In the problems that follow assume the l given or asked for is that within the thin layer. Two glass plates are placed together so as to leave a narrow air gap between them. Oil has an optical density between air and water. I hope you've noticed an apparent inconsistency. The term interference is used to describe the interaction of waves.
Destructive interference is the cancellation on wave energy.
Constructive interference is the reinforcement of waves. What we have been looking at interference from very thin layers of material is called thin film interference . You have had contact with it in your own experience. Soap bubbles are colored because certain wavelengths of light will be reflected when the soap film reaches a particular thickness. It isn't the soapy water itself that is colored, merely the light reflected from it.