How does one measure the frequency of a laser? I am taking a short introduction course on lasers. I never knew that the frequency of a laser is so high for a given wavelength (e.g. 780nm). I am wondering how people can measure the frequency of the laser. Do they downconvert the frequency of the light so that we could measure it with scope?
 A: The laser light 'vibration' (or any sort of light close to the visible range), is too fast for any electronics. However,  the frequencies at which electrons can 'vibrate' around atoms or within a solid are close to that. This means that visible light can be absorbed by materials which have 'transitions' in that frequency range. For example, molecule vibration or chemical bonds can be of this sort. That's also what makes these material have a particular color.
To measure precisely the frequency of light, however, the general technique is to use a dispersive material, such as a prism, or a diffraction technique, such as gratings. Concerning the first option, the prism, it is able to separate light of different frequencies because of the way electrons can vibrate within the prism. To explain it very briefly, light at different frequencies has to travel at different velocities inside the prism to match this electron vibration. In the end, different frequencies go different directions. For the second option, gratings, they make interferences between multiple sub-beams of the initial light beam in order to have most of the frequencies cancel out with themselves. Only one remaining wavelength remains.
A: Well  you measure the wavelength by any one of many means, and calculate the frequency from    c
You can measure the wavelength with a Fabry-Perot etalon.
A: If the frequency of one laser is known with precision (or is assumed to be the standard frequency), it is possible to directly measure the frequency of another laser whose frequency is within a few GHz of the standard laser, without measuring wavelengths.
This is accomplished by shining both lasers onto a very fast photodetector.  The frequency difference between the lasers produces a beat frequency which can be measured very precisely.  If the beat frequency is $\delta _{\nu}$ then the frequency of the second laser is $\nu _ {ref} + \delta _{\nu}$.
