Light of wavelength `lamda` from a small 0.5 mW He-Ne laser source, used in the school laboratory, shines from a spacecraft of mass 1000 kg. Estimate the time needed for the spacecraft to reach a velocity of `1.0km^(-1)` from rest. The momentum p of a photon of wavelength `lamda` is given by `p=(h)/(lamda)`, where h is Planck's constant.

Answer

`6 xx 10^(17)` sec