In 1927, Heisenberg put forward a principle known as Heisenberg’s uncertainty principle.
According it, “it is not possible to measure simultaneously both the position and momentum (or velocity) of a microscopic particle, with absolute accuracy.”
Mathematically, this principle is expressed as:
∆x * ∆p = h/4 π
∆x = uncertainty in position
∆p = uncertainty in momentum
The constancy of the product of uncertainties means that, if the position of the particle is known with more accuracy, there will be large uncertainty in momentum and vice versa.
This uncertainty arises, as all observations are made by impact of light, the microscopic objects suffer a change in position or velocity as a result of impact of light. So there is a disturbance in them due to the measurement.
The principle does not affect the measurement of large objects as in these cases impact of light does not created any appreciable change in their position or velocity.