A thin prism having refracting angle 10^o is made of glass of refractive infex 1.42. This prism is combined with another thin prism of glass of refractive index 1.7. This combination producess dispersion without deviation. The refracting angle of second prism should be

Two identical glass ($$\mu $$_g = 3/2) equiconvex lenses of focal length $$f$$ each are kept in contact. The space between the two lenses is filled with water $$\left( {{\mu _w} = 4/3} \right)$$. The focal length of the combination is

An air bubble in a glass slab with refractive index 1.5 (near normal incidence) is 5 cm deep when viewed from one surface and 3 cm deep when viewed from the opposite face. The thickness (in cm) of the slab is

A person can see clearly objects only when they lie between 50 cm and 400 cm from his eyes . In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens, the person has to use. will be