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Kaori Costelloe

on 23 October 2013

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Diffraction Experiment

Calibration curves using Mercury Light source
1st order
2nd order
In more detail
To determine the wavelength of the sodium light from the 1st and 2nd order calibration curves.

Summary of results - Sodium

Using a table of emission lines, the above graphs were obtained by plotting mercury's emission line wavelengths versus Sine of the measured angle.
Introduction/ Background information
Calibration curve [METHOD 1]
1. Set up experiment as shown in the diagram.

2.Adjust the line on the lens so that it directly matches the light passing through the medium.
3. slowly slide the telescope until a streak of colour is seen. (e.g.Violet)
4. Record the angle at which the colour was detected.
5. slide the telescope to the opposite side until the same streak of colour is reached.
6. Record the angle.
7. Subtract the small angle from the larger angle and divide this number by 2.
8. Repeat steps 4,5,6 &7 with a few more colours.
9. Plot the 1st order calibration curve given the angle and wavelength. (Wavelength VS Sinθ)
10. Carry out 2 to 9 for the 2nd order.

Sodium light
1. Repeat the steps as taken in method 1.
2. Determine the wavelength of the sodium light using the two calibration curves.

By Victor Tse, Jaimie Lai & Kaori Costelloe
The Balmer Series n.d., viewed 19th October 2013, <http://web.utk.edu/~cnattras/Phys250Fall2012/Laboratories/Balmer.htm> .
OSC Physics maintainer 9th September 2013, Multiple slit diffraction, viewed 21st October 2013, <http://cnx.org/content/m42512/latest/?collection=col11406/latest>.
The diffraction grating spectrometer n.d., viewed 19th October 2013, <http://www.tass-survey.org/classes/manual273/diffract.html>
In celebration of Psalm Nineteen:Gods handiwork in creation, January 2011, Viewed 22nd October 2013, <http://19thpsalm.org/Ch01/>
Atomic Spectra and Models of the atom, n.d., 22nd October 2013, <http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0/s10-03-atomic-spectra-and-models-of-t.html>.
What is the differences between low pressure and high pressure, n.d., viewed 22nd October 2013, <http://www.wolffsystem.com/pressure.html>.
LP: Less than 1 atm, slight vacuum, Implode.
HP:More than 1 atm, explode.
Using sodium
Sodium is used for yellow fireworks due to the 589nm emission lines.
This also applies for other elements.

What is diffraction grating?
An optical device containing thousands of closely spaced parallel lines
Separates incident light into its individual wavelengths
d sin (theta) = n (lambda)
While the large number of lines are opaque, the spacings allow light to transmit. In this case, both diffraction and interference occurs.
Applying Huygen's princple,every slit act as a new source. The waves interfere constuctively, where we are able to see the bright colours of the spectrum from the light.
A calibration graph can be drawn from a known wavelength and a particular order. Then used to read off the value of lamda for other light sources.

The interpolated wavelengths and colours of the sodium bands were found to be semi consistent with what we observed during the experiment. The green band we observed at the angle of 17.5 and 36.5 degrees did not match with the blue/cyan colour at those angles using the calibration graphs. Indicating a significant amount of uncertainty in our measurements.

The main source of uncertainty is the movement of our diffraction grating when a piece of cloth was used to cover the spectrometer. When the telescope was adjusted to larger angles the cloth folded and moved the grating causing us to potentially miss colour bands as shown by the lack of a second order violet measurement.

For future repetition holding down the diffraction grating with adhesive or a darker environment with no interfering sources of light will produce better results as in a darker environment there will be no need of a cloth and the weaker colour bands will be more visible allowing us to produce more accurate angle measurements and calibration curves.
The colour bands and wavelengths of a sodium light were determined to be: violet at 430, green at 500 and yellow at 585 nm, using a calibration graph produced with a mercury light source.
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