In wave diffraction, which two factors primarily determine how much a wave spreads after passing through an opening (or around an obstacle)?

A diffraction grating has$500\text{lines}/\mathrm{mm}.$ If it is illuminated by a laser at$532\mathrm{nm},$ how many bright spots will be observed on a screen placed behind the grating?

A particular kind of oil with an index of refraction of $1.46$ has spilled on water. The different thicknesses of the oil slick result in different colors being strongly reflected at different parts of the spill. But near the edges, you identify the thinnest part of the oil layer that strongly reflects green light with a wavelength of 550 nm when you are directly overhead. What is the thickness of the oil at this point? Assume light is incident normal to the oil surface.

You place one glass slide on top of another, with an animal hair between them at one end. You illuminate the slides with blue light of wavelength $450\mathrm{nm}.$ You count along the slide and find 300 transitions from bright to dark and back to light. How thick is the hair?

A carbon dioxide laser produces $10.6\mathrm{\mu }\mathrm{m}$ electromagnetic waves, which pass through a circular aperture with diameter $1.0\mathrm{mm}$. What is the approximate width of the laser beam when it strikes a target $3.0\mathrm{m}$ away?