A ray of yellow light (f = 5.09 * 1014 hz) travels at a speed of 2.04 * 108 meters per second in

Velocity = fλ

where f is frequency in Hz, and λ is wavelength in meters.

2.04 * 10⁸ m/s = 5.09 * 10¹⁴ Hz * λ

(2.04 * 10⁸ m/s) / (5.09 * 10¹⁴ Hz) = λ

4.007*10⁻⁷ m = λ

The wavelength of the yellow light = 4.007*10⁻⁷ m