First of all, the condition of light interference is: 1, and the frequency of light should be as same as possible. 2. When two or more light waves interfere, their vibration directions should be as parallel as possible. 3. The phase difference should be stable.

These are the conditions for interference. Please note that the following are the conditions for "stable" interference. To be stable, the conditions will be much harsher: 1, the frequency of light should be "the same", in short, the frequency is the same. 2, the vibration direction should be consistent. 3. The phase difference should be fixed.

Well, if the above three harsh conditions are not met, interference can occur, but the clarity and stability of the disturbed image will be compromised. The farther away from this condition, for example, the direction of vibration is uneven and becomes vertical, and the interference becomes less clear and invisible until no interference occurs. Interference pattern is usually that we use a spectrum or something visible to receive the light intensity distribution in space, just like imaging, except that the light converges at the end when imaging, and interference is the regular separation of light.

For φ in the equation, it is called the phase of the wave, whether you are a mechanical wave or a light wave. And this phase φ = kδ, where k is the wave loss, k=2π/λ, and δ is the optical path. So in fact, what does he mean when the phase φ = 2π δ/λ? As you can see, δ is the distance from the point where you observe the interference to your light source, and λ is your wavelength. If you divide the two by one, it means how many wavelengths there are from your position to your light source. By calculation, you can know how your instantaneous vibration is. In this way, if you already know the phase of two light waves at this point (it can be calculated by dividing the distance by the wavelength and then multiplying it by 2π), then at this time, the position of two light waves, who vibrates high, will know who vibrates low, so that you can directly add and subtract the position when superposition (learning simple harmonic vibration, the displacement of a simple harmonic oscillator from the central position is a vector, and if the two waves vibrate in the same direction, they will therefore, your first formula is not written correctly. If it is written correctly, it should be asin (wt+2pai * δ/λ), and the second formula is no problem. There is no difference between the two, but the expression is different. If you have only studied mechanical waves, then I think your interference with light waves can be analogized from the interference of mechanical waves. There's no need to dig deeper. If you are lucky enough to be an undergraduate majoring in physics in the future, you will learn more about the interference of light. If you are a high school student, it is enough for high school students to know that light has interference diffraction phenomenon.