Introduction:
This lab is designed to provide a basic understanding of whether and how the
wavelengths of different spectral lines, the angle of the diffracted waves, and the width of the
grating are correlated with each other. That is, in this lab we will use the transmission grating,
which is a type of diffraction spectrometer, in conjunction with a spectrometer. Obviously the
formula that will be used to corroborate this correlation will be n 2/ d = sin 0. We should note
that if AB (illustrated in the picture below) is equal to 2, or 22, or 32., etc. where ^ is the
wavelength of the light, waves from all the slits will constructively interfere at F and we shall get
a bright image. In addition, if the light is not monochromatic, there will be the same amount of
images coming from the slit in every order as there are various wavelengths in the light that is
coming from the source.
Moving on to the second part of the experiment, the experiment is designed to illustrate
interference at a wedge. That is, in this experiment the thin film consists of the air space between
two approximately plane parallel plate's pf glass, separated at one end by a thin strip of paper of
thickness T, as shown in the figure.2. We should note that if we move from toward the end of the
film that is thicker, the fringe of light that is encountered thereafter, should be related to the next
number of wavelengths of phase difference which, indeed, is an integral. Thus, if the travelling
between fringes corresponds to an increase in film thickness of one half wavelength, then,
travelling the entire wavelength of the film, would result in an increase in film thickness of N
half-wavelengths. The equation used to assess our data and get the wavelengths needed will be N
2/2 = T
Obiectives:
The main purpose of this experiment is to measure the wavelengths of several spectral
lines. Also, this lab is designed to help students visualize and get better understanding of
diffraction grating and interference. Students were also able to see the many geometric properties
of light waves and view the different spectrums of light through the uses of the lamps provided.
Procedure:
Part 1:
First off, the grating spectrometer is adjusted so that the slit is illuminated. Thereafter,
while looking through the telescope, you should focus on the cross-hairs by moving the tube
holding the eyepiece lens. At that point, adjust the tube that holds the slit without disturbing the
focus of the cross hairs. It is then that the spectrometer would be ready to be used for this
experiment. Now, one of the members must focus the cross-hairs at one of the colors or images
and read one of the verniers. Then, the telescope must be turned to either the direction right or
left and focus the cross hairs on a line of the spectrum and read the Vernier for the angle. The
difference between the two angles is the angle of diffraction. Use equation 1 to measure the
wavelength. The same procedure is to be repeated in order to calculate the wavelengths of at
least four lines. Finally, the data obtained will be compared with those given at the end of the
manual.
Part 2:
Make sure the glass plates are clean. Place a strip of paper between the ends of the glass
plates. Then, place the surfaces that have the rules lines on the inside. The length L must then be
measured. At that point, you are ready to place the aluminum stand which contains the lens and
the mirror over the center of the air film and the sodium lamp should then be placed at a
reasonable and convenient distance in front of the whole in the side of the stand. You should then
be looking for interference patterns consisting of a series of dark lines across the dark image of
the flame. Count two lengths of two counts of the number of dark lines. Finally, the wavelength
of the yellow light can easily be calculated using the formulas.
Part 3:
At this stage, the paper strip is to be replaced by a strip of steel. The thickness with the
aid of the sodium lamp is to be measured and the assumption should be made that the values of
the wavelength of the light are correct.
Part 4:
If time permits, measure the wavelengths of the three bright lines of the hydrogen
spectrum, using the spectrometer.
We were able to see from our results, as the color of the light changed (wavelength
increased) the fringe distance also increased. For diffraction grating: As the grating lines increase
we witness that the fringe decreases. Violet 400 nm, Green 535 nm, and Red 650 nm, the fringe
distance increased from Violet 21 mm to Green 29.8 mm to 34.5 mm.
Part 1:
Part 2:
L= 0.012 m
Length 1 = 0.005 m, number of dark lines = 3
Length 2 = 0.007 , number of dark lines = 2
Calculation of wavelength of yellow light= 580 +/- 5 nm
Analysis: As we can see the wavelength attained through part 2, considering the uncertainty, is
the same as that given in the manual. Thus, we can see that the formula provided in the manual
holds true and the data in part 1 are also true.
Part 3:
The thickness measured or the strip of steel is 0.008 m
QUESTION: In deriving Equation 2, an expression of the type "phase difference
corresponding to 14 wavelengths" was used instead of "path difference of 14 wavelengths". Why
do these two expressions differ in meaning for this experiment?
Answer: Path difference is the difference in path traversed by the two waves, measured in terms
of the wavelength of the associated wave. It has a direct relation with phase difference. However,