As the wavelength decreases, the frequency

The product of (wavelength) x (frequency) is always

the same number. (It's the wave speed.)

So if either one of them changes, the other one has to change by

the same factor in the other direction, to keep the product constant.

If the wavelength decreases, the frequency must increase.

In fact, the numbers are easy. If the wave shrinks to, say,

half as long as it started out, then the frequency must double.

As the wavelength decreases the frequency increases.

Think of the formula c = f (lambda)

When you divide c by a larger lambda (wavelength) then the value of f will be smaller, but if you divide c by a smaller lambda then the frequency will be larger