Radar (radio detection and ranging) is used in weather for detecting the presence of precipitation. This is achieved by emitting radio waves into the atmosphere which have high frequencies, so that small droplets can be detected.
Precipitation detection with radar is achieved by measuring the strength of the reflected radio wave.
A different technology that can be used for weather radar purposes is Lidar, which stands for light detection and ranging. While using the same principles as radar, (ultraviolet) light with very short wavelengths is used instead of regular microwaves.
Lidar based weather radar is capable of detecting very small droplets, insects and aerosols. It can be used for detecting cloud base altitude, but by consequence of its wavelength it cannot see anything in the cloud.