The two waves interfere with each other if they vibrate in the sale plane and travel along the same path. Retardation is the distance that one wave lags behind the other. This Retardation can be described either in terms of the number of wavelengths that one wave lags the other, or in terms of the distance in nanometers that one wave lags the other. When the Retardation is found to be equal to the integral number of wavelengths involving an integer *i*, then the two waves are considered in phase and they are usually found constructively interfering with each other. The resultant wave will be the sum of the two waves. When two waves are found out of phase, then they destructively interfere and cancel each other. When the Retardation is some intermediate value, the light is partially in phase and the interference is partially constructive. But then it can be also possible that in this case, light is partially out of phase if that is what one preferred and the interference is partially destructive.

The two waves can be resolved into a resultant vibration direction by means of vector addition if the two waves vibrate at an angle to each other. By constructing a parallelogram whose sides are parallel to the vibration directions of wave A and wave B, the resultant vibration direction can be obtained. Similarly, a component of a single wave may be resolved into any arbitrary vibration direction. By constructing a right triangle with X as the hypotenuse, the component of wave X, which can be resolved into a new vibration direction *V*, can be obtained. The vibration direction V is a product of the amplitude of wave X and the angle between the vibration direction X and the new vibration direction V. It must be noted that if V is 90 degrees to the original vibration direction, the resolved component must be zero. This is considered as one of the important observations and it accounts for a number of optical properties discussed in optical mineralogy.