$2-$ A laser Doppler anemometer made up of $He-Ne$ laser $)=(632\mathrm{.}8nm$ is used to measure the velocity of water at a point in a flow. A $150-$ lens, with $=11$ degrees, is used to operate the LDA in a dual beam mode. If an average Doppler frequency of $1\mathrm{.}41MHz$ is measured, estimate the velocity of water.