# Reversal of Photoinduced Bending of Crystals Due to Internal Refraction of Light
## 3 Experimental Results
The obtained result of changing the direction of the crystal bend with a change in the irradiation wavelength is phenomenologically similar to the phenomenon observed in diarylethene crystals [11], when a change in the wavelength from 365 nm to 380 nm also led to a change in the bend direction to the opposite, but was observed only at some initial stage of transformation. It is noteworthy that, similar to the effect discussed here, the 380 nm radiation is also characterized by weaker absorption in diarylethene crystals compared to the 365 nm radiation [11]. As the explanation of the photoinduced bending is based on the traditional consideration of BBL law in [11], the authors assume a different sign of strain \(\varepsilon_{0}\) at a small degree of transformation as the reason of the bend direction reversal; when the strain sign becomes usual upon further transformation, the direction of the bend returns to normal. The results of the study proposed here offer a different look at the effect observed in [11]. The dependences of the crystal curvature on time are shown in Figure 4. Information of primary interest is in the sign of the curvature, as well as in the magnitude of its change, while the characteristic duration of the process is determined mainly by the intensity of a particular source and the quantum yield of the reaction at the corresponding wavelength. The largest bend reversal effect was obtained for 403 nm light source, since light absorption by the substance is the weakest at this wavelength, and the expected refractive factor should be most pronounced. The lower magnitude of the effect at 465 nm is explained by the higher absorption at this wavelength. The absorption of 523 nm light by the initial nitro isomer is comparable to the absorption at 403 nm, thus a similar bending magnitude would be expected for these light sources. However, the transformation to the nitrito form leads, according to [31; 39], to an approximately twofold increase in the extinction at 523 nm, so the effect of refraction is reduced for this light source. The low conversion rate at \(\lambda=523\) nm is due to low quantum yield, approximately an order of magnitude lower than at 403 nm [39]. The assumption about the influence of refraction providing the concentration of light on the back side of the crystal due to the lens effect requires theoretical verification. It is intuitively clear that the lens effect should require convex shape of the irradiated crystal surface, while light falling on a flat crystal face should not lead to its concentration in the bulk. However, the diffraction effects may play important role for crystals of wavelength range thickness, independently of the cross-section shape. For this, a direct calculation of the electromagnetic wave penetrating crystals of various cross sections was carried out for various ratios of crystal thickness, wavelength and absorption depth. Figure 3: Combined image of the crystal at three different moments of transformation at \(\lambda=350\) nm: (1)—initial state of the crystal, (2)—position of maximum bending (after 15 min of irradiation), (3)—final shape of the completely transformed crystal (80 min of irradiation, the image is shifted vertically up). The arrows indicate the direction of light.