|Newton's ring, a familiar interference phenomenon,
can be seen whenever a transparent object or film closely overlies
a reflecting surface. Examples include a convex lens on an optical
flat, a film of oil floating on water and, all too frequently, a transparent
film in a glass slide mounting. Coherent light (light from a single
source) reflected from both sides of a thin air gap or oil film interference
with itself, giving rise to a pattern of colored or, for monochromatic
light, light and dark fringes.