The energy stored in a 90 cm length of a laser beam operating at 10 mW is  _________×10−12J

Time taken by laser beam to move through a distance 90 cm is, t=90C  t=90cm3×1010cm/s ⇒t=30×10−10s The energy contained in 90  cmlength of laser beam is  U=power× time=10×10-3×3×10−9 J So,  U=30×10−12J