Question

Take a thin thermocol sheet. Cut it in circular discs of different radii like $2cm,3cm,4cm,4.5cm,5cm$ etc and mark centers with sketch pen. Now take needles of length nearly $6cm$. Pin a needle to each disc at its centre vertically. Take water in a large opaque tray and place the disc with $2cm$ radius in such a way that the needle is inside the water as shown in fig. Now try to view the free end (head) of the needle from surface of the water.
Are you able to see the head of the needle?

Answer 1

We will be able to see the end of the needle,till the angle of incidence is less than critical angle and since we know that critical angle for water is ${48.59}^0$ .Hence if i> ${48.59}^0$ then the head of needle will not be visible.