a, Picture of the sample used. To visualize the outline of the lens clearly, the picture was taken before filling the cuvette with index-matching viscous polymer. The imaged area is 3.2 mm (x) × 1.3 mm (z). b, Image based on measured frequency shifts, corresponding to a cross-sectional map of elastic modulus. c, Image created by using Brillouin scattering magnitudes as contrast. d, Representative cross-sectional line profiles taken along the dotted line in b and c. Arrows indicate which y axis scale applies.