Main Page/PHYS 3220/Speed of Light - Revision history

Gloria at 20:08, 22 October 2020

2020-10-22T20:08:51Z

Gloria at 20:03, 2 September 2020

2020-09-02T20:03:49Z

Gloria at 19:50, 2 September 2020

2020-09-02T19:50:28Z

Gloria at 19:41, 2 September 2020

2020-09-02T19:41:18Z

Mgeorge at 12:50, 12 October 2011

2011-10-12T12:50:03Z

Mgeorge at 12:49, 12 October 2011

2011-10-12T12:49:19Z

Mgeorge: Protected "Main Page/PHYS 3220/Speed of Light" ([edit=sysop] (indefinite) [move=sysop] (indefinite))

2011-07-20T18:12:47Z

Protected "Main Page/PHYS 3220/Speed of Light" ([edit=sysop] (indefinite) [move=sysop] (indefinite))

Mgeorge at 17:39, 19 July 2011

2011-07-19T17:39:12Z

Mgeorge at 17:38, 19 July 2011

2011-07-19T17:38:53Z

Mgeorge at 17:31, 19 July 2011

2011-07-19T17:31:46Z

@@ Line 88: / Line 88: @@
 <ul>
 <li><p>With the motor off, mark the location of the beam on the graph paper.</p></li>
-<li><p>To give a good angular displacement of the light beam, the mirror should be rotating at about 400 cps (= 24,000 rpm). Accelerate the motor by slowly turning the dial on the control box. At this recorded frequency of rotation, which can be observed directly on the oscilloscope, of mirror M1, mark the new beam location on the graph paper. Notice that it is not the bright line you have to mark, but the dim spot above it. (What is the reason for the bright line?)</p></li>
+<li><p>To give a good angular displacement of the light beam, the mirror should be rotating at about 400 cps (= 24,000 rpm). Accelerate the motor by slowly turning the dial on the control box. At this recorded frequency of rotation, which can be observed directly on the oscilloscope, of mirror M1, mark the new beam location on the graph paper. Notice that it is not the bright line you have to mark, but the dim spot above or below it depending on your alignment. (What is the reason for the bright line?)</p></li>
 <li><p>From the measured displacement x and frequency f calculate the speed of light. Include a discussion of the uncertainties involved in this set-up. How could this set-up be improved to yield more accurate results?</p>
 </li>

← Older revision		Revision as of 20:03, 2 September 2020
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	<p>Should you use the origin (i.e., location of the spot when the mirror is stationary) to obtain an absolute scale for the displacements or not? If yes, then you could constrain the fit to go through the origin. </p>		<p>Should you use the origin (i.e., location of the spot when the mirror is stationary) to obtain an absolute scale for the displacements or not? If yes, then you could constrain the fit to go through the origin. </p>
		+
		+	<p> Some useful information in preparation for this experiment can be found on [https://www.ld-didactic.de/literatur/hb/e/p5/p5611_e.pdf FM1] and [https://www.ld-didactic.de/literatur/hb/e/p5/p5612_e.pdf FM2].

@@ Line 88: / Line 88: @@
 <ul>
 <li><p>With the motor off, mark the location of the beam on the graph paper.</p></li>
-<li><p>To give a good angular displacement of the light beam, the mirror should be rotating at about 400 cps (= 24,000 rpm). Accelerate the motor by slowly turning the dial on the control box.<b>Do not use the “max Rev/sec” button.</b> At this recorded frequency of rotation of mirror M1, mark the new beam location on the graph paper. Notice that it is not the bright line you have to mark, but the dim spot underneath it. (What is the reason for the bright line?)</p></li>
+<li><p>To give a good angular displacement of the light beam, the mirror should be rotating at about 400 cps (= 24,000 rpm). Accelerate the motor by slowly turning the dial on the control box. At this recorded frequency of rotation, which can be observed directly on the oscilloscope, of mirror M1, mark the new beam location on the graph paper. Notice that it is not the bright line you have to mark, but the dim spot above it. (What is the reason for the bright line?)</p></li>
-<li><p>From the measured displacement x and frequency f calculate the speed of light. Include a discussion of the uncertainties involved in this set-up. How could this set-up be improved yield more accurate results?</p>
+<li><p>From the measured displacement x and frequency f calculate the speed of light. Include a discussion of the uncertainties involved in this set-up. How could this set-up be improved to yield more accurate results?</p>
 </li>
 </ul>
@@ Line 98: / Line 98: @@
 <ul>
 <li>Replace the graph paper with a photodiode detector, mounted on a leveled travelling microscope, as shown in Fig 2. The oscilloscope measures both the pulse amplitude and rotation frequency directly. The travelling microscope allows both horizontal and vertical measurements of image movement.</li>
-<li>At a number of frequencies, record the pulse-amplitude distributions, both horizontal and vertical. From this, determine the position of the pulse maxima. Graph these x values as a function of frequency. Include an error analysis in your report. Your least-squares analysis should give a value and precision estimate of x. Is the least-squares fit a satisfactory method?  Measure exactly the position of the optical components. Substitute these values to determine c. Why do you see a spot image?   (Why is it not a line image?)</li>
+<li>At a number of frequencies, record the position of the pulse-amplitude distributions, both horizontal and vertical. Adjust the x and y position to find the pulse maxima at each frequency.  Record the position of the pulse maxima. Graph these x values as a function of frequency. Include an error analysis in your report. Your least-squares analysis should give a value and precision estimate of x. Is the least-squares fit a satisfactory method?  Measure exactly the position of the optical components. Substitute these values to determine c. Why do you see a spot image?   (Why is it not a line image?)</li>
 </ul>

@@ Line 74: / Line 74: @@
 <p><b>CAUTION: Laser light is dangerous. Always wear the laser safety goggles provided when the laser is on. Failure to do so will result in you being asked to leave. Understand the beam path of the laser source before switching the laser on.</b></p>
-<p>Set up the apparatus as shown in Fig. 1. <b>(Check if it has already been aligned for you!)</b>. The beam from the laser travels a long path to the fixed mirror and back again, reflecting in both directions from the rotating mirror. It finally reflects from BS, a beam splitter, onto the scale. Notice that the lens positioned perpendicular to the S2 beam path introduces a negligible variation in Δs but adjusts the resulting beam-size to a more useable magnitude. The rotation frequency of the mirror is controlled by a control box with a digital display of the rotation frequency. <b>(Do not run the motor at maximum frequency for long time periods!)</b></p>
+<p>Set up the apparatus as shown in Fig. 1. <b>(Check if it has already been aligned for you!)</b>. The beam from the laser travels a long path to the fixed mirror and back again, reflecting in both directions from the rotating mirror. It finally reflects from the beam splitter, BS, onto the scale. Notice that the lens positioned perpendicular to the S2 beam path introduces a negligible variation in Δs but adjusts the resulting beam-size to a more useable magnitude. The rotation frequency of the mirror is controlled by a Variac transformer.  A photodiode is placed near the rotating mirror to measure the rotation frequency using an oscilloscope. <b>(Do not run the motor at maximum frequency for long time periods! Note maximum speed is reached at 60 on the Variac transformer dial.)</b></p>
-<p>The laser beam must be optically aligned at a  slight upward angle with respect to all the other components. Once a component is aligned, do not disturb it, even slightly, or the entire process may have to be repeated!  This can amount to a few hours of your time.</p>
+<p>The laser beam must be optically aligned at a slight upward angle with respect to all the other components. Once a component is aligned, do not disturb it, even slightly, or the entire process may have to be repeated!  This can amount to a few hours of your time.</p>
-<p>Turn the mirror motor off. Aim the laser so that it strikes the rotating mirror (M1). By gently turning the wheel under the motor, turn mirror M1 until the beam is reflected back just above the beam origin. Adjust either the actual position or the levelling screws as necessary. Then turn mirror M1 until the beam reflects off of mirror M2. Adjust mirror M2 until the beam is reflected back onto mirror M1 and the beam origin. <b>(Avoid adjusting the vertical position of the apparatus, aside from M4 and M5, unless absolutely necessary!)</b> Repeat these steps until the beam strikes mirror M5 and reflects back the whole way to mirror M1 and the beam origin. You will need to adjust both mirrors M4 and M5 until the beam retraces its path, that is, with no secondary images. Finally, align the beam splitter (BS), with equal distances to the laser and graph scale, placing the beam in a convenient location for your measurements. Can you think of a procedure to optimise this step? Hint: think in terms of any secondary images. </p>
+<p>Ensure the mirror motor off. Aim the laser so that it strikes the rotating mirror (M1). By gently using the attached key turn the mirror M1 until the beam is reflected back just above the beam origin. Adjust the height of the beam, if necessary, using the levelling screws of the rotating mirror mount. Then turn mirror M1 until the beam reflects off of mirror M2. Adjust mirror M2 until the beam is reflected back onto mirror M1 and the beam origin. <b>(Avoid adjusting the vertical position of the apparatus, aside from M4 and M5, unless absolutely necessary!)</b> Repeat these steps until the beam strikes mirror M5 and reflects back the whole way to mirror M1 and the beam origin. You will need to adjust both mirrors M4 and M5 until the beam retraces its path, that is, with no secondary images. Finally, align the beam splitter (BS), '''with equal distances to the laser and graph scale''', placing the beam in a convenient location for your measurements. Can you think of a procedure to optimise this step? Hint: think in terms of any secondary images. </p>
 <p>Do not in any way disturb the tables or you will have to begin again.</p>

@@ Line 72: / Line 72: @@
 <h1>Procedure</h1>
-<p><b>CAUTION: Laser light is dangerous.Always be wearing the laser safety goggles provided when the laser is switched on. Failure to do so will result in you being asked to leave. Understand the beam path of the laser source before switching the laser on.</b></p>
+<p><b>CAUTION: Laser light is dangerous. Always wear the laser safety goggles provided when the laser is on. Failure to do so will result in you being asked to leave. Understand the beam path of the laser source before switching the laser on.</b></p>
 <p>Set up the apparatus as shown in Fig. 1. <b>(Check if it has already been aligned for you!)</b>. The beam from the laser travels a long path to the fixed mirror and back again, reflecting in both directions from the rotating mirror. It finally reflects from BS, a beam splitter, onto the scale. Notice that the lens positioned perpendicular to the S2 beam path introduces a negligible variation in Δs but adjusts the resulting beam-size to a more useable magnitude. The rotation frequency of the mirror is controlled by a control box with a digital display of the rotation frequency. <b>(Do not run the motor at maximum frequency for long time periods!)</b></p>

← Older revision	Revision as of 18:12, 20 July 2011
(No difference)

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 <p>Referring to Fig. 1, the distance travelled by the light pulse is</p>
 <table width=600 align=center><tr><td>
-<p align=justify>[[File:C-eqn4.png|300px|center]]
+<p align=justify>[[File:C-eqn4.png|270px|center]]
 </p>
 </td><td><b>(1)</b></td></tr></table>