If the angle of incidence is twice the angle of refraction in a medium of refractive index $\mu$,then the angle of incidence is

$A$ magician during a show makes a glass lens with $n = 1.47$ disappear in a trough of liquid. What is the refractive index of the liquid? Could the liquid be water?

Light takes $8$ min $20$ sec to reach the Earth from the Sun. If the whole atmosphere were filled with water,how much time would the light take to reach the Earth? (Given refractive index of water,$_a\mu_w = 4/3$)

$A$ student sees the top edge and the bottom centre $C$ of a pool simultaneously from an angle $\theta$ above the horizontal as shown in the figure. The refractive index of water which fills up to the top edge of the pool is $\frac{4}{3}$. If $\frac{h}{x}=\frac{7}{4}$,then $\cos \theta$ is

$A$ light ray is incident on a spherical glass surface at an angle $\alpha$ and is refracted at an angle $\beta$. What is the angle of deviation between the emergent ray and the incident ray?

$A$ tank is filled with benzene to a height of $120 \,mm$. The apparent depth of a needle lying at the bottom of the tank is measured by a microscope to be $80 \,mm$. The refractive index of benzene is: (in $.5$)