One aspect of this disclosure relates to a computer-implemented method for determining a force acting on at least part of a structure, for example a biological structure, such as a DNA molecule. The method comprises controlling a light-sensitive system, e.g. of a microscope, to determine light information based on light from the structure. The light is incident on at least a part of the light sensitive system. The light-sensitive system may be said to capture the light from the structure. The at least part of the structure comprises one or more optically active entities, such as DNA intercalator molecules and donor/acceptor fluorophores. At least one of (i) an optical activity of the entities and (ii) a quantity of the entities depends on the force acting on the at least part of the structure. Furthermore, the light information defines a light property value associated with said at least part of the structure. The method further comprises determining the force acting on the at least part of the structure on the basis of said light property value and a reference light property value.