Two points on a wave are in phase if they are both at the same point of the wave cycle, they will have the same displacement and velocity and their phase difference will be a multiple of 360° (2π radians), they do not need the same amplitude, only the same frequency and wavelength
Two points are completely out of phase when they're an odd integer of half cycles apart e.g. 5 half cycles apart where one half cycle is 180° (π radians)
Occurs when one wave has positive displacement and the other has negative displacement, if the waves have equal but opposite displacements, total destructive interference occurs
Formed from the superposition of 2 progressive waves travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude
You can double the first harmonic frequency to find the second harmonic where there are 2 antinodes, you triple the first harmonic frequency to get the third harmonic where there are 3 antinodes, and so on for the nth harmonic
Using white light instead of monochromatic laser light gives wider maxima and a less intense diffraction pattern with a central white fringe with alternating bright fringes which are spectra, violet is closest to the central maximum and red furthest
The type of interference described above can also be demonstrated in sound waves through a very similar process, however instead of using a double slit, you could use two speakers connected to the same signal generator. And the intensity of the wave can be measured using a microphone to find the maxima (equivalent to light fringes), and minima (equivalent to dark fringes)
Evidence for the wave nature of light was provided by Young's double slit experiment because diffraction and interference are wave properties, and so proved that EM radiation must act as a wave (at least some of the time)
Knowledge and understanding of any scientific concept changes over time in accordance to the experimental evidence gathered by the scientific community