Analog of the Umov experiment in a birefringent medium

Abstract

Classical demonstration experiment on visualization of light polarization plane rotation in an optically active medium is the Umov experiment [1–4]. When a collimated polarized light beam passes through an optical cell filled with a sugar solution in water and radiation scattered at an angle of 90o to the direction of beam propagation is observed, a bright dashed line typical of light scattered in an isotropic medium is seen rather than a solid line. The essence of the phenomenon is that when the beam passes through the active medium, its polarization plane is rotated. Due to the fact that the electromagnetic waves are transverse in character, the scattered light intensity vanishes when the direction of light polarization plane coincides with the observation direction. Because the effect is rather weak, to observe several tracks, the cell length must be no smaller than ∼1 m (even when it is filled with the concentrated sugar solution). Analogous character of scattering is observed in crystalline media [5–7] in which the beam polarization state also changes during beam propagation. However, because the birefringence effect in crystals is generally relatively strong, the similar scattering pattern can be seen with naked eye only in the direction close to the optical crystal axis where its intensity is low. In the present work, experiments on light scattering in an Icelandic spar crystal and a Plexiglas bar are described. Linearly polarized He-Ne laser or laser pointer radiation with power 10 mW P ≤ passed through the spar crystal of low optical quality (red and green pointers were used). To observe a scattering track stripe in the crystal, the laser beam was incident in the direction close to the optical crystal axis (Z-cut). Figure 1a shows a photograph of the scattering pattern in the spar crystal 4.2 cm long. Since the birefringence intensity in the crystal depends on the direction of light propagation, smooth crystal rotation causes changes in the scattering stripe period. They were observed in our experiment. The bright dashed scattering straight line can also be observed in plexiglass where, as a rule, small residual birefringence is always presented [7]. Figure 1b shows a photograph of light scattering pattern in the organic glass rod 50 cm long. In both cases, no professional polishing of specimen sides is required for clear visibility of tracks. Their simple coating is sufficient. The birefringence intensity of the medium n Δ is related to the track period Λ by the formula / n Δ = λ Λ ; it is determined exactly enough. Based on observations of the scattering pattern through a polaroid that is rotated, it is easy to verify that radiation scattered at an angle of 90o is almost completely polarized. The above-described experiments whose ideology and observed pattern are analogous to the Umov experiment are performed with simple accessible materials. The track stripe of scattered light in both cases is well seen by naked eye in slightly turbid specimens in a darkened room. The experiments on measuring the optical anisotropy of the medium for light scattering can provide the basis for a laboratory work or a demonstration experiments for a large audience.