5. Light with frequency 7.5 x 10¹⁴ Hz is

b. Blue.

This is second of two questions with some calculations involved:

The wavelength of this light is:

Using:

v=fl

We can divide both sides by f to get:

l=v/f

Or:

l=(3 x 10⁸ m/s) /(7.5 x 10¹⁴  1/sec) = 400 x 10^-9 m, or 400 nm. That's near the blue end of the spectrum (See tutorial 1, chapter 4 of FEU text.)