Question

How do you calculate light frequency?

Answer 1

Speed of light $\left(c\right)$ over wavelength $\left(\lambda \right)$
Explanation:
Considering that light travels at the speed of light (after all it's not called the speed of light for nothing), then the only thing that affects frequency (or number of waves per second) would be the wavelength. This occurs only in a vacuum by the way. In a non-vacuum , the speed of light does not equal $c$ so numbers might not be exactly the same, frequency still equals the speed of light over wavelength , it is just the speed of light is ever so slightly lower.