Which of the following combinations should be selected for better tuning of an L-C-R circuit used for combination ?

A linear aperture whose width is 0.02 cm is placed immediately in front of a lens of focal length 60 cm. The aparture is illuminated normally by a parallel beam of wavelength 5 $$ \times $$ 10^$$-$$5 cm. The distance of the first dark band of the diffraction pattern from the centre of the screen is

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

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