Wave Phase

Interference fringe, a bright or dark band (as shown in image-1a) caused by beams of light that are in phase or out of phase (ref. image 2) with one another.

Light waves and similar wave propagation, when superimposed, will add their crests if they meet in the same phase (the waves are both increasing or both decreasing); or the troughs will cancel the crests if they are out of phase; these phenomena are called constructive and destructive interference, respectively. If a beam of monochromatic light (all waves having the same wavelength) is passed through two narrow slits (ref. image 3)(an experiment first performed in 1801 by Thomas Young, an English scientist, who inferred from the phenomenon the wavelike nature of light), the two resulting light beams can be directed to a flat screen on which, instead of forming two patches of overlapping light, they will form interference fringes, a pattern of evenly spaced alternating bright and dark bands.

Phase of a wave: In Physics, the phase of a wave or periodic function of some real variable like time is an angle like quantity representing the fraction of the cycle covered up to time t. It may be measured in any angular unit such as degrees or radians, thus increasing by 360 deg or 2π as the variable t completes a full period.

Image-1(a)