Anh sang phat photon theo hinh cau Nhu vay tinh mat do pho ton la
$n=\frac{P.\lambda }{hc.4\pi .R^{2}}$
Neu goi dien tich phan may ma pho ton cham vao ma may cam nhan dk la S va so photon cham vao ma may van cam nhan dk la N thi
$N=\frac{P\lambda _{1}}{hc.4\pi .R_{1}^{2}}=\frac{P\lambda _{2}}{hc.4\pi .R_{2}^{2}}=>\frac{R_{1}}{R_{2}}=\sqrt{\frac{\lambda _{1}}{\lambda _{2}}}=\frac{5}{6}$
Vay de dang suy ra dk $R_{1}=150km$