Light

The separation between the dark lines in an interference pattern is given by the formula: Where: - is the separation between the dark lines, - is the wavelength of the monochromatic light, - is the distance between the slits and the screen, - is the distance between the slits. From the formula, it is evident that the separation between the dark lines is directly proportional to the wavelength .
Thus, to increase the separation between the dark lines, we can increase the wavelength .
Therefore, using monochromatic light of a longer wavelength will increase the separation between the dark lines of the interference pattern.

The lens is a plano-convex lens, where:
- (radius of curvature for the convex face),
- (radius of curvature for the concave face, negative because it's opposite in direction).
Using the lens maker's formula: Substitute the given values: Simplifying this:
Thus, the focal length .