Biologists often study the patterns of migratory birds by using radar (1–10 GHz electromagnetic waves) to track their flight. To check whether radar waves influence the birds’ flight, researchers tracked the birds visually, both with the radar on and with it off. The 9 GHz radar waves had an intensity of 400 W/m² at 250 m. What was the amplitude of the electric field at this distance? (The experiments showed that the radar did not affect the birds.)