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author | Eugeniy Mikhailov <evgmik@gmail.com> | 2013-08-30 17:38:34 -0400 |
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committer | Eugeniy Mikhailov <evgmik@gmail.com> | 2013-08-30 17:42:57 -0400 |
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tree | 047333fc81ffd6c08e54e1593eb86f13307e5a4d /manual/chapters/fabry-perot.tex | |
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diff --git a/manual/chapters/fabry-perot.tex b/manual/chapters/fabry-perot.tex deleted file mode 100644 index d74745f..0000000 --- a/manual/chapters/fabry-perot.tex +++ /dev/null @@ -1,298 +0,0 @@ -%\chapter*{Fabry-Perot Interferometer and the Sodium Doublet} -%\addcontentsline{toc}{chapter}{Fabry-Perot Interferometer} -\documentclass{article} -\usepackage{tabularx,amsmath,boxedminipage,epsfig} - \oddsidemargin 0.0in - \evensidemargin 0.0in - \textwidth 6.5in - \headheight 0.0in - \topmargin 0.0in - \textheight=9.0in - -\begin{document} -\title{Fabry-Perot Interferometer and the Sodium Doublet} -\date {} -\maketitle - - -\noindent - \textbf{Experiment objectives}: Assemble and align Fabry-Perot interferometer, - and use it to measure differential wavelength for the Na doublet. - - \section*{Theory} - -\subsection*{The Fabry-Perot Interferometer} - -Any interferometer relies on interference between two or more light field. In a Fabry-Perot configuration input -light field bounces between two closely spaced partially reflecting surfaces, creating a large number of -reflections. Interference of these multiple beams produces sharp spikes in the transmission for certain light -frequencies. Thanks to the large number of interfering rays, this type of interferometer has extremely high -resolution, much better than, for example, a Michelson interferometer. For that reason Fabry-Perot -interferometers are widely used in telecommunications, lasers and spectroscopy to control and measure the -wavelengths of light. In this experiment we will take advantage of high spectral resolution of the Fabry-Perot -interferometer to resolve two very closely-spaces emission lines in Na spectra by observing changes in a -overlapping interference fringes from these two lines. -\begin{figure}[h] -\begin{center} -\includegraphics[width=0.8\linewidth]{./pdf_figs/pfig1} -\caption{\label{fpfig1}Sequence of Reflection and -Transmission for a ray arriving at the treated inner surfaces $P_1 \& P_2$.} -\end{figure} -\end{figure} - -A Fabry-Perot interferometer consists of two parallel glass plates, flat to better than 1/4 of an optical -wavelength $\lambda$, and coated on the inner surfaces with a partially transmitting metallic layer. Such -two-mirror arrangement is normally called an {\it optical cavity}. The light in a cavity by definition bounces -back and forth many time before escaping; the idea of such a cavity is crucial for the construction of a laser. -Any light transmitted through such cavity is a product of interference between beams transmitted at each bounce -as diagrammed in Figure~\ref{fpfig1}. When the incident ray arrives at interface point $A$, a fraction $t$ is -transmitted and the remaining fraction $r$ is reflected, such that $t + r = 1$ ( this assumes no light is lost -inside the cavity). The same thing happens at each of the points $A,B,C,D,E,F,G,H\ldots$, splitting the initial -ray into parallel rays $AB,CD,EF,GH,$ etc. Between adjacent ray pairs, say $AB$ and $CD$, there is a path -difference of : -\begin{equation} - \delta = BC+CK -\end{equation}%eq1 - where $BK$ is normal to $CD$. In a development -similar to that used for the Michelson interferometer, you can show -that: -\begin{equation} - \delta = 2d\cos\theta -\end{equation}%eq.2 - If this path difference produces -constructive interference, then $\delta$ is some integer multiple of , -$\lambda$ namely, -\begin{equation} - m\lambda = 2d\cos\theta %eq.3 -\end{equation}%eq.3 - -This applies equally to ray pairs $CD$ and $EF, EF$ and $GH$, etc, so that all parallel rays to the right of -$P2$ will constructively interfere with one another when brought together. - -Issues of intensity of fringes \& contrast between fringes and dark background -are addressed in Melissinos, {\it Experiments in Modern Physics}, pp.309-312. - -\subsection*{The Sodium Doublet} - - In this lab you will measure the separation between the two - famous ``sodium doublet'' lines, the two closely spaced lines - which occur at 589 $nm$ and 589.59 $nm$, respectively. This ``doublet'' - emission is evidence that the atomic electron has the property - of intrinsic angular momentum, or spin S. As you are learning - in Modern Physics 201, the discrete spectral lines in atomic - emission are due to the quantization of electron energies in - the atom. As Niels Bohr postulated, electrons in atoms are - only allowed to absorb and emit energy in discrete - quantities. When an electron moves from one orbit to another - in an atom, a well-defined amount of energy is emitted as - light at a fixed wavelength. Later in this class we will - explore the spectra of various atomic gases. -\begin{figure}[h] -\centerline{\epsfig{height=5cm, file=fpfig2.eps}} \caption{\label{fpfig2.fig}Fine Structure Splitting in sodium -giving rise to the sodium doublet lines} -\end{figure} - For many - atoms, {\bf atomic levels are further split}, for example, - by interactions of electrons with each other (Russell-Saunders - coupling), external magnetic fields (Zeeman effect), and even - the interaction between the spin of an electron and the - magnetic field created by its orbital angular momentum - (spin-orbit coupling). This is known as fine structure - splitting (FSS). The fine structure splitting for the sodium - 3P state is due to spin-orbit coupling, and is illustrated in - Figure \ref{fpfig2.fig}. The "3P" state refers to sodium's -valence electron - which has a principal quantum number of $n=3$ and an orbital - quantum number of $l=1$ (a P-state). Further, the electron has - an intrinsic spin (like a top), described by a spin quantum - number $S$, which is either +1/2 or -1/2. The electron has a - magnetic moment due to its intrinsic spin, given by $m_S$. Due to -its orbital angular momentum around a charged nucleus, it - senses a magnetic field ${\mathbf H}$. The energy of interaction of a - magnetic moment in a field is given by $E = -\mu\cdot {\mathbf H}$. -This gives - rise to the splitting and two spectral emission lines. - - -\section*{Procedure} - -\subsection*{Set Up} -\textbf{Equipment needed}: Pasco precision interferometry kit, Na lamp, -adjustable-hight platform. - -\begin{figure} -\centerline{\epsfig{width=0.7\linewidth,file=fpfig3new.eps}} \caption{\label{fpfig3.fig}The Fabry-Perot -Interferometer} -\end{figure} - -The interferometer layout is shown in Figure \ref{fpfig3.fig}. The inner spacing $d$ between two -partially-reflecting mirrors ($P1$ and $P2$) can be roughly adjusted by loosening the screw that mounts $P2$ to -its base. It is important that the plates are as closely spaced as possible. Move the plates to within $1.0 - -1.5$~mm of each other, but make sure the mirrors do not touch! - -\subsection*{Data acquisition} - -\begin{enumerate} -\item \textbf{Turn on the sodium lamp as soon as you arrive. It should warm up for about 20 minutes -before starting}. -\item Turn the micrometer close to or at 0.00. -\item Remove the diffuser sheet from in front of the lamp. Look through -plate $P2$ towards the lamp. If the plates are not parallel, you will see -multiple images of the lamp filament. Adjust the knobs on $P1$ until the images -collapse into one. At this point, you should see faint circular interference -fringes appear. -\item Place the diffuser sheet in -front of the lamp so you will now only see the fringes. Continue to adjust the -knobs on one plate (not the knobs to move the plate back and forth, but the -ones to bring it parallel) to get the best fringe pattern. It is the most -convenient to view the interference picture directly. -\item -Seek the START condition illustrated in Fig.(\ref{fpfig4.fig}), in which all bright fringes are evenly spaced. -You do this by moving the micrometer. Note that alternate fringes may be of somewhat different intensities, one -corresponding to fringes of $\lambda_1$, the other to those for $\lambda_2$. If you do not see this condition, -try moving the mirror $P2$ across the range of micrometer screw. If you still cannot find them, you can also -move the whole plate by loosening one plate and sliding it a little. -\item Practice going through the fringe conditions as shown in Fig.(\ref{fpfig4.fig}) -by turning the micrometer and viewing the fringes. Do not be surprised if you -have to move the micrometer quite a bit to go back to the original condition. -\item Find a place on the micrometer ($d_1$) where you -have the ``START'' condition for fringes shown in Fig.(\ref{fpfig4.fig}). Now -advance the micrometer rapidly while viewing the fringe pattern ( NO COUNTING -OF FRINGES IS REQUIRED ). Note how the fringes of one intensity are moving to -overtake those of the other intensity (in the manner of -Fig.(\ref{fpfig4.fig})). Keep turning until the ``STOP'' pattern is achieved -(the same condition you started with). Record the micrometer reading as $d_2$. -\emph{Remember that 1 tick mark is 1 micrometer ($10^{-6}m$). That means if you -read 1.24, your really move 124 $\mu m$.} -\end{enumerate} - -\noindent \fbox{\parbox{\linewidth}{\textbf{Experimental tip:} You may have to -``home in'' on the best START and STOP conditions. Let's say that the even -fringe spacing for the START condition ($d_1$) is not exactly in view. Now move -the micrometer, looking to see if the pattern moves toward even spacing. If so, -stop and read the micrometer for $d_1$. Move a bit more. If this second fringe -spacing looks better than the first, then accept this for $d_1$. The same -``homing in'' procedure should be used to select the reading for $d_2$. In -other words as you approach the even spacing condition of the STOP pattern, -start writing down the micrometer positions. Eventually you will favor one -reading over all the others.}} - -\section*{Analysis} - - Since the condition we are seeking above for ``START'' places - the bright fringes of $\lambda_1$ at the point of destructive - interference for $\lambda_2$, we can express this for the bull's eye - center ($\theta= 0 $) as: -\begin{equation} -2d_1=m_1\lambda_1=\left(m_1+n+\frac{1}{2}\right)\lambda_2 -\end{equation} - - Here the integer n accounts for the - fact that $\lambda_1 > \lambda_2$ , and the $1/2$ for the -condition of - destructive interference for $\lambda_2$ at the center. Since the - net action of advancing by many fringes has been to increment - the fringe count of $\lambda_2$ by one more than that of -$\lambda_1$ , - then we express the ``STOP'' condition as: -\begin{equation} -2d_2=m_2\lambda_1=\left(m_2+n+\frac{3}{2}\right)\lambda_2 -\end{equation} - Subtracting the - two interference equations gives us: -\begin{equation} -2(d_2-d_1)=(m_2-m_1)\lambda_1=(m_2-m_1)\lambda_2+\lambda_2 -\end{equation} - Eliminating $(m_2-m_1)$ - in this equation we obtain: - -\begin{equation} -2(d_2-d_1)=\frac{\lambda_1\lambda_2}{(\lambda_1-\lambda_2)} -\end{equation} - - Solving this for $\Delta \lambda = \lambda_1-\lambda_2$, and - accepting as valid the approximation that $\lambda_1\lambda_2\approx -\lambda^2$ ( where $\lambda$ is the - average of $\lambda_1$ and $\lambda_2 \approx 589.26 nm$ ), we obtain: -\begin{equation} -\boxed{\Delta\lambda=\frac{\lambda^2}{2(d_2-d_1)}} -\end{equation} - -Each lab partner should independently align the interferometer and make at least \textit{two} measurements of -``START'' and ``STOP'' positions. A sample table to record the data is shown below. \\{\large -\begin{tabular}{|p{27mm}|p{27mm}|p{27mm}|p{27mm}|} -\hline - $d_1$ $\pm \dots$ & $d _2$ $\pm \dots$& $(d_1-d_2)$ $\pm \dots$& -$\Delta \lambda(nm) $ $\pm \dots$\\ -\hline -&&&\\ -\hline &&&\\ \hline &&&\\ \hline &&&\\ \hline &&&\\ \hline -\end{tabular} -} - -\vspace{1cm} Calculate average value of Na doublet splitting and its standard deviation. Compare your result -with the established value of $\Delta \lambda_{Na}=0.598$~nm. - - -\begin{figure}[h] -\centerline{\epsfig{width=0.8\linewidth,file=fpfig4.eps}} \caption{\label{fpfig4.fig}The Sequence of fringe -patterns encountered in the course of the FSS measurements. Note false colors: in your experiment the background -is black, and both sets of rings are bright yellow.} -\end{figure} - -\end{document} - -\newpage -\noindent -Physics 251 Section:\\ -\hskip 4.5in Name:\\ -\hskip 4.5in Partners:\\ -\vskip 0.5in -\subsection*{The Fabry-Perot Interferometer} -1. Briefly describe how the Fabry-Perot interferometer gives and interference -pattern (in one or two sentences):\\ -\vskip 1.2in -2. How does the interferometer's resolving power of the fringes depend on the -reflectivity of plates, r ? That is, does the sharpness of the fringes increase -or decrease with r ? Consult Melissinos or Professor Kane's Mathview program. -(The reflectivity of the plates defines the {\it finess} of the cavity).\\ -\vskip 1in. - - -{\large -\noindent -Fill in: - -The sodium doublet lines arise because an atomic$\underline{\hskip 1.in}$ -is split into two by$\underline{\hskip 1.in}$ coupling. -The electron has -intrinsic $\underline{\hskip 1.in}$, like a top, with values of -$\underline{\hskip 1.in}$ or$\underline{\hskip 1.in}$. Because of -this, the electron has in intrinsic magnetic$\underline{\hskip 1.in}$ and -has magnetic -energy in a magnetic field given by E=$\underline{\hskip 1.in}$ -case comes from the electron's $\underline{\hskip 1.in}$ motion. - -} - -\subsection*{DATA:} -{\large -\begin{tabular}{|p{27mm}|p{27mm}|p{27mm}|p{27mm}|} -\hline - $d_1$ $\pm \dots$ & $D _2$ $\pm \dots$& $(d_1-d_2)$ $\pm \dots$& -$\Delta \lambda(nm) $ $\pm \dots$\\ -\hline -&&&\\ -\hline -&&&\\ \hline -&&&\\ \hline -&&&\\ \hline -&&&\\ \hline -\end{tabular} -} -\vskip .2in -$\Delta \lambda=$\hskip 1.5in nm\\ -\vskip .2in -Standard deviation= \hskip 1.5in nm -\newpage -\end{document} |