Gratings
Instructor:
Pro. Lab4
Diffraction Tsai
Jiajie Huo(Jacky)
Member:
Yao Bin Yu
Zeyuan (Seth) Guan
1. introduction.
In this experiment, we are generally focusing on information
about inference and diffraction.
2. Procedure.
a. In section 1, we align the diode laser and shine it through one
of the one- slit gratings on the 3*5 grating plate. Then we choose
the 7th bright spots and the distance from the central maximum
l=1cm.
In Q1, we did the sketch pattern on the graph paper. The orders
we are seeing aroused from both interference and diffraction.
In Q2, we got the experimental width of it was d=0.11mm,
while the real width of it was do=0.088mm, so the difference of
them is 25%.
In Q3, we predicted that when the spectrum would spread out
when the width of the slit decrease and spectrum would be
condense when the width of the slit increase. We changed the
width of the slit to d0=0.176mm and the experimental one was
d=0.228mm, so the difference was 29.5%.
In Q4, the prediction was correct because when we increased the
width of the slit, the spectrum would be condense.
b. Then we shine the laser through a double-slit grating.
In Q5a, we sketched spectrum from the double-slit and the graph
of the intensity of light vs distance from center of the screen. The
experimental slit separation was 0.185mm, the real slit separation
was 0.175mm, so the difference of them was 5.7%.
In Q5b, we predicted that spectrum would spread out when the
slit separation decreased and spectrum would be condensed when
the slit separation increased. We used the double-slit grating with
slit separation of 0.35mm, and the experimental slit separation was
0.31mm, so the difference was 11.4%.
In Q6, the prediction was correct.
c. We shined the laser through several two-slit gratings until we
saw one with a clearly missing order. We record the slit separation
d was 1.35cm and it was the 5th dark spot and 11th bright spot.
In Q7a, the ratio of a/d is 5/11.
In Q7b, the ratio of the slit separation and width we used was
2:1, the constructive and maxima diffraction minima should be at
the point where the n:m was 2:1, so the difference of the
experimental and real ratio is 10%.
In Q8, we got the wave length of the laser is 4789A, so its
difference with the real number was 25%.
d. Then we held the high-quality transmission grating in front of
our eye and looked at a white light source. We could see the
spectrum.
In Q9, the reason we saw the rainbow spectrum is a diffraction
grating spreads out light into its component wavelength.
In Q10, according to the theory, the peak separation length of the
light vary according to its wavelength, so the bright peaks of
different wavelength located in different places, they are widely
spaced.
4. Data Sheet
Attached.
6. Discussion and conclusion.
In this experiment, we have four sections. First, we shined the
laser through a one- slit gratings, and got the experimental width of
it was d=0.11mm, while the real width of it was do=0.088mm, so
the difference of them is 25%. The prediction we made: when the
spectrum would spread out when the width of the slit decrease and
spectrum would be condensed when the width of the slit increase
was correct. Second, we shined the laser through a double-slit
grating, we got the experimental slit separation was 0.185mm, the
real slit separation was 0.175mm, so the difference of them was
5.7%. And the prediction was proved too. Third, we shined the
laser through several two-slit gratings until we saw one with a
clearly missing order and it was the 5th dark spot and 11th bright
spot. Then we got the wave length of the laser is 4789A, so its
difference with the real number was 25%. Fourth, we could see the
rainbow spectrum.
During the experiment, the major errors we would came across
would be as follow: first, we shined the laser through a one or two
slit gratings, the points appeared on the screen were too closed and
too small, which were very hard to measure. Second, there were
always the systematic errors existing in the whole experiment
because we had used approximation method.