HOMEMADE SPECTRA

These are the very first attempt at producing visible light spectra from a "toilet paper tube spectrometer." We've improved the process quite a bit by using entrance slits cut with a laser engraver! These, however, use the original aluminum-foil-cut-with-a-knife slits.

This is the spectrum of an energy saving fluorescent bulb, as seen looking through a 100 line/mm grating:

 

Note that the diffraction orders overlap and that the reflection from the lamp's harp also appears in multiple orders. (In the original photo, pictures on the wall also appear in multiple overlapping orders. It's very confusing!) The brightest bulb image is the undiffracted light (zero order) from the bulb itself. Note that the orders on the right are brighter than the corresponding orders on the left. (The first order on the right is nearly as bright as the zero order in the center.)

A grating with 300 lines/mm separates the orders so that there is less overlap:

Again, the zero order is the undiffracted light from the bulb (which looks overly yellow in this photo- I was playing with the exposure controls without benefit of the camera manual!) The first order on either side no longer overlaps the second order, although the colored bulb images in each order still overlap (a slit will take care of that- see below). There were additional orders, but the camera field of view wasn't wide enough to capture them.

A grating with 600 lines/mm does an even better job of separating the orders:

 Again, the field of view of the camera could only capture the central and first orders, and the order on the right is cut off a bit.

 

Still, it's difficult to see the individual colors of the lamp's spectrum because the large bulb images overlap. Putting a slit in front of the bulb helps a lot:

 

This sequence of photos shows the same bulb and three different gratings, this time with a piece of cardboard blocking the lamp except for a narrow slit. Notice that the light reflecting from the chair (as well as from other objects in the room) is diffracted by the grating. The individual colors in each order show clearly, especially in the bottom photo, which uses the 600 line/mm grating.

 

Finally, by constructing an improvised "spectroscope" out of a cardboard tube (yup, a toilet paper tube) and some aluminum foil, the stray light is blocked and the spectrum appears uncluttered:

This is the 300 line/mm tube. Note that the spectral lines are colored images of the slit, which is a bit irregular. (I convinced myself that a sloppy slit would make this point better than one made with some care.) To create this photo, the slit was actually pointed not at the lamp (which caused an annoying glare) but at the reflection of the lamps's light from a piece of white paper.

The photos below show the lamp and the "spectroscope" (grating-end (left) and slit-end views). The stack of books was the base for the slit used in the middle sequence of photos. In the photo on the left, you can see the sheet of paper used to observe the spectra above.

 

 

 

 

 

 

 

 

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