3.13.3 Experimental principles
To adjust the telescope well is the key to adjusting the spectrometer,and other adjustments are based on the telescope and meet the following requirements:
(1)The collimator should emit parallel light.
(2)The parallel light can be focused by the telescope.
(3)The stage and the optical axis of the telescope and collimator are perpendicular to the main axis of the spectrometer.
1)Adjusting the focal length of the eyepiece
The purpose is to observe clearly the scratch on the reticule of the eyepiece,as shown in Figure 3.13.4.The focusing method is to gently rotate the focusing hand wheel of the eyepiece in or out along the optical axis until the image of the reticule line is clear.
Figure 3.13.4 The reticle seen in the eyepiece
Figure 3.13.5 The top view of the double-sided mirror placed on the stage
2)Adjusting the focal length of the telescope
Focusing the telescope aims to make the cross line on the reticule of the eyepiece to the focal plane of the objective lens,and the specific methods are as follows:
(1)Connect the lamp source,and connect the lamp source plug of the eyepiece with the transformer socket,and illuminate the eyepiece.
(2)Place the plane mirror on the stage.The plane mirror should be placed along the diameter direction of the stage and through one of the adjusting screws,as shown in Figure 3.13.5.
(3)Roughly adjust the optical axis of the telescope to be placed horizontally.Adjust the leveling screw of the stage and turning the stage so that the plane mirror and the optical axis of the telescope are vertical,and the reflected image of the telescope is in the identical line to the telescope.
(4)When observing in the eyepiece,you can see a bright spot.Move the eyepiece sleeve back and forth to focus the objective lens of the telescope to make the bright green cross line image clear.Then use the leveling screw on the stage and the fine-tuning mechanism to adjust the bright green cross line to coincide with the cross line above the reticule.Move repeatedly the eyepiece back and forth to make the bright green cross line coincide with the cross without parallax.
3)Adjust the optical axis of the telescope to make it perpendicular to the main axis of the instrument
When the mirror is perpendicular to the optical axis of the telescope,its reflected image should fall on the center of the upper cross line symmetrical to the lower cross window on the reticule of the eyepiece,as shown in Figure 3.13.4.After the plane mirror rotates 180° around the axis,if the reflected image of another mirror also falls here,it means that the mirror is parallel to the main axis of the instrument.Of course,the optical axis of the telescope perpendicular to the mirror is also perpendicular to the main axis of the instrument.What are the reasons for some phenomena during adjusting?What needs to be adjusted?How to adjust it?All these issues should be analyzed clearly.
(1)Representation for non-adjusted stage and the adjustment method.Supposing that the optical axis of the telescope is perpendicular to the main axis of the instrument,but the tilt angle of the stage is not adjusted properly,as shown in Figure 3.13.6.The reflected light from surface A of the plane mirror is on the upper side,and the reflected light from surface B is on the lower side after the stage rotates 180°.Therefore,what can be seen in the eyepiece is that the reflection image from surface A is above the reflection image from surface B.Obviously,the adjustment method is to jump up(or down)the image from surface B(or the image from surface A)to half of the distance between the two image points,so that the images of surfaces A and B fall at the same height on the reticle.
Figure 3.13.6 Performance of stage non-adjusted well and corresponding adjustment principle(https://www.daowen.com)
(2)Representation for non-adjusted optical axis of telescope and the adjustment method.Supposing that the stage has been adjusted,but the optical axis of the telescope is not perpendicular to the main axis of the instrument,as shown in Figure 3.13.7.In Figure 3.13.7(a),whether the reflected light from surface A or plane B is upward,and the reflected image falls above the cross line on the reticule.In Figure 3.13.7(b),the specular reflection light is lower,and the reflected image falls below the upper cross.Obviously,we only need to adjust the elevation adjustment screw(12 in Figure 3.13.1)of the telescope to make the line coincide with the upper cross line,as shown in Figure 3.13.7(c).
Figure 3.13.7 Performance of telescope with not adjusted well optical axis and corresponding adjustment principle
Representation for the axis of the telescope and stage non-adjusted well and the adjustment method.
If the reflection image from one mirror is up and that from another mirror is down,we should firstly adjust the screw of the stage to make the reflected image points of the two mirror surfaces equal in height(but the image points do not fall on the upper cross line),and then adjust the image to the upper cross line,as shown in Figure 3.13.7(c).
4)Adjusting collimator
The emitted parallel light is perpe ndicular to the axis of the instrument,and then adjust the illuminated slit to the focal plane of the collimator objective lens to obtain the parallel light emitted from the objective lens.
Figure 3.13.8 The optical axis of collimator is coaxial with that of telescope
The adjustment method is as follows:remove the lighting source of the mirror and the eyepiece,and align the slit with the front mercury lamp light source.Turn the telescope to the direction of the collimator to observe the slit image through the eyepiece.Move continuously the slit tube along the axial direction until the image is clear,which indicates that the light tube has emitted parallel light.(Why?)Then turn the slit to transverse direction and adjust the screw(25 in Figure 3.13.1)to make the image coincide with the central transverse line,as shown in Figure 3.13.8(a).This suggests that the optical axis of the collimator is collinear with the optical axis of the telescope,so it is also perpendicular to the main axis of the instrument.At present,the screw(25)can no longer be moved.(Why?)Adjust the slit to be vertical and lock the screw,as shown in Figure 3.13.8(b).
5)Determining the refractive index of prism by the minimum deflection angle method
As shown in Figure 3.13.9,a beam of monochromatic light incident on the AB plane at i1 angle,and is refracted from the AC plane at an exit angle i′2 after it is refracted twice by the prism.The angle between the incident light and the emergent light is called the deflection angle.When the vertex angle A of the prism is fixed,the deflection angle δ changes with the incident angle i1.When the incident angle i1 is equal to the exit angle i′2,δ is the minimum(please refer to the relevant contents in the optical textbook).At this time,the deflection angle is called the minimum deflection angle,which is recorded as δmin.
Figure 3.13.9 The schematic diagram of the minimum deflection angle of prism
As displayed in Figure 3.13.9,when
we can get the formula:
Thus,we can get:
Assuming the refractive index of the prism as n,according to the law of refraction,we can get the formula:
Therefore,the vertex angle A and the minimum deflection angle δmin must be measured to obtain the refractive index of the prism.
