Answer: This phenomenon happens due to upthrust exerted by water.
Step-by-step explanation:
We know that,
Liquid Pressure is directly proportional to the height of the vertical column in the liquid.(P∝h)
When a bucket filled water is sunk into the water container, there occur difference in the pressure in top and bottom of the water container. Due to this, water exerts an upward force on the bucket filled with water. This is called Uprthrust.
Upthrust on the bucket makes the bucket filled with water lose some of it's weight and causes apparent loss in weight.
Hence, the bucket filled with water seems light while it sinks into water.