The objective lens in a DVD player must move quickly to keep the laser beam focused on the disc's reflective layer. Why must this lens have a very small mass?

The small mass of lens in a DVD player accounts for small inertia of the lens. Hence, even on application of force of small magnitude, the lens is rapidly accelerated. Moreover, the small mass of lens also accounts for lower resistance to change in its motion and therefore, again can be rapidly accelerated back and forth.