# Reversal of Photoinduced Bending of Crystals Due to Internal Refraction of Light
## 3 Experimental Results  
In all previous studies of photoinduced bending of [Co(NH\({}_{3}\))\({}_{5}\)NO\({}_{2}\)]Cl(NO\({}_{3}\)), crystals with a thickness of more than ~20 \(\upmu\)m were used, and bending away from the light source was always observed, which corresponds to the positive curvature, according to the accepted convention about the curvature sign. This fully agrees with Equation (7), taking into account the negative light intensity gradient \(dI/dx<0\) due to its absorption in the bulk of the crystal [24; 35; 36; 39]. The light penetration depth in this substance, ~10 \(\upmu\)m at \(\lambda\) = 465 nm, and ~30 \(\upmu\)m at \(\lambda\) = 403 nm and at 523 nm, always turned out to be less or slightly more than the thickness of the crystals used. As a consequence, the light absorption was the main factor determining the distribution of light intensity in the volume of the crystal. Therefore, in order to experimentally detect the effect of any other factors, it was necessary to use crystals with a thickness much smaller than the characteristic absorption depth, \(h\eta<<1\). The thickness of the crystal obtained by dissolution is at least five-times less than the characteristic absorption depth for the most strongly absorbed light in the visible range, 456 nm. For all three sources of visible light, it turns out that the studied crystal bends in the opposite direction, compared to ordinary thicker crystals. At the same time, the crystal lengthens by \(\varepsilon_{0}\)\(\sim\)4% in all cases, which confirms that we are dealing with the same substance experiencing nitro-nitrito isomerization. An example of a combined image of a crystal in three successive moments of the process at \(\lambda\) = 465 nm is shown in Figure 2. Since crystal bending is strongly associated with an inhomogeneous distribution of the degree of transformation, then from Figure 2 follows the conclusion about the abnormal situation when the opposite side of the crystal isomerizes faster than the irradiated surface. The only possibility of such an effect must be associated with some factors that lead to the concentration of light intensity on the reverse side of the crystal. The most natural assumption is the lens effect, the refraction of light inside the crystal, which turns out to be stronger than absorption for a sufficiently thin crystal. To test this hypothesis, an experiment was carried out with the UV light source \(\lambda\) = 350 nm. According to the absorption spectra of [Co(NH\({}_{3}\))\({}_{5}\)NO\({}_{2}\)]Cl\({}_{2}\) solution given in [31], the extinction of this compound at 350 nm is approximately an order of magnitude higher than at 465 nm. Therefore, the characteristic absorption depth can be assessed to ~1 \(\upmu\)m at 350 nm. Three successive states of the crystal under the UV light source are combined in Figure 3: the initial, the maximum bend at the intermediate transformation, and the completely transformed crystal (the latter is shifted vertically not to overlap the original crystal). As can be seen, the direction of crystal bending returns to the normal when using strongly absorbed 350 nm light. It is also seen that the crystal bend amplitude turned out to be much stronger with the UV, compared to \(\lambda\) = 465 nm. This means that the absorption of light has a greater effect on the intensity non-uniformity than the expected refraction. The free end of the crystal left the focal plane when bent, so that the absolute values of the curvature are unreliable. Nevertheless, the theoretical estimate of the  
Figure 2: Combined image of the crystal at three different moments of transformation at \(\lambda\) = 465 nm: (1)—initial state of the crystal, (2)—position of maximum bend (irradiation for 3 min), (3)—final shape of the crystal (after 36 min of irradiation). The arrows indicate the direction of light. minimum radius of curvature for this crystal at \(\varepsilon_{0}=0.04\) and \(\mu^{-1}=1\)\(\upmu\)m is about 70 \(\upmu\)m, according to [1], which is consistent with the observation.