of course, you need to know some math:
7.628 cm * 2 = 15.256 cm
Now, since the speed of light is equal to the wavelength times the frequency, we can figure out the speed of light. But we don't know the frequency of the microwaves. Apparently, most microwaves operate at 2.45 gigahertz, or 2,450,000,000 Hz. So, we take the the product of the wavelength and the frequency:
15.256 cm * 2,450,000,000 Hz = 37,377,200,000 cm/s which, given that we are doing this in a kitchen (and a small error our measurements are multiplied by 4,900,000,000), is shockingly close to the actual speed of light, which is 29,979,245,800 cm/s, or, as it is typically defined, 299,792,458 meters per second.
and, although the US military hesitates to use their warm and fuzzy death ray, you can find how to make a HERF gun at instructables too...