Vaisesika Sūtra 5.2.20

तेजसो द्रव्यान्तरेणावरणाच्च ॥२०॥

tējasō dravyāntarēṇa āvaraṇāc ca ॥20॥

[तेजसः (tejasah) - of light; द्रव्य-अन्तरेण (dravya-antareṇa) - by another substance; आवरणात्-च (āvaraṇāt-ca) - and by covering;]

Light is obstructed by another substance and also by covering.