Initial momentum is 200p_{m}
Explaination:
\({d\lambda\over\lambda}\times 100 =0.5 \)
\({d\lambda\over\lambda}\times 100 ={0.5\over100}={1\over200}\) and \(\triangle p=p_m\)
\(p={h\over\lambda},\) differentiating
\({dp\over d\lambda}=-{h\over \lambda^2}={-h\over \lambda}\times{1\over\lambda}={-p\over \lambda}\)
\({|dp|\over p}={d\lambda\over \lambda} \therefore {p_m\over p}={1\over 200}\)
p=200 p_{m}