prsim-spectrometer.ppt

Prism spectrometerCOLLEGE PHYSICS EXPERIMENT CENTER OF SCIENCE SCHOOL XIAN SHIYOU UNIVERSITY 1.Understand configuration of spectrometer and the function of all parts,and the principle of measure the index of refraction of the tri-prism.2.Master method of adjusting spectrometer.3.Determine the index of refraction of a glass prism by the angle of minimum deviation.PurposeApparatus Prism,spectrometer,Sodium discharge lamp,Mirror,etc TheoryA light ray incident on a plane surface that forms the boundary between two transparent media can be reflected at the surface or transmitted into the second medium.Theoryand An optical prism is a transparent,optically dispersive material which is typically cut in the shape of the triangular wedge.Consider a light ray obliquely incident on left hand side of a triangular prism as shown in left Figure.This figures shows a typical path for the light ray transiting this material.The angles 1,2,and are,respectively,the incident angle from medium 1 to medium 2,the refraction angle within medium 2,the incident angle from medium 2 to medium 1,and the refraction angle out into medium 1.TheoryThe deviation angle is defined as the angle between the initial direction of the light ray and its final direction.=2+It can be further shown that=1+2 whereis the angle formed by the leading edge of the prism TheoryIf the angle of incidence1 is varied from some arbitrary starting value,either by moving the light source or rotating the prism,the deviation anglewill become smaller until it reaches a minimum value,after which continued adjustment of 1 will cause the deviation angle to increase again.This special,smallest,value ofis called the angle of minimum deviation which is denoted by m.Taking the index of refraction of air to be n1=1,n2,in terms of the angle of minimum deviation Prism spectrometerAdjusting the spectrometer In order to conduct precise measurements,the apparatus must be carefully adjusted The slit and the crosshairs must be positioned exactly in the focal plane of the respective objective lens(telescopic beam path).The slit and the prism surfaces must be aligned parallel to the rotary axis of the telescope.Adjusting the spectrometer1.Remove the prism from the spectrometer.2.Telescope Setup.Preliminary adjustment:The inclination of the prism table can be varied only to a limited extent.To ensure that enough play is left for adjustment,align the table as horizontally as possible(by eye)during preliminary adjustment.Align the telescope,the prism table and the slit tube(collimator)horizontally by eye.Center the telescope and the collimator laterally using the screws for lateral adjustment,and then screw them tight.Do not loosen the adjusting screws too much on one side,as these support the telescope and the collimator.Adjusting the spectrometerWhile looking through the telescope,slide the eyepiece in and out until the cross-hairs come into sharp focus.Loosen the graticule lock ring,and rotate the graticule until one of the cross-hairs is vertical.Retighten the lock ring and then refocus if necessary.Position the spectrometer so that the telescope can be pointing at some distant object(e.g.building outside the window).View the distant object through the telescope and turn the focus knob until the image is sharp.The telescope is now focused for parallel light rays.Adjusting the spectrometerAligning the optical axis of the telescope perpendicular to the spectrometer axis:Place the flat glass plate in its holder(g)in the center of the prism table at an angle of 45to the collimator(e)so that the(imaginary)line between two of the adjusting screws of the prism table is parallel to the side surfaces of the flat glass plate(see Fig.12-6).Align the telescope(a)perpendicular to one of the side faces of the flat glass plate so that the cross-hairs are reflected in this surface.Adjust the horizontal cross-hair so that is coincident with its reflection.In making this adjustment,correct half the difference using the height-adjustment screw of the telescope(a6)(see Fig.12-5)and the other half using the leveling screw(c1)of the prism table.Adjusting the spectrometerRepeat the following two steps until the horizontal crosshair and its mirror image coincide on both sides of the flat glass plate:1)Turn the telescope 180as shown in Fig.12-6，so that the cross-hairs are reflected on the opposite side of the flat glass plate.2)Check whether the cross-hair and its mirror image coincide.If not,correct half the difference using the height-adjustment screw of the telescope(a6)(see Fig.12-5)and the other half using the leveling screw(c1)of the prism table,as described above.Secure the height-adjustment screw of the telescope(a6)using a lock nut.Remove the flat glass plate with holder from the prism tableAdjusting the spectrometerFig.12-5 Telescope with illumination sourcea1 Focus adjustment knoba2 Arresting screw for illumination sourcea3 Illumination sourcea4 Oculara5 Opening for illumination source(not visible)a6 Height adjustment screw for telescopeAdjusting the spectrometerCollimator Setup.Check that the collimator slit is partially open(use the slit width adjust screw).Align the telescope directly opposite the collimator as shown in Figure 12-7.Looking through the telescope,adjust the focus of the collimator and,if necessary,the rotation of the telescope until the slit comes into sharp focus.DO NOT change the focus of the telescope.Adjusting the spectrometerSpectrometer Table Setup.Read the angle Experimental procedure 1.Adjusting prism spectrometer following up steps.2.Measurement of the Prism Angle.Rotate the spectrometer table until the refracting edge of the prism is approximately pointing towards the collimator as in Figure 12-10.Rotate the telescope to one side to receive the cross light which is reflected from the face ZX of the prism.Carefully position the telescope until the cross image is centered on the cross-wires.Record the angular position of the telescope as 1 in Data Table 12-1.Rotate the telescope around to receive cross light reflected off the other face ZY of the prism and record the angular position of the telescope as 2 in Data Table 12-1.Rotate the spectrometer table slightly by a few degrees.Repeat the above steps one more times to have a total of TWO trials in estimating the prism angle.Record all your results in Data Table 12-1.Experimental procedure3.Setting the angle of minimum deviation:The resolution increases with decreasing slit widths;however,the light intensity of the spectrum is correspondingly reduced:Using the micrometer screw for slit width,set the slit to a suitable width.Slowly turn the prism table and observe the shift of the spectral lines with the telescope until a“center”line of the spectrum(e.g.yellow,l=587.6 nm)just passes through a reversing point(minimum setting).Fix the prism table and telescope in the minimum position using the corresponding arresting screws.Record all your results in Data Table 12-2.Repeat the above steps three more times to have a total of THREE trials.RecordCalculationReflection index Standard deviation SummaryWrite a paper about 1000 words,it includes the following contents.1.Your learning from experiment.2.your recommendation for experimental class.Hand in all your homework before 20th December to 2#309(Teachers office)-Table No.19