University physics experiment report
Experiment name Determination of refractive index of liquid by Abbe refractometer
- Understand the principle of the Abbe refractometer and learn how to adjust and use the Abbe refractometer.
- Master the method of measuring the refractive index of a substance using an Abbe refractometer.
- Determine the concentration of the glucose solution by measuring its refractive index.
Honey refractometer, liquid to be tested, glucose solution (several different concentrations), absolute alcohol, distilled water, lens paper, dropper (three).
If the object to be tested is a transparent concentrate, the refractive index nx is generally measured by a transmitted light, that is, a grazing incidence method.
The Abbe prism group in the Abbe refractometer consists of two right-angle prisms (refractive string rate n), one is a light-increasing prism, and its chord surface is frosted, which functions to form a uniform extended surface light source. The other is a refractive prism. The liquid to be tested (nx<n) is sandwiched between the chord faces of the two prisms to form a film.
As shown in Fig. 1, the light is first incident on the entrance prism, and the matte chords A, B are generated to generate diffused light through the liquid layer into the refracting prism (ABC in the figure). Therefore, the light rays (e.g., 1, 2, 3, etc.) reaching any point E on the contact surface (AB surface) of the liquid and the refractive prism have various incident angles, and the maximum incident angle is 90 degrees. Incidents in this direction are called grazing incidence. For a certain light in the incident light in different directions, it is directed to the AB surface at the incident angle i, and after being refracted twice by the prism, it is emitted from the AC surface at an angle. If, then, the law of refraction is obtained.
Where α is the angle of refraction on the AB plane and β is the angle of incidence on the AC plane. The relationship between the apex angle A of the prism and the angle α and angle β is
It can be seen from Fig. 1 that for the light “1”, there are i → 90°, sini → 1, →, then the above formula becomes
Therefore, if the refractive index n and the refractive apex angle 95 of the refractive prism are known, the refractive index of the liquid to be tested can be obtained by measuring the exit angle.
If, at this time, the exiting ray and the apex angle A are on the same side of the AC surface normal, the upper formula becomes
How does the Abbe refractometer measure the exit angle corresponding to the light “1”? As can be seen from Fig. 1, except for the light “1”, the incident angles of the other light rays “2”, “3”, etc. on the AB plane are less than 90 degrees. Therefore, when the light of the extended light source is emitted from the various directions to the AB surface, the light having an incident angle of less than 90 degrees, the exit angle of the prism refracted by the prism must be larger than the angle and deflected to the left side of the “1′” to form a bright field of view. Another measure of “1” forms a dark field without light. Obviously, the dividing line of the light and dark field of view is the outgoing direction of the grazing incident beam “1” (“1).
The Abbe refracting mirror indicates the refractive index value corresponding to the angle. When the brightness boundary line is aligned with the telescope crosshair, the nx value can be read from the refractive index scale in the field of view.
The outer structure of the Abbe refractometer is shown in Figure 1.
First, the calibration instrument
The instrument is calibrated before measurement. It can be calibrated with distilled water (“n” _”D” ^”20″ =1.3330) or standard glass block (standard glass block marked with refractive index)
- Calibration with distilled water
(1) Loosen the prism locking wrench 12 and wipe the prism clean (Note: dry with lens paper with absolute alcohol or other volatile solvent).
(2) Use a dropper to drop 2 to 3 drops of distilled water into the middle of the two prisms, close and lock.
(3) Adjust the prism to turn the handwheel 2 so that the refractive index reading is just 1.3330.
(4) Observe whether the black and white boundary line coincides with the intersection of the crosshairs from the measuring barrel. If it is not coincident, adjust the scale adjusting screw 9 so that the intersection of the crosshairs accurately coincides with the boundary line. If the field of view shows dispersion, the Amiss prism handwheel 10 can be adjusted to disappear.
- Calibration with standard glass block
(1) Loosen the prism locking wrench and pull the light-emitting prism apart.
(2) Drop brominated naphthalene (high refractive index liquid) on the polished bottom surface of the glass block and attach it to the DE surface of the refractive prism. The polished side of the glass block should be upward to receive the light, so that the measuring tube field of view Bright.
(3) Adjust handwheel 2 so that the refractive index reading is exactly the refractive index value known for standard glass blocks.
(4) Observing from the measuring barrel, if the dividing line does not coincide with the intersection of the cross wires, the adjusting screws 9 make them coincide. If there is dispersion, the hand wheel 10 is adjusted to eliminate the dispersion.
Second, determine the refractive index of a liquid
- Wipe the entrance prism and the refractive prism clean.
- Drop 2 to 3 drops of the liquid to be tested on the frosted surface of the light-increasing prism and lock it. (If the solution is volatile, replenish it in a small hole on the side of the prism group)
- Rotate the handwheel 2. In the measuring tube, the black and white boundary line will be observed to move up and down (if there is color, turn the handwheel 10 to eliminate the dispersion, so that the dividing line is black and white), to the black and white boundary line and the crosshair in the field of view. The intersection points coincide.
- In the reading barrel, read the data indicated by the horizontal line on the right side of the reticle, that is, the refractive index n of the liquid to be tested, and record the well.
- Repeat the measurement three times to find the average value of the refractive index.
- Record room temperature.
Note: If you need to measure the refractive index of the liquid at different temperatures, screw the thermometer into the socket, connect the thermostat, and adjust to the desired temperature. After stabilization, measure according to the above steps.
- Measure the refractive index n and concentration c of the glucose solution and make an n-c curve.
The experimental procedure is the same as “two”, and the glucose solution of different concentrations is replaced, and the corresponding n and c (values of 8 to 10 groups) are measured, and then c is the abscissa and n is the ordinate, and the glucose solution is made on the coordinate paper. -c relationship curve.
[Data Recording and Processing]
- Determine the refractive index of a liquid. Graduation value: _ _ Experimental temperature: t=_ _°C
- Measure the refractive index and concentration of the glucose solution for the n-c curve.
[original experimental data paste]
Can I use an Abbe refractometer to measure a liquid with a refractive index greater than the refractive index of the refractive prism? why?
No, because the Abbe refractometer is designed based on the critical angle. If the condition of the refractive index is changed, that is, the prism is regarded as a light-dissipating medium, the bright area and the dark area cannot be formed, and only the bright area can be seen.
Why do you need to drop a drop of high refractive index liquid when calibrating with a standard glass block?
Because the refractive index of glass is larger than that of air, a drop of high refractive index liquid, the light that is transmitted from the glass will have a relatively small angle of refraction into the liquid, and the surface of the liquid is spherical, and the light from the liquid does not. Refraction angles are generated again, and the human reaction is light entering the eyeball, which reduces the angle of refraction due to the large difference in refractive index between air and glass, so the visual error is reduced. (The refractive index liquid is mainly used in this experiment to calibrate the Abbe refractometer, and its refractive index must be smaller than the refractive index of the prism, so the refractive index of the liquid is higher.)
3. When measuring the refractive index and temperature experimental curve of distilled water, what happens if the water is evaporated? why?
Because the refractive index of water is greater than the refractive index of air, it is found that the boundary between light and dark will move to the bright area.