Rainbow Analysis

Adapted from Project Astro Activities

Summary:

Students are introduced to the scientific tool of spectroscopy. They will each build a simple spectroscope so that they can examine the light from different light sources, particularly the Sun (if logistically feasible) and artificial lights (fluorescent or sodium lamps, discharge lamps, or whatever is available locally). The solar spectrum will appear continuous at the resolution typical of plastic diffraction gratings; however the fluorescent or sodium room lights and discharge lamps will show clear lines (students often describe the spectra as "broken up"). These lines represent the "fingerprint" of the element contained in the lights and are always the same, no matter where the element appears or how much of the element is present. Appearance of a fingerprint in the spectrum of a distant astronomical object demonstrates the existence of that substance in the object. The discharge lamps provide the opportunity to show students a variety of spectral fingerprints.

Purpose:

To teach students how astronomers determine the composition of distant objects.

Audience:

~ 20 students (grade range 6th-9th) in a group works well

Objectives:

Understand that light is composed of different wavelengths
Recognize that light can be separated by wavelength, which is equivalent to color
Build an astronomical tool, a spectroscope, to study light
Learn that elements and molecules each have a unique "fingerprint" of lines at different wavelengths

Materials:

empty paper towel tubes (1 per student)
aluminum foil (2 pieces ~ 3 x 5 inches and 2 strips ~ 1 x 3 inches per student)
masking tape
diffraction grating (~ 1 inch square of material per student)
example spectrum (e.g. poster from Spitzer Science Center)
discharge lamps (optional, H, He, Ar, O, and CO2)
completely blacked-out room (optional)

Kit and Room Preparation:

Prepare parts of spectroscope: ~ 30 minutes
Cut pieces of foil and diffraction grating. Collect a paper towel tube, 2 pieces of foil 3 x 5 inches and 2 pieces of foil 1 x 3 inches, and a 1 inch square of diffraction grating for each student. You can put each kit in a Ziploc bag.

A room set up with spectroscope kits on each desk

Darken the room: ~ 10 minutes
The room should be capable of going from brightly lit to dark so that both the overhead fluorescent lamps and the narrow discharge lamps can be seen effectively. Sometimes this means lights or light leaks must be covered. Dark black plastic trash bags and duct tape have proved useful for this.