Diffraction grating is an optical element that uses the principle of multi-slit diffraction to disperse light waves. It consists of a large number of slits or notches that are parallel to each other, equal in width and equal in spacing. Grating is widely used in various spectral instruments because of its large dispersion and high resolution. Modern high-tech technology can produce gratings with tens of thousands of slits per cm, which is not only suitable for analyzing visible light components, but also suitable for infrared and ultraviolet bands. There are plane grating and concave grating in structure, and gratings are divided into transmission type and reflection type. The grating used in this experiment is a transmission grating.

The diffraction and scattering of light in the process of propagation, as well as the reflection and refraction of light are all related to the angle. Some optical quantities such as refractive index, wavelength and the maximum and minimum positions of diffraction fringes can be determined by measuring the relevant angles. In optical technology, it is of great significance to accurately measure the deflection angle of light.

Spectrometer is a typical precision angle measuring optical instrument, which is often used to measure wavelength, refractive index, dispersion rate, observation spectrum and so on. Because the device is precise and the operation and control parts are numerous and complicated, it must be used in strict accordance with the requirements. Beginners, in particular, often find it difficult. However, as long as the adjustment requirements are clearly defined, the phenomena are observed, and the existing theoretical knowledge is used to analyze and guide the operation, it is generally easy to master. The adjustment idea, method and skill of spectrometer are representative in optical instruments. Learning how to adjust and use is helpful to master the complicated optical instruments.

The main instruments used in this experiment are spectrometer and measuring microscope, which measure grating diffraction angle and grating constant respectively. The key training methods and skills are: (1) adjustment of spectrometer, including adjustment and focusing of telescope eyepiece, adjustment of collimator, etc. (2) The principle and reading method of spectrometer angle vernier. (3) Adjustment and use of measuring microscope.

This is a basic physical optics experiment. In the experiment, we should pay attention to the idea and method of adjusting the precision optical instrument from coarse to fine, the method of eliminating the eccentricity difference of spectrometer, the method of eliminating parallax and the method of eliminating nodal difference. It is difficult to use and adjust the spectrometer, which is also the focus and difficulty of this experiment. Data processing is not difficult, and it is suitable for many students majoring in physics, machinery, computer and automation. The difficulty coefficient is: 1. 10.

Specific contents and requirements of the experiment

1, adjustment of spectrometer

Before adjustment, you should be familiar with the positions of the following screws in the spectrometer:

(1) Eyepiece focusing handwheel (see the reticle clearly);

(2) the handwheel (or screw) of the focusing telescope (see the object clearly);

(3) Adjust the horizontal screw of the telescope;

(4) screws (together with dial) for controlling the rotation of the telescope;

(5) Three screws for adjusting the horizontal state of the stage;

(6) Brake screw for controlling the rotation of the stage;

(7) Screw for adjusting the slit width of collimator;

(8) screw for adjusting the level of collimator;

(9) Slit sleeve brake screw for collimator focusing.

The adjustment content and process of spectrometer are roughly as follows:

(1) Visual coarse adjustment: coarsely adjust the telescope, stage and collimator to be horizontal and perpendicular to the central axis.

(2) Adjust the telescope to focus at infinity by the self-collimation method.

Adjust the eyepiece focusing handwheel until the reticle on the crosshair is clearly visible. Put the small flat mirror on the stage and gently rotate the stage, or gently adjust the level of the stage and the telescope, and observe the reflected green cross image (or blurred image spot) from the telescope. Adjust the position of the telescope eyepiece sleeve to make the cross image clear. Pay attention to eliminating parallax.

(3) Adjust the optical axis of the telescope to be perpendicular to the central axis of the spectrometer.

First, coarse adjustment, that is, first adjust the horizontal adjustment screws of the telescope and the stage by visual inspection, slowly rotate the stage, and then observe the cross images reflected on both sides of the small flat mirror from the telescope, and then fine-tune the level of the telescope and the stage to make the cross image coincide with the crossed line in the field of view of the telescope.

(4) Adjust the collimator.

Adjust the collimator with the adjusted telescope. Adjust the position of the slit device to make the slit image seen from the telescope clearest, and adjust the slit width to make the image width about1mm. Adjust the optical axis of the collimator to be perpendicular to the central axis of the spectrometer.

2. Measurement of diffraction angle

(1) Placement of grating: Place the grating on the stage and adjust it to be perpendicular to the collimator to meet the condition of direct parallel light.

(2) Align the diffraction fringes and use two angle cursors for accurate reading.

3. Measurement of grating constant

Correct use of reading microscope and cumulative amplification method are used to measure multiple grating constants at one time to reduce the measurement error.

4. Calculate the wavelength of sodium yellow light, compare it with the standard value, and calculate the relative percentage error.

Brief introduction of experimental instruments

1, the spectrometer is used to measure the diffraction angles of the diffraction lines of gratings at all levels. Spectrometer is a precise instrument for measuring angles, which is mainly composed of tripod base, collimator, stage, telescope, dial and vernier disk.

2. Measuring microscope for measuring grating constant.

3. Diffraction grating.

Preview basic requirements

1, understand the structure and adjustment of spectrometer.

2. Understand the formation principle of light diffraction fringes and the significance of grating equation.

3. Understand the use method of measuring microscope and the measurement method of grating constant.

4. Understand the method and process of measuring diffraction angle with spectrometer and the method of calculating the wavelength of sodium yellow light with diffraction equation. Frequently asked questions and answers

1. Spectrometer has complex structure and many adjusting parts. Don't be too anxious during the experiment. Follow the textbook and the teacher's explanation step by step, and you will soon master the adjustment and use methods.

2. When adjusting the focal length and level of the telescope, it often happens that the reflected cross image cannot be found. At this time, we should adopt the method of coarse adjustment first, then fine adjustment, that is to say, first make the telescope basically horizontal and the small plane mirror basically perpendicular to the telescope through visual adjustment, and then look for the cross image reflected from the telescope.

3. Through adjustment, the cross images reflected from both sides of the facet mirror coincide with the upper cross position in the field of view of the telescope.

There are many screws on the spectrometer. Generally, the vernier disk is fixed and the dial is connected to the telescope. When measuring the diffraction angle quantitatively, it is more convenient to fix the dial and use the fine-tuning screw.

5. When measuring the grating constant, make the grating notch perpendicular to the moving direction of the stage. When using spiral micrometer mechanism, pay attention to eliminate pitch difference.

Preview thinking problems

1. What is the main part of the spectrometer? Why is the adjustment of telescope the basis and key of spectrometer adjustment?

2. What state should the telescope of spectrometer be set to?

3. What state should the grating be adjusted to on the stage?

4. How to find and eliminate parallax when adjusting the telescope?

5. How to eliminate eccentricity in spectrometer design?

Matters needing attention in the experiment

1. Spectrometer is a precision instrument with many adjusting screws. Please don't touch these screws until you know their functions and usage, so as not to damage the spectrometer.

2. Grating is a precise optical element. It is forbidden to touch the notch by hand to avoid damage.

3. When measuring the diffraction angle, (1) it is better to fix the telescope, and it is more convenient to use fine-tuning rotation. (2) Measure the fringe positions of +3, +2,+1 and-1, -2 and -3 from left to right (or from right to left), and record the readings of the left and right cursors respectively.

4. When measuring grating constant with measuring microscope, pay attention to eliminate the influence of pitch difference.