CN110927108B

CN110927108B - Method for measuring refractive index of material by irradiating edges and bottom surfaces of double prisms

Info

Publication number: CN110927108B
Application number: CN201911211871.7A
Authority: CN
Inventors: 胡再国
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2019-12-02
Filing date: 2019-12-02
Publication date: 2021-07-20
Anticipated expiration: 2039-12-02
Also published as: CN110927108A

Abstract

A method for measuring the refractive index of the material by irradiating the edges and the bottom surface of the biprism; the laser shell is a cylinder, and the laser can rotate around the central axis; the emergent light of the two lasers is on the same straight line; each scale is provided with a square hole, so that an emergent light spot of the laser is changed from a round light spot into a square; the double prisms are positioned in the middle of the two lasers; the right laser irradiates the edge of the double prism, and the distance between the nearest ends of the two reflection light spots is S1; closing the right laser, opening the left laser, rotating the left laser, and moving the biprism backwards to enable the laser of the left laser to be perpendicular to the bottom surface of the biprism and irradiate the bottom surface corresponding to the prism surface of the front end, wherein the distance between the rear end of the refracted light spot and the rear end of the square hole is S2, namely the distance between the square hole and the same side of the refracted light spot is S2; the refractive index of the material corresponding to the laser wavelength on the left side is n =2S 2/S1. The measuring principle is clear, the measuring method is easy to understand, and the calculation is simple.

Description

Method for measuring refractive index of material by irradiating edges and bottom surfaces of double prisms

Technical Field

The present invention relates to the measurement of refractive index, in particular the measurement of refractive index of a double prism or a small angle wedge.

Background

The measurement of the refractive index is mainly based on the law of refraction, and the measurement is usually carried out by adopting total reflection; the used tools usually adopt a spectrometer or an Abbe refractometer, and the adjustment of the spectrometer and the Abbe refractive index is complicated; meanwhile, the Abbe refractometer has certain requirements on the shape, size and thickness of materials, and the measurement cannot be carried out on materials with certain shapes. Therefore, it is necessary to provide a suitable measuring method depending on the specific shape of the transparent material.

Disclosure of Invention

The invention mainly provides a method for measuring the refractive index of a small-angle wedge such as a biprism.

The invention adopts the technical scheme that the purpose of the invention is realized by: a method for measuring the refractive index of a material by irradiating edges and bottom surfaces of a biprism comprises two lasers and the biprism, wherein the biprism comprises two isosceles triangular surfaces and three rectangular surfaces, the largest rectangular surface is the bottom surface, the other two surfaces are edge surfaces, the edge intersected by the two edge surfaces is an edge, namely the included angles of the edge surfaces and the bottom surface are alpha; alpha is 0.5-1^oThe optical element is placed on the optical bench through the support, the support can slide on the guide rail of the optical bench and is fixed by the screw, the support shaft of the optical element can move in the front-back direction through the sliding groove nested on the upper surface of the support, so the thickness of the biprism can be ignored relative to the length of the optical bench, and the optical bench is characterized in that: the laser shell is a cylinder, and the laser can rotate around the central axis; the emergent light of the two lasers is in oneOn a straight line; a graduated scale is fixed on an emergent hole of each laser, and the graduated scale is provided with a square hole so that an emergent light spot of each laser is changed from a circular light spot into a square light spot; one end of the square hole is the zero point of the graduated scale, namely the lower side of the square hole is parallel to the lower side and the upper side of the graduated scale (because the graduation line of the graduated scale is vertical to the edges of the upper side and the lower side); the biprism is positioned in the middle of the two lasers, namely the distance from the biprism to the left laser is equal to the distance from the biprism to the right laser, and the distance is assumed to be L; the left laser is incident perpendicular to the bottom surface, namely the right laser is irradiated to the edge or the edge surface perpendicular to the bottom surface of the double prism; closing the left laser, opening the right laser, moving the double prism back and forth to enable the right laser to irradiate edges of the double prism, rotating the right laser to enable the distances from the lower ends of the two reflection light spots and the lower end of the square hole to the edge of the graduated scale to be equal, enabling the front end and the rear end of the front reflection light spot and the front end and the rear end of the rear reflection light spot to be parallel to the graduated line, enabling the distance between the rear end of the front reflection light spot and the front end of the rear reflection light spot to be S1, and enabling the distance between the nearest ends of the two reflection light spots to be S1; closing the right laser, opening the left laser, rotating the left laser to enable the front ends and the rear ends of two refraction light spots reflected and refracted from the two sides of the edge to be parallel to the scale marks, then moving the double prisms backwards to enable the laser of the left laser to be perpendicular to the bottom surfaces of the double prisms to irradiate the bottom surfaces corresponding to the edge surfaces of the front ends, enabling the light paths to be unchanged, irradiate the edge surfaces, reflect to the bottom surfaces, refract out the bottom surfaces to form refraction light spots, and enabling the distance between the rear ends of the refraction light spots and the rear ends of the square holes to be S2, namely the distance between the square holes and the same side of the refraction light spots to be S2; the refractive index of the material corresponding to the laser wavelength on the left side is n =2S 2/S1.

The invention has the beneficial effects that: according to the existing optical bench and the common laser; the existing laser is properly modified, a graduated scale is added, and the light spot of the laser is modified (a square or rectangular diaphragm is added, namely a hole of the graduated scale) for convenient positioning and observation; the measuring principle is clear, the measuring method is easy to understand, and the calculation is simple. The teaching aid is beneficial to the understanding of people with weak theoretical basis such as middle school students and the like, and is suitable for being used as a demonstration teaching aid. The main point of the invention is that the test skills of coaxial equal height adjustment, horizontal adjustment and the like are not the key points of the invention, and the optical element needing fine adjustment can be adjusted back and forth and left and right by arranging a support frame on the top of a support shaft of a support (the up-down adjustment can be realized by up-down adjustment of the support shaft instead).

Drawings

FIG. 1 is a schematic diagram of a double prism (where the dashed part of the label 3 indicates occlusion; because in this figure the bottom surface is occluded); FIG. 2 is a schematic of a laser exit aperture (dashed circle indicates exit aperture); FIG. 3 is a schematic diagram of the relationship between the scale and the exit aperture (the scale is opaque and has a square or rectangular hole); FIG. 4 is a schematic view of a spot striking a rib to produce a reflection (enlarged for ease of labeling); FIG. 5 is a schematic view (enlarged) of light rays perpendicularly illuminating the bottom surface, transmitted to the prism surface, reflected and refracted out of the bottom surface; FIG. 6 is a schematic view of a light ray that is transmitted through a prism surface and reflected back out of the bottom surface when it is illuminated perpendicular to the bottom surface (a true light path diagram, i.e., the difference between the incident light spot on the bottom surface and the exit point of the refracted light on the bottom surface can be ignored); FIG. 7 is a schematic view of reflection of a light spot on an edge (a schematic view of a real light path); FIG. 8 is a schematic diagram showing two lasers emitting light in a collinear manner; FIG. 9 is a schematic diagram of a double prism skew insertion (initial state, to facilitate identification of reflected and refracted light spots, with refracted light away from incident light relative to reflected light); FIG. 10 is a schematic view of a left side laser impinging perpendicularly on the base of a biprism (i.e., the right side laser impinging on the edges and facets of the biprism in a direction perpendicular to the base of the biprism); fig. 4-10 are top views.

Wherein, 1, a laser; 2. a double prism; 3. a bottom surface; 4. a prism surface; 5. a graduated scale; 6. a hole; 7. an exit aperture; 8. a ridge; 9. a side surface.

Detailed Description

Remarking: FIGS. 4-10 belong to the top views; the up-down relationship in fig. 4-10 is a front-back relationship in the horizontal plane, so the up-down direction adjustment of fig. 4-10 in the description is the same step as the front-back adjustment in the horizontal direction.

The laser 1 is used as a common optical element, a support of the laser is fixed on an optical bench, a hole is formed in the middle (center) of the support and can be used for inserting a support shaft, and the support shaft can be fixed through a fastening screw; the upper end of the supporting shaft is fixedly connected with a flat plate (called a fixing plate), the rear part of the fixing plate is connected with another flat plate (called a suspended plate) through three springs, the centers of the two flat plates are provided with a hole, a cylinder of the laser is positioned in the two holes (the diameter of the hole of the fixing plate is larger than that of the hole of the suspended plate), the laser is fixed in the hole of the suspended plate (can be fastened through screws or friction force, most of the existing lasers are fastened through friction force, and parts of the existing lasers are fixed through bonding.

The laser is perpendicular to the bottom surface 3 of the biprism 2 and is irradiated on the edge 8:

step 1, adjusting the outgoing light of the two lasers to be collinear, as shown in fig. 8, moving the two lasers together on an optical bench, adjusting the supporting shafts of the two lasers (rotating to make the fixing plates of the lasers approximately perpendicular to the length direction of the optical bench and adjusting the lifting to make the outgoing holes of the two lasers have the same height), making the outgoing holes 9 of the two lasers have the same height (which is a common optical element height adjustment), then fastening screws of a laser supporting shaft are fastened, then supports of the two lasers are moved to enable the two lasers to be respectively positioned at two ends of the optical bench, the supports are fixed on the optical bench through the fastening screws for fixing the supports (namely, the two lasers are fixed), the right laser is closed, the left laser is opened, the front and back and pitching adjusting screws of the left laser are adjusted (which belongs to fine adjustment), and light rays emitted by the left laser are made to irradiate an emergent hole of the right laser; similarly, the left laser is closed, the right laser is opened, and the front-back adjusting screw and the pitching adjusting screw of the right laser are adjusted, so that the light emitted by the right laser irradiates the emergent hole of the left laser, and the light of the two lasers is transmitted in the same straight line;

step 2: the biprism is obliquely inserted into the light path, as shown in fig. 9, the biprism is obviously obliquely inserted to generate backward reflected light and refracted light, the refracted light tends to the back, namely, the backmost light is refracted light, the biprism is rotated clockwise to reflect the reflected light to the exit hole of the left laser, namely, the left light is vertically incident to the bottom surface of the biprism, since the laser of the right laser and the laser of the left laser are collinear, the right laser irradiates the edge or the edge surface of the biprism in the direction perpendicular to the bottom surface of the biprism, at this time, the fastening screw of the support shaft of the biprism and the fastening screw of the biprism support are fastened (at this time, a device capable of moving in the front-back direction is generally nested on the upper surface of the support, the biprism is driven to move in the front-back direction, and the support capable of driving the support shaft to move back and forth is a prior art);

and 3, step 3:

adjusting the front-back direction of the biprism (part of the support has this function), as shown in fig. 10 (fig. 10 is a top view, in fig. 10, the up-down adjustment is performed, and the front-back adjustment is performed in the horizontal plane), the left laser is irradiated to the back surface of the front prism surface of the biprism (the back surface of the upper prism surface in the top view of fig. 10), at this time, the left laser is turned off, and the right laser is turned on, so that the right laser is irradiated to the prism surface or edge of the biprism perpendicularly to the bottom surface.

The scale 5 has a graduation line parallel to the edge 8 of the biprism, and can be made by a manufacturer, or the scale 5 can rotate (preferably, the laser is cylindrical and can rotate to irradiate the reflection light spot and the refraction light spot on the scale, and ensure that the lower end edge line of the reflection light spot and the lower end edge line of the square hole are collinear (i.e., the lower end edge line of the reflection light spot and the lower end edge line of the square hole are equidistant from the edge of the scale), and similarly, the lower edge of the refraction light spot and the lower edge of the square hole are collinear.

The distance between the nearest ends of the two reflected spots is S1, i.e., the distance between the rear end of the front reflected spot and the front end of the rear reflected spot S1. The distance between the refracted light spot and the same side of the exit aperture is S2, for example, the distance between the front end (rear end) of the refracted light spot and the front end (rear end) of the exit aperture is measured as S2.

Because the refractive index is related to the material and the wavelength of light, monochromatic light is adopted for measuring the refractive index, parallel light is generally adopted for observation, the divergence angle of laser is small, and the laser is approximately regarded as the parallel light; the wavelength of the laser is relatively stable, and the laser is relatively ideal monochromatic light.

Wherein, the laser irradiates on the edge 8 of the biprism 2, there are two reflection light spots and two refraction light spots, the reflected light irradiating on the edge surface 4 of one side in front is on the same side with the edge surface 4; the refracted light is on the other side of the prism surface 4 (i.e., the rear end prism surface side), one light screen is closed from the outside of the prism surface 4 to the prism 8, and of the two light spots which gradually disappear, the light spot on the prism surface 4 side is a reflected light spot, and the light spot on the other prism surface 4 side is a refracted light spot.

Example (b):

a method for measuring the refractive index of a material by irradiating edges and bottom surfaces of a double prism comprises two lasers 1 and a double prism 2, wherein the double prism 2 comprises two isosceles triangular surfaces and three rectangular surfaces, the largest rectangular surface is a bottom surface 3, the other two surfaces are edge surfaces 4, the edge intersected with the two edge surfaces 4 is an edge 8, the isosceles triangular surfaces are called side surfaces 9 (the two isosceles triangular surfaces are parallel to each other and perpendicular to the three rectangular surfaces), the included angles of two waists and the bottom edge of the isosceles triangle are both alpha, namely the included angle of the edge surfaces 4 and the bottom surface 3 is alpha; alpha is 0.5-1^oTherefore, the sine value, the tangent value and the angle arc value are approximately equal, the length of the double prism 2 is 40-60mm, the thickness of the double prism is 0.3-1mm, the length of the optical bench is generally more than 1000mm, and therefore the thickness of the double prism 2 can be ignored, and the double prism is characterized in that: the shell of the laser 1 is a cylinder, and the laser 1 can rotate around the central axis; the emergent light of the two lasers 1 is on the same straight line; a graduated scale 5 is fixed at the exit hole of each laser 1, the graduated scale 5 is provided with a square hole 6 which is aligned with the exit hole of the laser 1, the light spot of the laser is 2.5-3mm, the side length of the square hole 6 (diaphragm) is 1.5-2mm, and the square hole 6 is positionedThe scale covers the laser spot, namely the emergent spot of the laser is changed into a square from a round spot; one end of the square hole 6 is the zero point of the graduated scale 5; the biprism 2 is located in the middle of the two lasers 1, that is, the distance from the biprism 2 to the left laser 1 is equal to the distance from the biprism 2 to the right laser 1, and is assumed to be L; the laser is perpendicular to the bottom surface 3 of the biprism 2 and irradiates the edge 8; turning off the left laser, turning on the right laser, irradiating the right laser to the edge 8 of the biprism, rotating the right laser to make the centers of the two reflected light spots and the center of the square hole be in a straight line (in fig. 7, fig. 7 is a top view, that is, the triangular surface of the biprism is horizontally placed, and the light spots are vertically symmetrical, that is, the light spots are equal in height in the vertical direction, so that the lower ends of the two reflected light spots and the lower end of the square hole are in a straight line, and in the same way, the upper ends of the two reflected light spots and the upper end of the square hole are also in a straight line, theoretically, the biprism needs to have a pitching adjustment function, that is, the clamping device of the biprism has a pitching adjustment, which can be achieved because the design of the invention also has certain degree of approximation, and any measurement has a certain error, so that the edge surface of the biprism is parallel to the support shaft, therefore, many optical elements in the laboratory now omit the pitch adjustment function, and rely on the plane where the outline of the optical element is parallel to the support shaft to make up for to some extent), the distances from the lower ends of the two reflected light spots and the lower end of the square hole to the edge of the scale are equal (generally estimated by an observer, or a straight line is drawn on the scale by a manufacturer to pass through the edge of the lower end of the square hole, and the straight line is used as a reference line; this is the expression of incident light, reflected light, normal in a plane), the distance between the rear end of the front reflected light spot and the front end of the rear reflected light spot is S1, i.e. the distance between the nearest ends of the two reflected light spots is S1, and the angle between the connecting line of the two ends of S1 and the edge is 4 α; turning off the right laser, turning on the left laser, moving the biprism backward to make the laser of the left laser 1 perpendicular to the bottom 3 of the biprism 2 irradiate the bottom 3 corresponding to the front edge surface 4, so that the light does not change the light path and irradiates the edge surface 4, reflects to the bottom 3, refracts out of the bottom to form a refraction light spot, and rotatingThe left laser has the lower end of the refracted light spot and the lower edge of the square hole 6 in a straight line (generally estimated by an observer, or a straight line drawn by a manufacturer on a scale and passing through the lower edge of the square hole is taken as a reference line), and the distance between the rear end of the refracted light spot and the rear end of the square hole 6 is S2, that is, the distance between the square hole 6 and the same side of the refracted light spot is S2; an included angle between a connecting line of two ends of S2 and an incident point is a refraction angle β, and an incident angle corresponding to the refraction angle β is 2 α, so that the refraction index is n = sin (β)/sin (2 α) = tg (β)/tg (2 α) = (S2/L)/(S1/2/L) =2S 2/S1.

Claims

1. The method for measuring the refractive index of the material by irradiating the edges and the bottom surface of the double prism comprises two lasers (1) and the double prism (2), wherein the double prism (2) comprises two isosceles triangular surfaces and three rectangular surfaces, the largest rectangular surface is the bottom surface (3), the other two surfaces are edge surfaces (4), the edge intersected by the two edge surfaces (4) is an edge (8), namely the included angles of the edge surfaces (4) and the bottom surface (3) are alpha; alpha is 0.5-1^oSo that the thickness of the biprism (2) is negligible with respect to the length of the optical bench, the edges (8) of the biprism (2) being parallel to the central axis of the support shaft of the biprism support, characterized in that: the shell of the laser (1) is a cylinder, and the laser (1) can rotate around the central axis; the emergent light of the two lasers (1) is on the same straight line; a graduated scale (5) is fixed at an emergent hole of each laser (1), and the graduated scale (5) is provided with a square hole (6) so that an emergent light spot of each laser is changed from a round light spot into a square light spot; one end of the square hole (6) is the zero point of the graduated scale (5), namely the lower side of the square hole is parallel to the lower side and the upper side of the graduated scale; the double prism (2) is positioned in the middle of the two lasers (1), namely the distance from the double prism (2) to the left laser (1) is equal to the distance from the double prism (2) to the right laser (1); the left laser is incident perpendicular to the bottom surface, namely the right laser is irradiated to the edge (8) or the edge surface (4) perpendicular to the bottom surface (3) of the double prism (2); turning off the left laser, turning on the right laser, moving the biprism back and forth to cause the right laser to strike the prism (8) of the biprism, rotating the right laser to cause the front and rear ends of the front and rear reflected spots to be parallel to the graduation marksThe distance between the rear end of the front reflection light spot and the front end of the rear reflection light spot is S1, namely the distance between the nearest ends of the two reflection light spots is S1; closing the right laser, opening the left laser, rotating the left laser to enable the front ends and the rear ends of two refraction light spots reflected from the two sides of the prism and refracted back to be parallel to the scale marks, then moving the double prism backwards to enable the laser of the left laser (1) to be perpendicular to the bottom surface (3) of the double prism (2) to irradiate the bottom surface (3) corresponding to the prism surface (4) at the front end, enabling the light path to be unchanged to irradiate the prism surface (4), be reflected to the bottom surface (3) and be refracted out of the bottom surface to form refraction light spots, enabling the distance between the rear end of the refraction light spots and the rear end of the square hole (6) to be S2, namely enabling the distance between the square hole (6) and the same side of the refraction light spots to be S2; the refractive index of the material corresponding to the laser wavelength on the left side is n =2S 2/S1.