Two vectors a and b are called perpendicular to each other if-

We know that the angle between the  two vectors \(\vec{a}\)  & \(\vec{b}\)  is given by

cos(θ) = (\(\vec{a}\)  .\(\vec{b}\))/ (|\(\vec{a}\)|).(|\(\vec{a}\)|) 

If the two vectors are perpendicular to each other then angle between them is 90º.

  ⇒cos(90º)  =(\(\vec{a}\) .\(\vec{b}\))/ (|\(\vec{a}\)|).(|\(\vec{a}\)|)

  ⇒  0  =(\(\vec{a}\) .\(\vec{b}\))/ (|\(\vec{a}\)|).(|\(\vec{a}\)|)

  \(\vec{a}\) .\(\vec{b}\) =0.