A vessel is half filled with a liquid of refractive index $$\mu$$. The other half of the vessel is filled with an immiscible liquid of refractive index $$2 \mu$$. The apparent depth of the vessel is $$50 \%$$ of the actual depth. Then $$\mu$$ is

A mosquito O is sitting infront of a glass rod having spherical end of radius of curvature 40 cm. The image would be formed at

An achromatic convergent doublet of two lenses in contact has a power of +5 D. The power of converging lens is +6 D. The ratio of the dispersive power of the convergent and divergent lenses is

The property of light used in optical fibre cables is