A spaceship from a friendly, extragalactic planet flies toward Earth at 0.203 0.203 times the speed of light and shines a powerful laser beam toward Earth to signal its approach. The emitted wavelength of the laser light is 691 nm . 691 nm. Find the light's observed wavelength on Earth.

The observed wavelenght on earth will be 5.51x10^-7 m

Explanation:

Speed of light c = 3x10^8 m/s

Ship speed u = 0.203 x 3x10^8 = 60900000 = 6.09x10^7 m/s

Wavelenght of laser w = 691x10^-9 m

Observed wavelenght w' = w(1 - u/c)

u/c = 0.203

w' = 691x10^-9(1 - 0.203)

w' = 5.51x10^-7 m

at right angles to each other