structure and adjustments to the spectrograph.


The transmission gratings are mounted in photographic slide holders. Hold the grating with the ES symbol facing you at the bottom left. Like this:

We are going to look at various light sources. I will give you a few suggestion of how to look at the light source along the way.You should also do some exploring of your spectrograph behavior as you change things.

Flourescent Light Fixture

Holding the grating close to your eye, look up at the fluorescent light fixture at an angle of 45 degrees and diagonally to the fixture. You should see something like this:

Is this what you see? 
What do you see when you look at the fluorescent fitting from different angles?
What happens if you rotate the grating?
We call the bottom set of colors -1st order, middle white zeroth order, upper colored 1st order.

Mercury Spectrum

Fluorescent light tubes contain mercury vapor that is ionized by an electrical potential. The mercury gas is optically thin so an emission line spectrum is produced. The phosphor (white stuff around the edge of the tube) converts the blue and near-UV photons a broad redder distribution. For efficiency modern fluorescent light tubes have a thin phosphor so the individual spectral lines (colors) are more visible.

Small old flourescent light tube with slits

What do you see now, how does the image vary with the different slit widths?
How is the image different from the big light fixture?
What is the difference between the spectra from the phosphor covered region compared with the non-phosphor region?

Lab exercises

Do the following using either the mercury lamp or the non-phosphor end regions of the old fluorescent light tube (do both if you have time!):

See the drawing on white board       


Think about your results using these supplemental questions following the lab and as the mathematical derivation of spectrograph
behavior is derived in subsequent lectures:

Defining Orders

Which are really the positive and negative orders?  There are conventions about angles for transmission and reflection gratings.  Usually the angle of the incident on the grating is called alpha and the angle of diffracted light is called beta.

On the images below Thetai = alpha, and Thetam = beta