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:
- Slits: each light source has a slit
- Collimator: available but variable form very poor to reasonable.
- Grating: a small handheld transmission grating
- Camera: the lenses in your eye
- Detector: your retina (with image processing provided by your brain)
Your arms and legs provide the structure and adjustments to the spectrograph.
GratingsThe 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 FixtureHolding 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.
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 slitsWhat 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?
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!):
- Initially look at the lamp with the grating normal to the line between your eye and the slit, with eye close to grating and approximately 5 feet from lamp. Find the positive and negative orders. Which is brighter, positive or negative orders?
- Can you see any higher orders?
- Hold the grating 6" from your eye and estimate the true angle subtended by the spectrum from the red to violet light. Note: at 6" from your eye, the grating subtends about 15 degrees from top to bottom.
- Keeping the grating in the same position as before (6" away from eye), view the lamp from across the room. Note the angle subtended by the spectra and by the length of the slit, and compare those values to those you observed when the lamp was nearby. Do the results agree with your preconceptions? If you have time, repeat the exercise with the lamp at half way across the room.
- How does the sharpness of the lines change with slit width and distance from the lamp? Can you resolve any of the lines into doubles?
- Estimate the angular separation between the same wavelength in first and second order.
- Now tilt the top of grating away from you and notice how the positive and negative orders of the spectra change. Try and keep your eye the same distance away from slide. Repeat with bottom of grating tipped away.
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:
- What do you see with you spectrograph if there is no grating, vs grating?
- What do you see if there is no slit or a very wide slit?
- What do you see if the slit defines the light?
- Why does the resolution improve when the lamp is viewed from further away?
- How does the relative grating and viewing angle change?
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