No need for a Raspberry Pi! I once made a spectrometer at home out of just a camera, a white LED and a diffraction grating (and some tape and a wood base to hold it together). If you don’t have a diffraction grating, it can be replaced by a CD — only difference is that the CD operates using reflection vs transmission. The idea is that you shine the light from the LED through your sample (which I bought some cuvettes to hold); the light then transmits through the grating / off the CD, which splits it into wavelengths. One can then take a photo of the spectrum and analyse it using a program such as [0]. Of course, an LED is a pretty terrible light source, but with some sort of baseline correction I suspect it could actually become pretty reasonable as a spectrophotometer.

[0] http://scheeline.scs.illinois.edu/~asweb/CPS/