Wave optics demonstration experiment

Oleg A. Povalyayev, candidate of technical science, phone 2746230,snark@corbina.ru,
Sergey V. Homenko, candidate of physical and mathematical science, phone 2746230, snark@corbina.ru,"Rosuchpribor", Moscow

Teachers apply special methods to determine the efficiency of educational process. The most efficient method is demonstrational experiment. A student should apprehend the experiment easily; it should be short, easy to perform and provide with good subject learning. The wave optics experiment requires producing rather sharp and bright lines of diffraction or interference, a spectrum or image drawn by polarized light on screen. The students should see all the optical elements which meet the light. This work was done because of need to demonstrate wave optics experiment. The equipment utilized is quite cheap and easy to use [1]. Students pay much unworthy attention to complex equipment. As a result, they do not understand the phenomena observed at experiment. Usage of more simple equipment results in advantages such as introducing new educational units based on demonstration physical experiment. Natural light is acceptable when the phenomena of spectrum expansion, polarization, diffraction and interference are studied. Projection apparatus meets these requirements. Such projection apparatus demonstrates the experiments which are hard to watch directly. LASER is required for staging the experiments on interference and diffraction. Semiconductor laser emits radiation at wave length of 670 nm. The wave optics package includes this laser. AC electric power network feeds the laser via adapter. The laser is mounted at magnet holder or frame. Optical elements of package are made up in the same style. Conceptually, the element consists of thin frame plastic cylinder with the optical unit put inside (e.g. lens, gitter etc.). The cylinder is closed at edge by metal ring. Small-sized optical units have outer diameter of 35 mm meanwhile big-sized ones have outer diameter of 70 mm. The holder fixes optical units which are installed into frames or fixed in a tripod. They also might be fixed in a magnet holder standing at metal surface. The frames are shaped as squares with side length of 11 cm. The frames are made of dark plastic. The package includes: 1) two frames for lenses and polaroids; 2) a frame of "Newton rings"; 3) a frame for small optical units. The "Flint" glass prism is mounted at its own stand. The stand is conceptually plastic cylinder having 23 mm height and 42 mm diameter. The magnets are pressed into the stand's foundation. Glass plate and flat mirror contain steel plates being stuck. That allows fixing them at magnet holder. Magnet holder is the 45 mm high stand which is mounted to a foundation (70 mm x 35 mm). The foundation and the stand are made of plastic. The foundation contains magnets necessary to fix it at metal surface. The stand contains magnets necessary to fix optical units. The magnets provide with precise installing and reconfiguring units. Thus, the optical system is adjusted. A special optical table is installed at projector's gate aperture. This table is utilized for optical units' fixation. Optical table is made of steel sheet. It has a round aperture of 100 mm diameter necessary to select bit core of projector's beam. he tripod stand can be fixed by threaded bush which is put into optical table. Metal sheet of 20 cm x 30 cm is utilized as an operating area for experimental purposes. The sheet is fixed at tripod stand in vertical plane. Optical units are fixed at operating area by magnet holders. The problem of units' multipurpose usage was solved at a stage of designing. The "Wave optics" package is easy to use. The teacher is able to construct a plant and explain properties of each unit simultaneously. The experiments might be sorted as listed below:

• dispersion and spectrum expansion of light;
• polarization;
• polarization;
• diffraction.

Dispersion and spectrum expansion of light

This series includes the phenomena of dispersion, white light synthesis by adding different spectral components and demonstration of impossibility to expand monochromatic light into spectrum. Several experiments on substance absorption of selective white light components are also considered. Fig. 1 illustrates the optical scheme of experimental device. Fig. 2 illustrates the device's view. The projector forms a beam of light as described below. Optical table and tripod stand are installed at projector's screening window. Operating area is fixed vertically at the stand. Optical table's gate aperture is closed by slit diaphragm. The slit must be perpendicular to vertically fixed operating area. Flat mirror is fixed near the top edge of operating area. Ray of light is directed to screen or a prism which is put at bottom part of operating area. The light passes the prism so that the light is directed horizontally. The second slit diaphragm is installed very closely to prism. Full lighting pf prism's plane is provided with correct configuration of prism and second slit diaphragm. That leads to maximal spectrum's brightness on demonstration screen. The demonstration screen is fixed vertically in (1-2) m distance from projection apparatus. Below are the descriptions of experiments which utilize the optical complex being modified. The experiment of spectrum colors supplement, when beams having different wavelengths are mixed, is done with lens utilization. Minimal spot's filling as achieved by lens's adjustment. Lack of colors in stain means that the eye comprehends them as white color. The experiment becomes more evident if the area of beam mixing is formed gradually. A sheet of white paper is bent at 45° due to light direction. Students may see the stain well. The sheet of paper is moved from lens to screen. The students may see that light is extended into spectrum just behind the lens but white stripe in the middle of sheet appears again when the lens approaches to screen. This stripe widens and spectrum disappears near the screen. The slit diaphragm lying on gate aperture is closed by red color filter to demonstrate the phenomena of monochromatic's light non-expansion to spectrum. A ray of light is directed to screen via prism. The students see initial color. Then mirror is turned so that ray passes the prism. The students make sure that spot's geometrical sizes have changed but its color remained the same. Since there are no new tints one has to conclude that monochromatic light can not be expanded to spectrum. The prism adds no new colors to light having initially only one color component. The same effect can be demonstrated with semiconductor laser utilization. The laser is installed to operating area instead of mirror. The red stain's shape and color at a screen do not depend on beam's way whether it has passed prism or not. The red color filter transmits only certain range of color's wavelength. The other waves having other wavelength are absorbed by filter's material. That explains why spectrum has red component only. Light absorption is demonstrated by putting a dish with KMnO4 solution onto a slit diaphragm. That solution absorbs light of green spectrum region. Dark sections appear in the appropriate spectrum region. The students may also be demonstrated the light absorption in any colored solutions, e.g. colored iodine water solution. The experiment becomes more evident when absorption spectrum and source's spectrum are compared. If the dish separates light to let it shine at the side then the spectrum consists of two appropriate components.

Polarization

Fig. 3 illustrates optical scheme of experiment on polarization. The experiment can not do without projector. Optical units are installed at tripod stand above the projector's gate aperture. The experiment on illumination variety submitted by polaroid spinning does not require any tuning. Polaroids both should stand close to each other. That is the only requirement. A high resolution polaroids' image screening of is possible in this case only. The labels of their orientation are seen quite well. Another experiment demonstrating tensions in substance meets the same requirements. The shapes on screen should be sharp. In this case only, the students may note and associate change of color screen and tensions. Optical system is designed to demonstrate polarization effects. A beaker filled with sugar solution is utilized to demonstrate plane-of-polarization rotation. Fig. 4 illustrates configuration of prisms and beaker.

Interference

Two coherent sources are created by beam's separation into two beams which originate from two different positions. Two symmetric images of emissive core are created by Fresnel's double prism. The light of imaginary source at flat mirror is added to source's light due to Lloyd's scheme. Two slits are expected to supply the light. They are lit by another source of light. The interference is observed in that area of screen which is lit by two sources both simultaneously. The schemes listed were developed due to ordinary light sources. The laser utilization leads to easier adjustment. The fringe pattern becomes visible from anywhere in class even if the room is not getting darkened. Laser is focused by short-focus lens. The focus point becomes emissive one. The width of interference bands is inverse proportional to distance between the light sources. The images made by these sources must be approached to improve visual comprehension of interference phenomena. It could be provided with approaching Fresnel's prism or Lloyd's mirror to laser beam focus area. The distance between slits at Young's experiment is predetermined. However, the slits should be positioned in focus area because the slits' luminous flux increases in result. After all, the pattern becomes brighter. Fig. 5 illustrates the optical system's view. Projector's optical table is installed to demonstration table. The tripod stand is fixed at optical table. The operating area is fixed to tripod stand as high above the demonstration table as possible. The screen stands in 2 m distance from demonstration table. Semiconductor laser and lens are fixed by magnets at operating area. Lens's parameters are 5 cm focus and 1.5 cm diameter. An object necessary for two sources creation is installed in focal area. Package "Newton's rings" is utilized for demonstrating interference effects in natural light. Newton's rings are demonstrated in transmitted and reflected both rays. Fig. 6 illustrates optical scheme of the experiment. Optical table and tripod stand are fixed at projector's gate aperture. "Newton's rings" package is fixed at tripod stand not very high above gate aperture. "Newton's rings" package is oriented at 45° or higher angle with pass of light direction. Lower area of demonstration screen gathers light reflected at package. The lens of 12 cm focal distance is utilized for screening of Newton's rings. The lens is adjusted according to sharpening of the shapes screened. The light of lens is directed to the top of screen. Projector's object-glass is adjusted by its height variation. Students should note interchange of differently colored rings in transmitted and reflected luminous fluxes. Also the reasons of high contrast range of rings are given. The same package complex is utilizes in demonstrating the interference in soap film. Newton's rings are screened by transmitted and reflected luminous fluxes, and then the "Newton's rings" package is replaced by soap film. Thus, optical scheme can be adjusted by a subject being more stable than soap film. Newton's rings can be easily observed in monochromatic light of semiconductor laser. Fig. 7 illustrates the optical scheme of such experiment. Lens widens laser beam up to (4-5) mm diameter. This beam falls at package's surface with 45° angle. Both screens show interference pattern without any adjustment.

Diffraction

The wave optics package contains units utilized in diffraction studying. These are 0.3 mm and 0.6 mm width slits, 0.2 mm diameter thread, 0.8 mm diameter hole, two gitters (50 mm-1 and 100 mm-1) and two-dimensional structure model (a very fine material). All the units are utilized in experiments on parallel and divergent beams diffraction. Semiconductor laser is the source of beams. The gitters provide with observing several diffraction orders in natural light supplied by projector. An appropriate optical scheme is quite simple. Semiconductor laser and subject of diffraction are installed at desktop. The experiment illustrates diffraction pattern of each object. The experiment also studies how slit width and grating groove influence distance between diffraction maximums. Screen bending is recommended until the hade approaches to 45° if slit and thread diffraction is expected. An additional lens's installation is recommended if diffraction by aperture is expected. Fig. 8 illustrates optical schemes of diffraction experiments in divergent beam. The scheme certainly consists of converging lens which creates divergent beam behind the focus. Thread and lens stay approximately in focal area. The aperture moves away from focus in about 3 focal distances. The main criterion is high resolution pattern in screen. The experiments demonstrate so strong diffraction effects that the concept of geometrical shadow becomes senseless. Spectroscopy utilizes gitters more often than prisms. That is why experiments on light expansion to spectrum have certain practical importance. Fig. 9 illustrates optical scheme of spectrum's projection by projector. Lens and revolving mirror of projector are installed at (35-40) cm high above gate aperture. The gitter is fixed at appropriate frame directly above projector's aperture. Incident light's collimation is significant in experiments because it depends on spectrum's resolution at screen. Thus, optical scheme consists of two slit diaphragms. The width of first one fixed directly near gitter is 5 mm. Another one with width of (2-3) mm is fixed at optical table which is installed at projector's gate aperture. After the 50 mm-1 gitter is installed and the spectrum is drawn, one should note that slightly colored stripe in the center of diffraction gitter is the zeroth diffraction order. The light is not expanded to spectrum at the zeroth diffraction order. The first diffraction order spectrums can be seen above and below the zeroth diffraction order. The spectrum widens due to its order's increment. As a result, the spectrums of second, third and further orders are aliased. Color distortion is observed. Students should be helped in explaining the diffraction pattern in both cases, after the 150 mm-1 gitter is installed. It must be concluded that diffraction angle of color blue is less than diffraction angle of color red. Also gitter's resolution increases as well as the diffraction order and number grating groove increase too.

The packages described above does meet modern requirements to educational experiment. This package demonstrates as full as possible the statements represented in course of physics. The Package provides with fast and obvious experiment. It helps student in understanding the right conception of physics and developing skills.

Literature www.l-micro.ru