scientific
7
012-02675B
Interferometer
Exp 1: Measuring the Wavelength of Light
Theory
In many scientific and industrial uses of interferometers, a light source of a known wave-
length is used to measure incredibly small displacements - about 10
-6
meters. However, if
you know the distance of mirror movement, you can use the interferometer to measure the
wavelength of a light source. In this experiment you will use the interferometer to measure
the wavelength of your laser light source.
Procedure
➀
Align the laser and interferometer as described in the preceding section, so an interference
pattern of circular fringes is clearly visible on your viewing screen.
➁
Adjust the micrometer knob so the lever arm is approximately parallel with the edge of the
interferometer base. In this position the relationship between knob rotation and mirror
movement is most nearly linear.
➂
Turn the micrometer knob one full turn counterclockwise. Continue turning counterclock-
wise until the zero on the knob is aligned with the index mark.
➦
NOTE: Whenever you reverse the direction in which you turn the micrometer knob,
there is a small amount of give before the mirror begins to move. This is called
mechanical backlash, and is present in any mechanical system involving reversals in
direction of movement. By beginning with a full counterclockwise turn, and then turning
only counterclockwise when counting fringes, you can eliminate backlash in your
measurement.
If you are using a blank piece of paper as your viewing screen, make a reference mark on
the paper between two of the fringes. You will find it easier to count the fringes if the
reference mark is one or two fringes out from the center of the pattern.
➄
Rotate the micrometer knob slowly counterclockwise. Count the fringes as they pass your
reference mark. Continue until a predetermined number of fringes has passed your refer-
ence mark (count at least 20 fringes). As you finish your count, the fringes should be in the
same position with respect to your reference mark as they were when you started to count.
Record d
m
, the distance that the movable mirror moved toward the beam-splitter as you
turned the micrometer knob. Remember, each division on the micrometer knob corre-
sponds to one micron (10
-6
meters) of mirror movement.
d
m
= _______________
Record m, the number of fringes that crossed your reference mark.
m = _______________
➇
Calculate the wavelength of the laser light (
λ
= 2d
m
/m).
➈
Calculate the percentage difference between your measured value for the wavelength of the
laser light and the value recorded in the laser specifications. (Check with your teacher for
the laser specifications.)
➃
➅
➆
