The frequency of a given region of the electromagnetic spectrum ranges from

4 * 1014 to 7.5 * 1014 Hz. The speed of light is 2.998 * 108 m/s. Which waves are found in this region?

The ranges given are 4 * 10^14 to 7.5 * 10^14 Hz. At these frequencies, visible light type of waves are found in this region. This occurs at a wavelength of 400 - 700 nanometers and a color of violet through red. Outside this range, the naked eye cannot see the waves.