Optical fibre are based on

A disc is placed on a surface of pond which has refractive index 5/3. A source of light is placed 4 m below the surface of liquid. The minimum radius of disc needed so that light is not coming out is,

If specific resistance of a potentiometer wire is 10^-7$$\Omega $$ m and current flow through it is 0.1 amp., cross-sectional area of wire is 10^$$-$$6 m² then potential gradient will be

Which one among the following shows particle nature of light?