A striking feature of how classical Indian philosophy generally, and Samkhya specifically, models vision is that seeing is described as a form of contact rather than as passive reception. Where a modern account has light travel from an object to a receiving eye, several classical schools instead describe chakshu as extending toward its object, so that perception occurs at the point where the organ and rupa, form and colour, meet, the same contact-based model applied to the other four Jnanendriyas and their respective tanmatras.
Chakshu is produced, along with shrotra, tvak, rasana and ghrana, from the sattvic aspect of ahamkara, and like each of the other four, it is restricted to a single tanmatra. Rupa alone is chakshu's object; sound, touch, taste and smell fall entirely outside its operation, which is part of the discipline the whole eleven-organ scheme imposes, no organ is a general-purpose sensor, each is a dedicated channel for exactly one of the five basic qualities the system recognises.
Chakshu's tanmatra, rupa, occupies the middle position among the five, denser than shabda and sparsha, subtler than rasa and gandha, and this middling position matters for the elemental chain it feeds: rupa is the tanmatra from which tejas, the fire element, eventually condenses, and fire is traditionally credited in Indian thought with being the element responsible for form and visibility generally, a correspondence the tattva sequence makes structurally explicit rather than merely poetic.