Why is wavelength the main limiting factor on limit of resolution in light microscopy?

The resolution of a microscope is the smallest resolvable distance between two objects. It is defined as the shortest distance between two points on a specimen and the observer can still distinguish them.
The wavelength is a determining and limiting factor in the degree of resolution afforded by the microscope. The relationship between the wavelength and the resolution is:
Shorter wavelengths yield higher resolution and visa versa. 

Lx bxh