Interference is the phenomenon in which two or more waves combine to form a new wave with an amplitude that is the sum of the amplitudes of the individual waves.

Diffraction is the phenomenon in which a wave spreads out as it passes through an aperture or around an obstacle.

Constructive interference occurs when the path difference between the waves is an integer multiple of the wavelength, resulting in the waves reinforcing each other.

Destructive interference occurs when the path difference between the waves is a half-integer multiple of the wavelength, resulting in the waves canceling each other out.

The pattern produced by the interference of two coherent waves is called an interference pattern.

The pattern produced by the diffraction of a wave is called a diffraction pattern.

The wavelength of a wave is inversely proportional to the size of the interference or diffraction pattern, meaning that shorter wavelengths produce smaller patterns and longer wavelengths produce larger patterns.

Young's double-slit experiment demonstrated the wave-like nature of light by showing that light could interfere with itself, just like waves in water.

The Millikan oil drop experiment demonstrated the particle-like nature of light by showing that light could exert a force on charged particles, just like particles of matter.

Fermat's principle states that the path of a light ray is the path that minimizes the time taken to travel from one point to another.

Huygens' principle states that every point on a wavefront is a source of secondary wavelets, which spread out in all directions.

Young's principle states that the intensity of light at a point is proportional to the square of the amplitude of the light wave.

Fresnel's principle states that the diffraction pattern produced by a single slit is the same as the interference pattern produced by two slits separated by a distance equal to the width of the single slit.

Refraction is the phenomenon in which a wave changes direction when it passes from one medium to another.

Snell's law relates the angle of incidence of a wave to the angle of refraction.