What are wave lengths infrared radiatio

Infrared radiation extends from the red edge of the visible light
spectrum at 700 nanometers all the way to the shortest microwave
wavelength of 1 millimeter.

This range of wavelengths corresponds to a frequency range
of roughly 430 terrahertz at the edge of red light, down to the
highest microwave frequency, at 300 gigahertz.

Sorry i don’t know what to do

Lexy is correct because angle r is between the incident wave,  n  = 0, and a diffracted wave.