Projection Spectroscope

Description

Opaque cards are used to produce a slit or beam of light from an overhead projector. The light emitted from the projector is passed through a plastic grating sheet. Examination of the dispersed light resulting from this process gives clues regarding light absorption.

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Procedure

Place an opaque card with a slit on the stage of an overhead projector.
Clamp a sheet of plastic diffraction grating material in the light beam from the projector on the screen side of the projecting lens. The grating lines in the plastic should be parallel to the projected slit.
Place Petri dishes containing colored solutions over the slit on the projector stage.
Use an opaque card to block any stray light from the projector. The region below the grating material is most likely to cause problems.
Darken the room for best effect. Two sets of diffracted light will appear-one on each side of the slit projection.
Examine the projected light. If solutions are concentrated enough you will see preferential absorption of light by the colored solutions. One section should be uncovered for a comparison of the intensity of the light.

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Handout Makeup

Name _____________________________ Class _______

Teacher______________________________

DoChem 035 Projection Spectroscope

Watch the movie
Which color is absorbed by the blue solution?
What color is absorbed by a red solution?
If you were to use a spectronic instrument to measure the concentration of a blue solution, what wavelength would give the most sensitive results?

Color	wavelength
blue	400-500
green	500-600
red	600-750

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Teachers Guide

Purpose

To demonstrate the set-up and use of an overhead projection spectroscope.

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Materials

overhead projector
screen
15-cm x 15-cm plastic grating material (Edmund Scientific) mounted in cardboard transparency holder
opaque cards with slit (can be fabricated from a manila file folder)
2 (or more) Petri dishes containing colored liquids
support stand, single buret clamp
food coloring

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Lab Hints

Practice with the projector. One needs to see where the diffracted light projects, and to see the effects which colored solutions have upon the amount of light reaching the screen.