Labs from YSI 95 :
The Grating Spectrometer.

Jim Sweitzer
CARA Yerkes Summer Institute, August 1995

This is the staff copy of the lab.

Introduction

Goal: This lab is part of our spiraling curriculum to help students continue to add to their concepts of light and wave phenomena. In particular, this lab will focus on understanding the basics of how a grating spectrometer works.

Objectives: Make a paper model that demonstrates wave interference for three wavelengths from two sources. Using the real spectrometer, observe and measure the wavelengths of spectral lines from various sources of light. Observe the solar spectrum and identify as many dark lines as possible.

Media & Tools Required: "Project STAR" spectrometers. (These may be obtained from Learning Technologies Inc., 59 Walden St., Cambridge, MA 02140). Solar Spectrum and Spectrum Analysis posters are nice too. (They are available from Wabash Instrument Co., 1801 Grand St., P.O. Box 707, Wabash, IN 46992.). You will need various light sources: incandescent, window out to the sky (don't look directly at the Sun, whatever you do!), and fluorescent. If you can get other light sources of single element gasses, that's great, but unnecessary. Otherwise, this activity only requires manila folders, pins, pencils and colored pens.

INSTRUCTIONAL PLANS

1. Motivation

Show students common diffraction grating -- an audio CD. Ask them to compare what it does to light to a prism to probe their basic concepts. They should recall that white light is made up of all the colors. Introduce concept of being in phase by drawing at a picture of a crew team with its many rowers. (In doing the lab, the students preferred to use the term "in rhythm" to describe this phenomena.)

2. Objective

Tell students we will learn how the stickers work and use a more sophisticated form (the Project STAR spectrometers that they have used once before). When we use it today, they will actually measure the features in the light from the Sun and other sources.

3. Prerequisites

A. Have them recall what a wave is from previous activities. Draw the usual sine wave and emphasize that it is like a sequence of peaks and valleys. Relate to water waves.

B. Refresh their memory about what a wavelength is and then explain that visible wavelengths are quite small -- from 350 to 750 billionths of a meter (nm). The way we experience the different wavelengths is as colors.

4. Information Delivery

A. Draw the following diagram of the spectroscope to explain the parts and begin to understand light path through the instrument.

B. Draw sine wave and discuss its features, especially focusing on the wavelength, peaks and valleys.

C. Draw a picture of the crew rowers in rhythm. Explore the idea of waves being in phase and out of phase. (Students were found to do quite well with this, even adding and subtracting waves visually.)

5. Construction Activity

Here we construct a "simple" paper model to demonstrate constructive interference from two points. Parts below are made from a manila file folder. Two "white light rays" are made as follows. (Takes about 45 minutes to finish and get working properly.)

B. Assembly of interference demonstration device (see photo.)

The two completed "white light rays" are then pinned to the remaining portion of the manila folder at one end where the two points are. These points represent the two scratches in the grating and are the sources of the imaginary scattered light we will be investigating. To do this, first draw a line parallel to the longest direction of the folder down the middle. This is the direction that light passing straight through the grating spectrometer will take.

Near the end of this line, draw two dots, each 1 cm away from the line you just draw. An imaginary line drawn between these two dots should be perpendicular to the drawn line. These points are where the "white light rays" will be pinned so they can rotate freely.

Assemble the strips onto the backing manila paper so that they move freely. It's important to have the pins go through the very centers of the first two blue lines. What the students will now do is see how the phase relation of the waves represented on the strips change as the angles of the strips are made to move. Note that you have to keep the strips parallel to make this work.

C. Questions:

1. When the strips are aligned parallel and in the direction of the line you drew, this is the zero-order interference direction. All colors are in phase here. But, as you move the two strips to other directions, the phase relationship between the two sets of waves change. What are the angles of the directions when there is first seen constructive interference for each color? Draw a line in each direction. Record the wavelength too in the table below. What conclusion can you make?

Wavelength (mm)    Color         Angle for interference
Conclusion: Have the students summarize what's going on in their own words.

6. Observing Activity

A. Now use the real spectrometers. First relate what they did above to the real thing again. Review how to observe with them.

B. Use them on an incandescent bulb.

C. Point them at the blue sky or a cloud out the window. (Don't look directly at the Sun, whatever you do!) What's the difference?

D. Calibrate the spectrometers on the fluorescent lamp. (See instructions on top of the spectrometers.)

E. If there's time, use the spectrometers to measure the wavelengths of the following sources and try to identify them.

Source of Light   Line Measured   What is it?    Lines on Chart

7. Discussion

This can be an opportunity to then link to other astronomy concepts we've be working on.

8. Summary

Review how the property of waves called interference makes this possible.


Important Disclaimers and Caveats: