Describe an experiment to determine the critical angle for a water-air boundary. or Describe an experiment to determine refractive index of liquid using air cell

Describe an experiment to determine the critical angle for a water-air boundary.

or

Describe an experiment to determine refractive index of liquid using air cell.

• The air cell is immersed in a liquid under test.
• A beam of monochromatic light is directed onto the air cell and then observed through the cell from the opposite side at E.
• The cell is then rotated on one side until light is suddenly cut off and the angular position θ1 is no
• The cell is again rotated in the opposite direction until light is suddenly cut off and the angular position θ2 is noted
• The refractive index of the liquid can then be calculated from

n  = 1/sinθ      where θ =(θ1+θ2)/2