Question

The difference in wavelength between two neighbouring spectral line
can be calculated
2.
(a)
2-22
22
2d
(b) 21-12
2d
2d
c)
21-12
(d)
None of these
2​

Answer 1

Answer:

Measurement of the Sodium D Emission Lines Using a Michelson

Interferometer

R. Price

University of Arizona

12 March 2008

The Sodium D emission lines (commonly called the ‘Sodium Doublet’) are spectral

emission lines that have wavelengths within a few angstroms apart. The experiment

performed involved calibrating a Michelson interferometer and measuring the

wavelengths of both emission lines. This was accomplished by measuring both the

average and beat wavelengths of the interference pattern created. From the

experiment it was found that the emission lines were given as 603±37nm and

604±37nm for the D1 and D2 lines respectively.

1 Introduction

The purpose of the experiment was to measure

the wavelength of the Sodium D emission lines.

Sodium has two emission wavelengths that are

extremely close in wavelength and without

sensitive equipment cannot be distinguished.

These lines, designated the D2 and D1 Fraunhofer

lines, have wavelengths of 589.6 nm and 589.0nm

respectively1. Using sensitive interferometry, the

average wavelength of the sodium lines was

determined as well as the difference between the

two emission lines.

When a sodium lamp is used in conjunction with

an interferometer, the wavelengths of the two

emission line will create interference patterns.

The distance between fringes within the

interference patterns can be measured to

calculate the wavelength of the emission source.

By measuring the path length difference that

causes the movement of the fringe pattern by one

cycle, the average wavelength of the source can

be determined. However, because of the relative

proximity between the sodium D lines, it becomes

more difficult to directly observe the two

1

Standard D Line values from University Physics, 11th

Edition

interference patterns. Also as a consequence of

the proximity of the two lines, the interference

patterns together will form an observable beat

pattern. From the resulting beat pattern the

difference in the wavelengths can be determined.

1.1 Setup

For the experiment, a standard Michelson

interferometer was used to create interference

patterns which allowed measurements of the

wavelength of the light sources used. Below in

Figure 1 is a diagram of the experimental setup

used:

Figure 1 - Experimental Setup

In the experimental setup, a computer controlled

motor was used to move the adjustable mirror