Coherence and Path Difference - A Level Physics

Coherence refers to two or more wave sources that emit waves with a constant phase difference. For example, waves with different wavelengths, like red and blue light, are not coherent. Coherent light sources must have the same wavelength and thus the same color.

Phase difference is the fraction of a cycle between the oscillations of two particles. It is measured in degrees or radians.

Path difference is the difference in the distance traveled by two coherent waves from two different sources to a particular point. It is measured in meters or fractions of a wavelength. For example, if one wave travels four wavelengths and another travels five to reach a point, the path difference is one wavelength. Path difference is essential for understanding phenomena like interference using slits and diffraction gratings.

Constructive interference occurs when waves have a path difference that is a multiple of the wavelength (nλ), causing their peaks to coincide and reinforce each other. This results in an increased amplitude.

Destructive interference occurs when the path difference is a multiple of half a wavelength (n/2 λ, where n is an odd integer), causing the waves to cancel each other out. This results in a decreased or zero amplitude, such as where light cancels out other light.