Why is visible light not optimal for microscopes or computer chips? Short Answer: Visible Light has wave properties and the interactions of these waves are not always ideal. We can no longer use the ray model of light - we must use the wave model.
Diffraction: the behavior of a wave when it encounters an obstacle in its medium. Diffraction, in general causes a wave to bend around obstacles and make patterns of strong and weak waves. Diffraction can be useful (such as using x-rays to find bones) or unhelpful (such as light entering telescopes)
Incoherent light: When multiple parts of the light wave are out of step from with each other and have different phase constants (ex. light from the sun)
Coherent light: When all parts of the wave are in step and have the same phase constant (light from a laser beam)
Diffraction angles depend only on the unit-less ratio of λ/d in which λ = wavelength and d = center to center spacing between the slits.
When λ/d is small (< 10-4), the ray model of light and the wave model of light must give approximately the same result.
Huygen's principle is used to describe the constructing and destructing interference that occurs between waves in a double slit experiment
It states that "any wavefront can be broken down into many small side-by-side wave peaks, which then spread out as circular ripples, and by the principle of superposition, the result of adding up these sets of ripples must give the same result as allowing the wave to propagate forward.