[填空题]
A laser used to weldnt c cb zk)).m9bo9q7f del c:p8ki ga5d 8aqqg7k:c6p8ttached retinas puts out $28$-ms-long pulses of $640$-nm light which average $0.68$-W output during a pulse. How much energy can be deposited per pulse and how many photons does each pulse contain?
E = J
N = $\times 10^{16}\ \text{photons}$
