A closed-circuit television was being used in certain technical colleges for lecture purposes. Frequently, projection device application is provided to be the only one tool for performance of conventional demonstrations which are difficult or impossible to be shown simultaneously to a great number of students because of insufficient luminosity or image sizes. We can show, for example, the light diffraction with Airy pattern, Brownian motion, and suchlike.
In 1973 using a television technique we managed to demonstrate anomalous dispersion in sodium vapor to the students' audience. We assembled a facility with the Jamin interferometer and a spectrograph. Two quartz tubes of 35 cm in length closed by plane-parallel glass windows were placed into the interferometer. In one of them there was metal sodium and the other one was intended for compensation. We used the second-order diffraction spectrum which provided a dispersion of about 0.4 nm/mm.
The classical approach of crossed dispersions is being demonstrated (Fig. 1). Therein the interference fringes are plotting down on the display the graphic chart of refraction index from the wave length in immediate proximity to spectral lines of sodium 589, 589.6 nm. As the temperature increases, so does the sodium vapor density that leads to free spectral range extension (Fig.2). The students are to observe such kind of smooth changes.