Sāmkhya Kārikā Kārikā XIII

Sattva is considered to be light and bright, Rajas exciting and mobile, and Tamas is only heavy and enveloping. Like a lamp, their function is to gain an end. 

Sattva is light and bright. When Sattva predominates then the limbs become light, the intellect becomes bright and the organs become clear (i.e. acute).

 Rajas is exciting and mobile. Upaṣṭambhaka is that which excites or stimulates. Just as a bull is very much excited at the sight of another bull, so is the nature of Rajas. And Rajas is observed to be mobile. A man of the nature of Rajas is fickle-minded. 

Tamas is only heavy and enveloping. When Tamas predominates, then the limbs become heavy and the organs become enveloped (i.e., dull)—incapable of apprehending their objects.