Bhagavad Gita · Akshara Brahma YogaChapter 8, Shloka 26 (8.26)

Shankaracharya — bright = the light of knowledge. Jñāna-prakāśakatvāt śuklā, tad-abhāvāt kṛṣṇā — "bright" because it reveals knowledge, "dark" because of its absence; jagataḥ iti adhikṛtānāṃ jñāna-karmaṇoḥ na jagataḥ sarvasyaiva — "of the world" means of those qualified for knowledge and action, not of the entire world; śāśvate nitye saṃsārasya nityatvāt.

Acharya Ramanuja. Śuklā gatiḥ arcir-ādikā kṛṣṇā ca dhūmādikā — the bright = the path of light (archir-adi), the dark = the path of smoke; ete … jñāninām vividhānāṃ puṇya-karmaṇāṃ ca śrutau śāśvate mate — in the scriptures these are held to be eternal, for the wise and for the various doers of virtuous deeds; evidence Chhandogya 5.10.1, 5.10.3 — those who worship with faith and austerity in the forest go to the light, those who perform sacrifices, public works and charity in the village go to the smoke.