The term is 'alternating voltage', not 'alternate voltage'. For an a.c. voltage or current, the average value is taken over half its wavelength because, over a complete wavelength it is, of course, zero. For a sine wave, the average value (over half a wavelength) is 0.637 Vmax or 0.637 Imax.

That's called the wavelength.

The average wavelength of microwaves is typically around 12.2 centimeters, which corresponds to a frequency of 2.45 GHz. These wavelengths are commonly used in household microwave ovens for cooking food.

There isn't any. The amplitude gives no information about the

frequency or wavelength, and is completely unrelated to them.

What Wavelength