Q230: Prep Notes

Introduction

A Bausch & Lomb variable-wavelength interference filter is available for demonstration of the visible spectrum and absorption spectra.

Supplies

Short List

  •   Food coloring
  •   Interference filter
  •   Overhead projector
  •   Goggles and gloves

Chemicals

  •   optional: copper sulfate, ferric thiocyanate, potassium permanganate

Prior to Lecture  (prep time ~ ? min.)

  1. Provide dilute solutions of various food colors in water

      OR

  1. Prepare very dilute solutions of: 
  •   Copper(II) sulfate (CuSO4)
  •   Iron(III) thiocyanate (Fe(NCS)2+) by mixing Fe(NO3)3 and KSCN
  •   Potassium permanganate (KMnO4)

To Conduct Demonstration

  1. Place the interference filter on the stage of the overhead projector.
  2. Turn off the room lights.  Block any remaining light from the overhead projector.
  3. To demonstrate absorption spectra, place crystallizing dishes or beakers containing dilute, colored solutions over the filter and note which wavelengths of light are absorbed and which are transmitted.  Solutions of water with food coloring can be used, as well as chemical solutions (blue CuSO4 blocks red light, reddish Fe(NCS)2+ blocks blue and some green light, and pinkish KMnO4 blocks yellow and green).

Demo Time: ~? minutes

Safety and Disposal

Food coloring is preferred as it is sink-disposable. Copper(II) sulfate, iron(III) thiocyanate, and potassium permanganate should be tagged and submitted for hazardous waste disposal.