Can light cancel out light ? (Essence of Young's Double Slit Experiment )

 At first instance the thought of light canceling light might sound obsolete, but for Thomas Young it wasn't the case. 

Young believed in the wave nature of light so he tried to find evidence for the same and he used diffraction the property of waves to do so. His experiment was very simple to perform and understand but very crucial to determine nature of light.

Thomas young used a very simple technique to lock the phase of two light waves. For this he took a sodium lamp and in front of it placed an opaque screen with two slits at a very small distance. So that the two slits behave as two different sources having same phase thus locking the phase. The two slits will now emanate wavelets(as per Huygens principle). Now he observed that now on a screen in front of two slits there were dark and bright bands(fringes). This was the first time the concept of light canceling light was first observed.   

Actually if we try to understand the deeper meaning of this fringes we come to know that the fringes were a result of  constructive and destructive interference a characteristic property of waves. 

Actually the interference phenomenon and diffraction phenomenon are very much linked and in some cases almost same. Here I would like to quote Richard Feynman:

Richard Feynman said, “No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them.”

At the end I would like to suggest you to watch the veritassium's video of Young's experiment. It will give you more visual insights.