Light of wavelength l 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 1.0 kms-1 from rest. The momentum p of a photon of wavelength l is given by p = h/l, where h is Planck's constant

Photons have momentum (P=hλ) which they carry away, the spacecraft will acquire momentum in the opposite direction according to law of conservation of momentum. No. of photons per second from laser =n. Then, from energy consideration is 0.5×10-3=nh(cλ)      n=(0.5×10-3)λ(ch)Rate of change of momentum of spacecraftnp=nhλ=(0.5×10-3)λch(hλ)=0.5×10-3cFrom Newton's second law,nhλ=ma⇒1000a=0.5×10-33.00×108=16×10-11⇒v=at