Light cannot be accelerated. Its speed is fixed.
acceleration a =dv/dt in vacuum, the speed of light is maximum c in a medium, it can be < or c check out the fermi lab site
299792458km/s is the speed of light in vacuum.
It is actually a heavy question for me!
try this link it may help you understand better www.newton.dep.anl.gov/askasci/phy99/phy99117.htm