What is monochromatic light?

Does the focal length of a lens change if instead of blue light, monochromatic red light is used to measure it ?

"When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have same frequency as the incident frequency". Is this statement true? If yes, why? If you think it is not true, why?

In a double slit experiment, the two slits are 1 mm apart and the screen is place 1 m away. A monochromatic light of wavelength 500 nm is used. What will be the width of each slit for obtaining ten maxima of double slit within the central maxima of single slit pattern?

In a young's double slit experiment two slits are made one millimeter apart and the screen is placed one meter away. What is the fringe separation when monochromatic light of wavelenght 500nm is used?

What is refraction of light?